Article

Ingestion of Glucose or Sucrose Prevents Liver but not Muscle Glycogen Depletion During Prolonged Endurance-type Exercise in Trained Cyclists

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Purpose: To define the effect of glucose ingestion compared to sucrose ingestion on liver and muscle glycogen depletion during prolonged endurance-type exercise. Methods: Fourteen cyclists completed two 3-h bouts of cycling at 50% of peak power output while ingesting either glucose or sucrose at a rate of 1.7 g/min (102 g/h). Four cyclists performed an additional third test in which only water was consumed for reference. We employed 13C magnetic resonance spectroscopy to determine liver and muscle glycogen concentrations before and after exercise. Expired breath was sampled during exercise to estimate whole-body substrate use. Results: Following glucose and sucrose ingestion, liver glycogen levels did not show a significant decline following exercise (from 325±168 to 345±205 and 321±177 to 348±170 mmol/L, respectively; P>0.05) with no differences between treatments. Muscle glycogen concentrations declined (from 101±49 to 60±34 and 114±48 to 67±34 mmol/L, respectively; P<0.05), with no differences between treatments. Whole-body carbohydrate utilization was greater with sucrose (2.03±0.43 g/min) vs glucose ingestion (1.66±0.36 g/min; P<0.05). Both liver (from 454±33 to 283±82 mmol/L; P<0.05) and muscle (from 111±46 to 67±31 mmol/L; P<0.01) glycogen concentrations declined during exercise when only water was ingested. Conclusion: Both glucose and sucrose ingestion prevent liver glycogen depletion during prolonged endurance-type exercise. Sucrose ingestion does not preserve liver glycogen concentrations more than glucose ingestion. However, sucrose ingestion does increase whole-body carbohydrate utilization compared to glucose ingestion. This trial was registered at clinicaltrials.gov as NCT02110836.
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... Consequently, the active, excitable fibre pool becomes progressively less oxidative during prolonged exercise, and power production increasingly requires activation of type II fibres, which have lower mitochondrial protein content (Nielsen et al. 2024). Similarly, liver glycogen is depleted during prolonged exercise (Casey et al. 2000;Gonzalez et al. 2015;Stevenson et al. 2009), and liver glucose output provides an additional glucose source for contracting muscles and prevents hypoglycemia (Gonzalez et al. 2016). Preventing hypoglycemia is crucial for avoiding neuroprotective downregulation of motor unit recruitment (Elghobashy et al. 2024;Glace et al. 2019;Nybo 2003). ...
... In line with this hypothesis, carbohydrate ingestion mitigates the reduction in power output at the heavy-to-severeintensity transition during exercise (Clark et al. 2019a, b;Clark et al. 2019a, b). This could plausibly be attributed to muscle glycogen-sparing (Tsintzas et al. 1995(Tsintzas et al. , 1996, although many studies have not reported a muscle glycogensparing effect of carbohydrate ingestion during prolonged cycling (Coyle et al. 1986;Gonzalez et al. 2015;Jeukendrup et al. 1999). However, the effect of carbohydrate ingestion during prolonged exercise on subcellular glycogen stores has not been assessed. ...
... However, the effect of carbohydrate ingestion during prolonged exercise on subcellular glycogen stores has not been assessed. In contrast, carbohydrate ingestion during exercise has been consistently shown to reduce liver glucose output, preserve liver glycogen stores, and prevent hypoglycemia (Bosch et al. 1994;Gonzalez et al. 2015;Hargreaves et al. 1995;Jeukendrup et al. 1999). ...
Article
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Purpose To determine the effect of carbohydrate ingestion during prolonged exercise on durability of the moderate-to-heavy-intensity transition and severe-intensity performance. Methods Twelve trained cyclists and triathletes (10 males, 2 females; V˙O2\dot{V}{\text{O}}_{2}peak, 59 ± 5 mL kg⁻¹ min⁻¹; training volume, 14 ± 5 h week⁻¹) performed an incremental test and 5-min time trial (TT) without prior exercise (PRE), and after 150 min of moderate-intensity cycling, with (POSTCHO) and without (POSTCON) carbohydrate ingestion. Results Power output at the first ventilatory threshold (VT1) was lower in POSTCHO (225 ± 36 W, ∆ -3 ± 2%, P = 0.027, n = 11) and POSTCON (216 ± 35 W, ∆ -6 ± 4%, P = 0.001, n = 12) than PRE (229 ± 37 W, n = 12), and lower in POSTCON than POSTCHO (∆ -7 ± 9 W, ∆ -3 ± 4%, P = 0.019). Mean power output in the 5-min TT was lower in POSTCHO (351 ± 53 W, ∆ -4 ± 3%, P = 0.025) and POSTCON (328 ± 63 W, ∆ -10 ± 10%, P = 0.027) than PRE (363 ± 55 W), but POSTCHO and POSTCON were not significantly different (∆ 25 ± 37 W, ∆ 9 ± 13%, P = 0.186). Blood glucose concentration was maintained in POSTCHO, and was significantly lower at the 120 and 150-min timepoint in POSTCON (P < 0.05). Conclusion These data suggest that durability of the moderate-to-heavy-intensity transition is improved with carbohydrate ingestion. This has implications for training programming and load monitoring.
... In fasted conditions, endogenous CHO reserves, primarily from skeletal muscle glycogen and blood glucose (derived from liver glycogen stores and gluconeogenesis), are the predominant energy substrate utilized by the working muscles during endurance exercise at moderate-vigorous intensities (Romijn et al. 1993). However, endogenous CHO stores can be significantly depleted during endurance exercise, limiting CHO availability and oxidation during the later stages of prolonged exercise (Coyle et al. 1986;Gonzalez et al. 2015). Depletion of endogenous CHO stores has been shown to contribute to fatigue and impaired exercise capacity (Bergstrom et al. 1967;Coyle et al. 1986). ...
... CHO ingestion during exercise maintains higher rates of total CHO oxidation throughout prolonged exercise, which is associated with improved performance in prolonged endurance events (Coyle et al. 1986;Smith et al. 2010). Improved CHO availability in late-exercise is due to increased oxidation of the exogenous CHO itself (Jeukendrup 2008;Smith et al. 2010), combined with greater availability of endogenous reserves due to sparing of hepatic glycogen (Jeukendrup et al. 1999;Gonzalez et al. 2015), and possibly muscle glycogen (Stellingwerff et al. 2007;De Bock et al. 2007), though the later finding is not consistently observed in the literature (Coyle et al. 1986;Gonzalez et al. 2015). ...
... CHO ingestion during exercise maintains higher rates of total CHO oxidation throughout prolonged exercise, which is associated with improved performance in prolonged endurance events (Coyle et al. 1986;Smith et al. 2010). Improved CHO availability in late-exercise is due to increased oxidation of the exogenous CHO itself (Jeukendrup 2008;Smith et al. 2010), combined with greater availability of endogenous reserves due to sparing of hepatic glycogen (Jeukendrup et al. 1999;Gonzalez et al. 2015), and possibly muscle glycogen (Stellingwerff et al. 2007;De Bock et al. 2007), though the later finding is not consistently observed in the literature (Coyle et al. 1986;Gonzalez et al. 2015). ...
Article
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Purpose To critically examine the research on novel supplements and strategies designed to enhance carbohydrate delivery and/or availability. Methods Narrative review. Results Available data would suggest that there are varying levels of effectiveness based on the supplement/supplementation strategy in question and mechanism of action. Novel carbohydrate supplements including multiple transportable carbohydrate (MTC), modified carbohydrate (MC), and hydrogels (HGEL) have been generally effective at modifying gastric emptying and/or intestinal absorption. Moreover, these effects often correlate with altered fuel utilization patterns and/or glycogen storage. Nevertheless, performance effects differ widely based on supplement and study design. MTC consistently enhances performance, but the magnitude of the effect is yet to be fully elucidated. MC and HGEL seem unlikely to be beneficial when compared to supplementation strategies that align with current sport nutrition recommendations. Combining carbohydrate with other ergogenic substances may, in some cases, result in additive or synergistic effects on metabolism and/or performance; however, data are often lacking and results vary based on the quantity, timing, and inter-individual responses to different treatments. Altering dietary carbohydrate intake likely influences absorption, oxidation, and and/or storage of acutely ingested carbohydrate, but how this affects the ergogenicity of carbohydrate is still mostly unknown. Conclusions In conclusion, novel carbohydrate supplements and strategies alter carbohydrate delivery through various mechanisms. However, more research is needed to determine if/when interventions are ergogenic based on different contexts, populations, and applications.
... It is important to emphasise that CHO ingestion rates required to elicit peak exogenous oxidation rates are very high and relevant only to prolonged excise (2.5-3 h) performed by highly trained athletes [46] (Fig. 3). Enhancing intestinal CHO absorption rates increases exogenous CHO availability for key tissues such as the liver and muscle, and lowers the gastrointestinal distress associated with the ingestion of large quantities of CHO [47,48]. ...
... During exercise without CHO feeding, the liver produces glucose via glycogenolysis and gluconeogenesis to maintain plasma glucose concentrations [56]. During prolonged (120 min), moderate-intensity exercise in the fasted state; the liver produces glucose at a rate of ~ 0.5 g⋅min −1 (30 g⋅h −1 ; Fig. 1), resulting in substantial liver glycogen depletion during exercise [47,56]. Glucose ingestion during exercise suppresses hepatic glucose production in a dose-dependent manner. ...
... g⋅min −1 (~ 72 g⋅h −1 ; Fig. 2), suggesting that all of the ingested glucose that is absorbed across the intestine is oxidized. Consistent with this, the ingestion of glucose at ~ 1.7 g⋅min −1 (~ 102 g⋅h −1 ) during prolonged (180 min) exercise results in no net changes in liver glycogen concentrations [47]. Since glucose-fructose mixtures can circumvent the saturation of SGLT1 to increase exogenous CHO availability from ~ 1.2 up to ~ 1.7 g⋅min −1 (42%). ...
Article
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The purpose of this current opinion paper is to describe the journey of ingested carbohydrate from 'mouth to mitochondria' culminating in energy production in skeletal muscles during exercise. This journey is conveniently described as primary, secondary, and tertiary events. The primary stage is detection of ingested carbohydrate by receptors in the oral cavity and on the tongue that activate reward and other centers in the brain leading to insulin secretion. After digestion, the secondary stage is the transport of monosaccharides from the small intestine into the systemic circulation. The passage of these monosaccharides is facilitated by the presence of various transport proteins. The intestinal mucosa has carbohydrate sensors that stimulate the release of two 'incretin' hormones (GIP and GLP-1) whose actions range from the secretion of insulin to appetite regulation. Most of the ingested carbohydrate is taken up by the liver resulting in a transient inhibition of hepatic glucose release in a dose-dependent manner. Nonetheless, the subsequent increased hepatic glucose (and lactate) output can increase exogenous carbohydrate oxidation rates by 40-50%. The recognition and successful distribution of carbohydrate to the brain and skeletal muscles to maintain carbohydrate oxidation as well as prevent hypoglycaemia underpins the mechanisms to improve exercise performance.
... In the fasted state, the main forms of carbohydrate utilised during exercise are skeletal muscle glycogen and plasma glucose (primarily derived from liver glycogen and gluconeogenesis) (van Loon et al. 2001). However, these glycogen stores can be rapidly depleted (by ß40-60%) within 90 min of moderate to high-intensity exercise (Casey et al. 2000;Stevenson et al. 2009;Gonzalez et al. 2015). Low endogenous glycogen stores can contribute to fatigue, thereby reducing endurance exercise capacity (Bergstrom et al. 1967;Coyle et al. 1986;Ortenblad et al. 2011;Alghannam et al. 2016). ...
... Dietary carbohydrates come in many forms, with glucose (polymers) being the most ubiquitous carbohydrate in most people's diets (Gonzalez et al. 2017). Glucose is also the primary cellular fuel source in most human tissues. ...
... During exercise, exogenous carbohydrate oxidation rates differ depending on the type of carbohydrate that is consumed (Cermak & van Loon, 2013). It has been well established that the maximal exogenous carbohydrate oxidation rate increases in a curvilinear fashion with carbohydrate ingestion rate, reaching peak exogenous oxidation rates of ß1.1 g/min when ingesting glucose (polymers) only during exercise (Jeukendrup & Jentjens, 2000;Gonzalez et al. 2017). Several factors may determine the rate at which exogenous carbohydrates are taken up and oxidized by the working muscles during exercise. ...
Article
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Carbohydrate availability is important to maximize endurance performance during prolonged bouts of moderate‐ to high‐intensity exercise as well as for acute post‐exercise recovery. The primary form of carbohydrates that are typically ingested during and after exercise are glucose (polymers). However, intestinal glucose absorption can be limited by the capacity of the intestinal glucose transport system (SGLT1). Intestinal fructose uptake is not regulated by the same transport system, as it largely depends on GLUT5 as opposed to SGLT1 transporters. Combining the intake of glucose plus fructose can further increase total exogenous carbohydrate availability and, as such, allow higher exogenous carbohydrate oxidation rates. Ingesting a mixture of both glucose and fructose can improve endurance exercise performance compared to equivalent amounts of glucose (polymers) only. Fructose co‐ingestion can also accelerate post‐exercise (liver) glycogen repletion rates, which may be relevant when rapid (<24 h) recovery is required. Furthermore, fructose co‐ingestion can lower gastrointestinal distress when relatively large amounts of carbohydrate (>1.2 g/kg/h) are ingested during post‐exercise recovery. In conclusion, combined ingestion of fructose with glucose may be preferred over the ingestion of glucose (polymers) only to help trained athletes maximize endurance performance during prolonged moderate‐ to high‐intensity exercise sessions and accelerate post‐exercise (liver) glycogen repletion. image
... Keywords: carbohydrate, cycling, sports nutrition Carbohydrate ingestion during exercise is a well-established method of improving prolonged endurance exercise performance (Coyle et al., 1986;Jeukendrup, 2010;Rollo et al., 2020). The mechanisms by which carbohydrate ingestion can improve performance are hypothesized to include sparing of endogenous glycogen stores, preventing hypoglycemia, and/or maintaining high rates of carbohydrate oxidation (Gonzalez et al., 2015;Rollo et al., 2020). While early sports nutrition guidelines made some consideration of body size (Rodriguez et al., 2009), some (but not all) contemporary guidelines ignore body mass and express recommendations as a rate of carbohydrate per unit time. ...
... Another putative explanation for higher lactate responses could be that identification of lactate threshold was erroneous, and thus, exercise intensity was higher in the smaller athletes. However, it is unlikely that errors in identification of lactate threshold would be systematically biased toward the smaller athletes, and the transient increase in lactate at the onset of carbohydrate feeding during exercise is documented (Gonzalez et al., 2015). Furthermore, the higher insulin concentrations support the hypothesis that relative glucose dose, rather than exercise intensity, explains most of the between-group variance in lactate concentrations. ...
Article
There is little evidence that body size alters exogenous glucose oxidation rates during exercise. This study assessed whether larger people oxidize more exogenous glucose during exercise than smaller people. Fifteen cyclists were allocated into two groups based on body mass (SMALL, <70 kg body mass, n = 9, two females) or (LARGE, >70 kg body mass, n = 6) matched for lactate threshold (SMALL: 2.3 ± 0.4 W/kg, LARGE: 2.3 ± 0.3 W/kg). SMALL completed 120 min of cycling at 95% of lactate threshold 1 . LARGE completed two trials in a random order, one at 95% of lactate threshold 1 (thereby exercising at the same relative intensity [RELATIVE]) and one at an absolute intensity matched to SMALL (ABSOLUTE). In all trials, cyclists ingested 90 g/hr of ¹³ C-enriched glucose. Total exogenous glucose oxidation was (mean ± SD ) 33 ± 8 g/hr in SMALL versus 45 ± 13 g/hr in LARGE-RELATIVE (mean difference: 13 g/hr, 95% confidence interval [2, 24] g/hr, p = .03). Large positive correlations were observed for measures of exogenous carbohydrate oxidation versus body size (body mass, height, and body surface area; e.g., body surface area vs. peak exogenous glucose oxidation, r = .85, 95% confidence interval [.51, .95], p < .01). When larger athletes reduced the intensity from RELATIVE to ABSOLUTE, total exogenous glucose oxidation was 39 ± 7 g/hr ( p = .43 vs. LARGE-RELATIVE). In conclusion, the capacity for exogenous glucose oxidation is, on average, higher in larger athletes than smaller athletes during exercise. The extent to which this is due to higher absolute exercise intensity requires further research, but body size may be a consideration in tailoring sports nutrition guidelines for carbohydrate intake during exercise.
... Carbohydrate ingestion during exercise is a well-established method of improving prolonged, endurance exercise performance [1][2][3]. The mechanisms by which carbohydrate ingestion can improve performance are hypothesised to include sparing of endogenous glycogen stores, preventing hypoglycaemia and/or maintaining high rates of carbohydrate oxidation [2,4]. Whilst some early sports nutrition guidelines made some consideration of body size [5], contemporary guidelines ignore body mass and express recommendations as a rate of carbohydrate per unit time. ...
... Another putative explanation for higher lactate responses could be that identi cation of lactate threshold was erroneous and thus exercise intensity was higher in the smaller athletes. However, it is unlikely that errors in identi cation of lactate threshold would be systematically biased towards the smaller athletes, and the transient increase in lactate at the onset of carbohydrate feeding during exercise is documented [4]. Furthermore, the higher insulin concentrations support the hypothesis that relative glucose dose, rather than exercise intensity explains most of the between group variance in lactate concentrations. ...
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Current guidelines do not consider body size for carbohydrate intake during exercise. This study assessed whether larger people can oxidise more exogenous glucose during exercise than smaller people. Fifteen cyclists were allocated into two groups based on body mass (SMALL, < 70 kg body mass, n = 9, 2 female) or (LARGE, > 70 kg body mass, n = 6) matched for lactate threshold (SMALL: 2.3 ± 0.4 W⋅kg − 1 , LARGE: 2.3 ± 0.3 W⋅kg − 1 ). SMALL completed 120 min of cycling at 95% of lactate threshold 1 . LARGE completed two trials in a random order, one at 95% of lactate threshold 1 [thereby exercising at the same relative intensity (RELATIVE)], and one at an absolute intensity matched to SMALL (ABSOLUTE). In all trials, cyclists ingested 90 g⋅h − 1 of ¹³ C-enriched glucose. Total exogenous glucose oxidation was (mean ± SD) 33 ± 8 g⋅h − 1 in SMALL versus 45 ± 13 g⋅h − 1 in LARGE-RELATIVE (mean difference: 13 g⋅h − 1 , 95%CI 2 to 24 g⋅h − 1 , p = 0.03]. Large positive correlations were observed for measures of exogenous carbohydrate oxidation versus body size (body mass, height and body surface area; e.g. , body surface area versus peak exogenous glucose oxidation, r = 0.85,95%CI: 0.51 to 0.95, p < 0.01). When larger athletes reduced the intensity from RELATIVE to ABSOLUTE, total exogenous glucose oxidation was 39 ± 7 g⋅h − 1 ( p = 0.43 versus LARGE-RELATIVE). In conclusion, the capacity for exogenous glucose oxidation is, on average, higher in larger athletes than smaller athletes during exercise. Body size may therefore be a consideration in tailoring sports nutrition guidelines for carbohydrate intake during exercise.
... Prolonged endurance exercise depletes endogenous glycogen stores and is associated with the onset of fatigue (8,47,86,87). Nevertheless, the intake of exogenous CHO during exercise not only helps to conserve endogenous glycogen stores (47,86,88) but also enables maintenance of substrate supply, thereby sustaining higher rates of whole body metabolism for longer while mitigating fatigue accumulation. Classic research by Coggan and Coyle (87) showed exogenous CHO intake, either by ingestion or direct venous infusion, significantly increased exercise tolerance following glycogen depleting/fatigue-inducing exercise in seven competitive male cyclists and triathletes (V _ O 2max $4,620 mL·min À1 ). ...
... One key finding was that exogenous CHO intake helped prevent exercise-induced hypoglycemia and, thus, fatigue (87). Importantly, exogenous CHO intake during exercise preferentially spares hepatic glycogen stores rather than myocellular glycogen (47,50,86,88,89). Therefore, exogenous CHO intake throughout exercise helps bridge the gap between the total CHO need and endogenous glycogen availability by preventing hypoglycemia and sparing hepatic glycogen to postpone the onset or perception of fatigue. ...
Article
Introduction: Carbohydrate (CHO) availability sustains high metabolic demands during prolonged exercise. The adequacy of current CHO intake recommendations, 30-90 g•hr ⁻¹ dependent on CHO mixture and tolerability, to support elite marathon performance is unclear. Purpose: We sought to scrutinize the current upper limit recommendation for exogenous CHO intake to support modeled sub-2-hour marathon (S2M) attempts across elite male and female runners. Methods: Male and female runners (n = 120 each) were modeled from published literature with reference characteristics necessary to complete a S2M (e.g., body mass and running economy). Completion of a S2M was considered across a range of respiratory exchange rates, with maximal starting skeletal muscle and liver glycogen content predicted for elite male and female runners. Results: Modeled exogenous CHO bioavailability needed for male and female runners were 93 ± 26 and 108 ± 22 g•h ⁻¹ , respectively (p < 0.0001, d = 0.61). Without exogenous CHO, males were modeled to deplete glycogen in 84 ± 7 minutes, females in 71 ± 5 minutes (p < 0.0001, d = 2.21) despite higher estimated CHO oxidation rates in males (5.1 ± 0.5 g•h ⁻¹ ) than females (4.4 ± 0.5 g•h ⁻¹ ; p < 0.0001, d = 1.47). Conclusion: Exogenous CHO intakes < 90 g•h ⁻¹ are insufficient for 65% of modeled runners attempting a S2M. Current recommendations to support marathon performance appear inadequate for elite marathon runners but may be more suitable for male runners in pursuit of a S2M (56 of 120) than female runners (28 of 120).
... At the metabolic level, a peculiarity of these tests is the use of mixed energy metabolism, where both the aerobic and anaerobic lactic systems are used, depending on the intensities reached at any given moment in the trial [6,7]. In these competitions, the availability of carbohydrates (CHs)-the main source of energy-can reduce as the intensity of the exercise progresses [7]. ...
... Nutritional planning is highly relevant in most competitions, especially in endurance sports such as cycling [6,14,39]. A wide range of ergogenic aids with a variety of flavours, shapes, and textures are currently available [40]. ...
Article
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In cycling, a wide range of ergogenic foods with a variety of flavours, shapes, and textures are available. The timing of their consumption and their correct oral processing can influence the performance of athletes. Furthermore, the differences in the texture of energy bars could result in differences in the chewing required. Nonetheless, research in this area is still scarce. The aim of this study was to analyse how the consumption of two energy bars with different textures (viscous versus hard) influenced the variables of oral processing, pedalling intensity, and the perception of satisfaction among cyclists. Ten cyclists performed two 15 min sections on a cycle ergometer at a moderate intensity (120–130 W) and consumed one of the two energy bars at random in each of the sections. The results showed that a shorter chewing duration and a fewer number of chews were required to consume the softer bar (p < 0.05, ES > 0.7). However, no differences among the cyclists were observed in the intensity of pedalling or perception of satisfaction. Nevertheless, participants were able to distinguish between the two different textures while pedalling. In conclusion, the texture of energy bars altered the oral processing of cyclists but did not affect pedalling intensity or perception of satisfaction.
... Our analysis revealed a small yet significant influence of carbohydrate intake during exercise on RER. Carbohydrate ingestion during exercise maintains blood glucose levels, carbohydrate oxidation, and RER, and prevents the depletion of liver, but not muscle, glycogen [120][121][122][123]. Increasing the rate of carbohydrate ingestion during exercise decreases hepatic glucose output and increases the contribution of exogenous carbohydrate oxidation to total energy contribution in a dose-response manner [124], at least up to the point where gastrointestinal transport of sugars becomes saturated [125]. ...
... The type of carbohydrate ingested may influence rates of endogenous and exogenous carbohydrate oxidation, but total carbohydrate oxidation (and RER) appears less affected [128][129][130][131]. However, future research should examine the differences in carbohydrate type in the context of high (> 100 g h −1 ) ingestion rates, as contrasting findings have been reported [123,131]. Carbohydrate ingestion may fail to influence RER when exercise intensity is high [132], and/or in untrained participants [133], as RER may already be elevated in these circumstances. However, our models were not significantly improved by interaction effects between carbohydrate ingestion during exercise and either exercise intensity (p = 0.119) or VO 2max (p = 0.179). ...
Article
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Background: Multiple factors influence substrate oxidation during exercise including exercise duration and intensity, sex, and dietary intake before and during exercise. However, the relative influence and interaction between these factors is unclear. Objectives: Our aim was to investigate factors influencing the respiratory exchange ratio (RER) during continuous exercise and formulate multivariable regression models to determine which factors best explain RER during exercise, as well as their relative influence. Methods: Data were extracted from 434 studies reporting RER during continuous cycling exercise. General linear mixed-effect models were used to determine relationships between RER and factors purported to influence RER (e.g., exercise duration and intensity, muscle glycogen, dietary intake, age, and sex), and to examine which factors influenced RER, with standardized coefficients used to assess their relative influence. Results: The RER decreases with exercise duration, dietary fat intake, age, VO2max, and percentage of type I muscle fibers, and increases with dietary carbohydrate intake, exercise intensity, male sex, and carbohydrate intake before and during exercise. The modelling could explain up to 59% of the variation in RER, and a model using exclusively easily modified factors (exercise duration and intensity, and dietary intake before and during exercise) could only explain 36% of the variation in RER. Variables with the largest effect on RER were sex, dietary intake, and exercise duration. Among the diet-related factors, daily fat and carbohydrate intake have a larger influence than carbohydrate ingestion during exercise. Conclusion: Variability in RER during exercise cannot be fully accounted for by models incorporating a range of participant, diet, exercise, and physiological characteristics. To better understand what influences substrate oxidation during exercise further research is required on older subjects and females, and on other factors that could explain additional variability in RER.
... Consequently, it is possible that the rapid delivery of a hypotonic CHO solution to the duodenum could result in greater substrate availability for muscle (6,7). Potentially, this could increase exogenous CHO oxidation, thereby sparing endogenous glycogen stores and enhancing any exercise effort in which glycogen depletion limits performance (8). Yet, two previous reports involving ingestion of an HMM CHO (Vitargo; molecular mass 500-700 kDa) with either 1.8 g·min −1 (9) or 0.8 g·min −1 (10) during 150 min cycling observed either no difference or lower HMM CHO oxidation compared with maltodextrin. ...
... Participants rated whole-body and leg perceived exertion rate (RPE) using the Borg (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) scale (14) as well as any gastrointestinal (GI) concerns using a 10-point scale (1 = no complaints at all, 10 = very severe complaints) (3). Briefly, there were 6 questions related to upper GI symptoms (nausea, general stomach, belching, urge to vomit, heartburn, and stomach cramps), 4 questions related to lower GI symptoms (flatulence, urge to defecate, intestinal cramps, and diarrhea), and 4 questions related to central or other symptoms (dizziness, headache, urge to urinate, and bloated feeling). ...
Article
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Background Phytoglycogen (PHY; PhytoSpherix; Mirexus Biotechnologies), a highly branched polysaccharide extracted from sweet corn, has considerable potential for exercise oxidation due to its low viscosity in water, high water retention, and exceptional stability. Objectives Using gas chromatography–isotope ratio mass spectrometry, we investigated dose–response oxidation of ingested PHY during prolonged, moderate-intensity exercise. Methods Thirteen men (≥1 y endurance-training experience, ≥6 d·wk−1, ∼1–1.5 h·d−1; age, 25.7 ± 5.5 y; mass, 79.3 ± 10.0 kg; V̇O2max, 59.9 ± 5.5 mL·kg−1·min−1; means ± SDs) cycled for 150 min (50% maximal watt output) while ingesting PHY concentrations of 0.0% (0.0 g·min−1), 3.6% (0.5 g·min−1), 7.2% (1.0 g·min−1), 10.8% (1.5 g·min−1), or 14.4% (2 g·min−1) in water (2100 mL) (n = 7–10/dose). Substrate oxidation was determined using stable-isotope methods and indirect calorimetry. Results PHY oxidation plateaued between 60 and 150 min of exercise and increased (P < 0.001) from 0.49 to 0.72 g·min−1 with 0.5- and 1.0-g·min−1 doses without further increases (0.76 and 0.73 g·min−1; P > 0.05) with 1.5 or 2 g·min−1. Peak PHY oxidation (0.84 ± 0.04 g·min−1) occurred in the final 30 min of exercise with 2 g·min−1. Exercise blood glucose was greater (5.1 mmol·L−1) with 1.0-, 1.5-, and 2-g·min−1 doses compared with that of 0.5 (4.7 mmol·L−1) or 0.0 g·min−1 (4.2 mmol·L−1) (P < 0.0001). Gastrointestinal distress was minimal except with 2 g·min−1 (P < 0.001). Conclusions In male endurance athletes, PHY oxidation plateaued at 0.72–0.76 g·min−1 during 150 min of cycling at 50% Wmax (peak oxidation of 0.84 g·min−1 occurred during the final 30 min). This trial was registered at clinicaltrials.gov as NCT02909881.
... In prolonged strenuous exercise, such as in SOUT events, nutritional planning and suitable CHO intake during the event reduce fatigue time [18,19], and consequently, high CHO intake rates were significantly correlated with faster finishing times [20]. Moreover, although it seems there is no linear dose-response to CHO ingestion [21], an optimal CHO intake could maintain plasma glucose and CHO oxidation rates [22] and augment exercise performance via multiple mechanisms, consisting of muscle glycogen sparing [23], and liver glycogen sparing [24] deemed necessary for optimal sports performance [11,13,[25][26][27]. As such, different international nutrition societies recommend a 90 g/h intake of CHO in exercises of more than 3 h duration (with a combination of CHO that use different absorption transporters such as glucose or fructose) to improve athletic performance by gastric emptying, maximize their oxidation and reduce possible GI discomfort via suitable gut training [13,[26][27][28]. ...
... Thus, it has been demonstrated that it is possible to improve EIMD markers with an intake of 120 g CHO/h. The highest levels of these indicators (CK, LDH) usually appear after 24-72 h, requiring several days to return to reference values [24,25]. In the study by Viribay et al. [4] the blood markers of GOT, LDH, CK evidenced better values in the group that consumed 120 g CHO/h (HIGH) compared to groups with an intake of 90 or 60g CHO/h (CON and LOW respectively). ...
Article
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Due to the high metabolic and physical demands in single-stage one-day ultra-trail (SOUT) races, athletes should be properly prepared in both physical and nutritional aspects in order to delay fatigue and avoid associated difficulties. However, high carbohydrate (CHO) intake would seem to increase gastrointestinal (GI) problems. The main purpose of this systematic review was to evaluate CHO intake during SOUT events as well as its relationship with fatigue (in terms of internal exercise load, exercise-induced muscle damage (EIMD) and post-exercise recovery) and GI problems. A structured search was carried out in accordance with PRISMA guidelines in the following: Web of Science, Cochrane Library and Scopus databases up to 16 March 2021. After conducting the search and applying the inclusion/exclusion criteria, eight articles in total were included in this systematic review, in all of which CHO intake involved gels, energy bars and sports drinks. Two studies associated higher CHO consumption (120 g/h) with an improvement in internal exercise load. Likewise, these studies observed that SOUT runners whose intake was 120 g/h could benefit by limiting the EIMD observed by CK (creatine kinase), LDH (lactate dehydrogenase) and GOT (aspartate aminotransferase), and also improve recovery of high intensity running capacity 24 h after a trail marathon. In six studies, athletes had GI symptoms between 65–82%. In summary, most of the runners did not meet CHO intake standard recommendations for SOUT events (90 g/h), while athletes who consumed more CHO experienced a reduction in internal exercise load, limited EIMD and improvement in post-exercise recovery. Conversely, the GI symptoms were recurrent in SOUT athletes depending on altitude, environmental conditions and running speed. Therefore, a high CHO intake during SOUT events is important to delay fatigue and avoid GI complications, and to ensure high intake, it is necessary to implement intestinal training protocols.
... The contribution of carbohydrate to exercise metabolism can be altered by the intensity and duration of exercise, in addition to fitness status and recent diet, such as prior carbohydrate intake (28,29). When total energy expenditure is fixed, any shift in carbohydrate oxidation is typically replaced with a reciprocal change in fat oxidation (30). In other words, a 100kcal reduction in carbohydrate oxidation would typically be replaced by a 100-kcal increase in fat oxidation. ...
... However, it is still unknown whether it is absolute quantity of liver glycogen, or the rate of liver glycogen utilization that is sensed. This could have implications for energy balance responses to exercise versus fasting, since exercise (at least at moderate-intensity) would provide a more rapid decline in liver glycogen stores than fasting or low-carbohydrate diets (30,31). Nevertheless, the relevance of this mechanism to humans is still unclear, with potential variance in the species-specific hepatic glycogen metabolism and hepatic innervation (31). ...
Article
We explore the novel hypothesis that carbohydrate availability is involved in the regulation of energy balance with exercise, via hormonal and neural signals. We propose that carbohydrate availability could play a direct mechanistic role and partially explain previously-documented relationships between a more active lifestyle and tighter control of energy balance.
... However, exceeding intestinal saturation rates for glucose-fructose increased the reliance on pre-existing stores of muscle glycogen, which had a detrimental effect on performance. In this respect, the 90 g h −1 dose of glucose-fructose was optimal for substrate utilisation and performance and agrees with previous performance (Smith et al. 2013) and hepatic glycogen oxidation data (Gonzalez et al. 2015). In contrast, the latter of these studies reported no change in muscle glycogen oxidation. ...
... This may in part be due to a methodological limitation in quantifying glycogen oxidation post-exercise with NMRS, but also attributable to the lower exercise intensity (50% W peak ), where muscle glycogen may not be as crucial to prolonged exercise performance. Furthermore, the glucose-fructose dose used by Gonzalez et al. (2015) may have slightly exceeded intestinal transport, which may be detrimental to fuel utilisation and performance. It also remains to be seen if glucose:fructose doses can be optimised around the 90 g h −1 proposed upper limit and if any endogenous fuel utilisation effects remain at longer duration and lower intensity exercise. ...
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Purpose This study investigated the effect of small manipulations in carbohydrate (CHO) dose on exogenous and endogenous (liver and muscle) fuel selection during exercise. Method Eleven trained males cycled in a double-blind randomised order on 4 occasions at 60% V˙O2  max\dot {V}{{\text{O}}_{2\;\hbox{max} }} for 3 h, followed by a 30-min time-trial whilst ingesting either 80 g h⁻¹ or 90 g h⁻¹ or 100 g h−1 13C-glucose-¹³C-fructose [2:1] or placebo. CHO doses met, were marginally lower, or above previously reported intestinal saturation for glucose–fructose (90 g h⁻¹). Indirect calorimetry and stable mass isotope [¹³C] techniques were utilised to determine fuel use. Result Time-trial performance was 86.5 to 93%, ‘likely, probable’ improved with 90 g h⁻¹ compared 80 and 100 g h⁻¹. Exogenous CHO oxidation in the final hour was 9.8–10.0% higher with 100 g h⁻¹ compared with 80 and 90 g h⁻¹ (ES = 0.64–0.70, 95% CI 9.6, 1.4 to 17.7 and 8.2, 2.1 to 18.6). However, increasing CHO dose (100 g h⁻¹) increased muscle glycogen use (101.6 ± 16.6 g, ES = 0.60, 16.1, 0.9 to 31.4) and its relative contribution to energy expenditure (5.6 ± 8.4%, ES = 0.72, 5.6, 1.5 to 9.8 g) compared with 90 g h⁻¹. Absolute and relative muscle glycogen oxidation between 80 and 90 g h⁻¹ were similar (ES = 0.23 and 0.38) though a small absolute (85.4 ± 29.3 g, 6.2, − 23.5 to 11.1) and relative (34.9 ± 9.1 g, − 3.5, − 9.6 to 2.6) reduction was seen in 90 g h⁻¹ compared with 100 g h⁻¹. Liver glycogen oxidation was not significantly different between conditions (ES < 0.42). Total fat oxidation during the 3-h ride was similar in CHO conditions (ES < 0.28) but suppressed compared with placebo (ES = 1.05–1.51). Conclusion ‘Overdosing’ intestinal transport for glucose–fructose appears to increase muscle glycogen reliance and negatively impact subsequent TT performance.
... Estimated CHO contribution from liver glucose disappearance Panels A and B in the authors' Figure 4 (1) predict that liver glucose disappearance contributes 68 and 49 g CHO to total CHO oxidation in males and females respectively ( Table 1, Row I). This is likely overestimated since high rates of CHO ingestion suppress liver glucose disappearance (7)(8)(9). ...
... Rather than muscle glycogen sparing it has been demonstrated, during continuous exercise, that ingestion of carbohydrates and accompanying increases in plasma insulin levels inhibit hepatic glycogenolysis and thus may spare liver glycogen instead [37]. In addition, increased insulin levels impede free fatty acid mobilization, as was observed in the present study with markedly lower plasma FFA levels in the CHO condition in line with previous investigations [17,27]. ...
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Carbohydrates are critical for high‐intensity exercise performance. However, the effects of carbohydrate supplementation on muscle metabolism and performance during short‐duration high‐intensity intermittent exercise remain inadequately explored. Our aim was to address this aspect in a randomized, counterbalanced, double‐blinded crossover design. Eleven moderately‐to‐well‐trained males performed high‐intensity intermittent cycling receiving carbohydrate (CHO, ~55 g/h) or placebo (PLA) fluid supplementation. Three exercise periods (EX1‐EX3) were completed comprising 10 × 45 s at ~105% Wmax interspersed with 135 s rest between bouts and ~20 min between periods. Repeated sprint ability (5 × 6 s sprints with 24 s recovery) was assessed at baseline and after each period. Thigh muscle biopsies were obtained at baseline and before and after EX3 to determine whole‐muscle and fiber‐type‐specific glycogen depletion. No differences were found in muscle glycogen degradation at the whole‐muscle (p = 0.683) or fiber‐type‐specific level (p = 0.763–0.854) with similar post‐exercise whole‐muscle glycogen concentrations (146 ± 20 and 122 ± 15 mmol·kg⁻¹ dw in CHO and PLA, respectively). Repeated sprint ability declined by ~9% after EX3 with no between‐condition differences (p = 0.971) and no overall differences in ratings of perceived exertion (p = 0.550). This was despite distinctions in blood glucose concentrations throughout exercise, reaching post‐exercise levels of 5.3 ± 0.2 and 4.1 ± 0.2 mmol·L⁻¹ (p < 0.001) in CHO and PLA, respectively, accompanied by fivefold higher plasma insulin levels in CHO (p < 0.001). In conclusion, we observed no effects of carbohydrate ingestion on net muscle glycogen breakdown or sprint performance during short‐duration high‐intensity intermittent exercise despite elevated blood glucose and insulin levels. These results therefore question the efficacy of carbohydrate supplementation strategies in high‐intensity intermittent sports.
... elegantly demonstrated that when recovery from glycogen reducing contractions occurs in the absence of glucose and thus glycogen remains low, fiber bundles fatigue at a faster rate and show reduced tetanic Ca21 transients in a subsequent fatigue test. These findings have been subsequently confirmed in human skeletal muscle, where an impairment in SR Ca21 release rate and subsequent power output are apparent under conditions of low muscle glycogen (7). Intriguingly, this impairment appears to occur at muscle glycogen concentrations below 300 mmol · (kg d.w.)21, similar to the pro-posed critical threshold required to significantly activate cell-signaling pathways regulating mitochondrial biogenesis. ...
Article
This research was done with the aim of studying cell signaling processes in the process of body metabolism in 2024. Methodology: The research method of the present study is descriptive-survey in nature, and applied in terms of research purpose and cross-sectional in terms of time. The research design used in this research is the pre-test-post-test design. The statistical population of this research consists of 3 men and 3 women who are engaged in cycling professionally. In this research, to reach a representative sample, a non-random sampling method was used. In this research, two library and field methods were used to collect data. Sports exercises have been used in the field section. Results: The findings of this research show that the metabolic process regulates many cells signaling processes that are related to human health and performance. Now that the specific regulatory checkpoints for CHO metabolism are well documented, the precision of the molecular mechanisms regulating CHO transport, storage, and utilization that have not yet been fully identified can be increased. In the process of metabolism, consumed nutrients are analyzed and converted into smaller molecules. Conclusion: These molecules ultimately contribute to cellular structures such as proteins and nucleic acids. Also, this stored energy is released and transferred to all body activities, including movements and chemical compounds, in signaling processes.
... Nonetheless, a similar decline in muscle glycogen content exists during prolonged exercise when CHO or a PLA (i.e.,CHO-free solution) is consumed, yet the endurance time is extended with the CHO feeding (7). These findings (7), in addition to other supporting literature (7,(18)(19)(20), indicate that by increasing total energy substrate availability, CHO ingestion can independently augment fatigue resistance irrespective of any effects on muscle glycogen sparing. ...
Article
Introduction We aimed to investigate the neuromuscular contributions to enhanced fatigue resistance with carbohydrate ingestion, and to identify whether fatigue is associated with changes in interstitial glucose levels assessed using a continuous glucose monitor (CGM). Methods Twelve healthy participants (6 males, 6 females) performed isokinetic single-leg knee extensions (90°/s) at 20% of the maximal voluntary contraction (MVC) torque until MVC torque reached 60% of its initial value (i.e, task failure). Central and peripheral fatigue were evaluated every 15 min during the fatigue task using the interpolated twitch technique (ITT), and electrically evoked torque. Using a single-blinded cross-over design, participants ingested carbohydrates (CHO) (85 g sucrose/h), or a placebo (PLA), at regular intervals during the fatigue task. Minute-by-minute interstitial glucose levels measured via CGM, and whole blood glucose readings were obtained intermittently during the fatiguing task. Results CHO ingestion increased time to task failure over PLA (113 ± 69 vs. 81 ± 49 min; mean ± SD; p < 0.001) and was associated with higher glycemia as measured by CGM (106 ± 18 vs 88 ± 10 mg/dL, p < 0.001) and whole blood glucose sampling (104 ± 17 vs 89 ± 10 mg/dL, p < 0.001). When assessing the values in the CHO condition at a similar timepoint to those at task failure in the PLA condition (i.e., ~81 min), MVC torque, % voluntary activation, and 10 Hz torque were all better preserved in the CHO vs. PLA condition (p < 0.05). Conclusions Exogenous CHO intake mitigates neuromuscular fatigue at both the central and peripheral levels by raising glucose concentrations rather than by preventing hypoglycemia.
... The effect of carbohydrate intake on RPE in sport is an important consideration for athletes and coaches alike. Consumption of carbohydrates has been shown to improve exercise performance and reduce symptoms of fatigue during strenuous exercise (Gonzalez et al. 2015). Carbohydrate intake has also been linked to a decrease in RPE during exercise, as carbohydrates can serve as an energy source for exercise and may help delay the onset of fatigue (Greer et al. 2011). ...
Article
A systematic review with meta-analysis was conducted to analyze the effect of carbohydrate (CHO) intake during exercise and some variables that could moderate this effect on endurance performance. We included 136 studies examining the effect of CHO ingestion during endurance exercise in the meta-analysis. The overall effect on performance showed a significant increase after CHO intake compared to the placebo/control groups. A larger effect of CHO consumption is observed in time to exhaustion than in time trials performance test. Moreover, the effectiveness of CHO supplementation was greater the longer the duration of the events. Also, there seems to be a higher effect of CHO intake in lower trained than in higher trained participants. In contrast, the magnitude of performance change of CHO intake is not affected by the dosage, ergometer used, the type of intake of the CHO ingestion and the type of CHO. In addition, a lower rate of perceived exertion and higher power and heart rate are significantly associated with the ingestion of CHO during endurance exercise. These results reinforce that acute CHO feeding is an effective strategy for improving endurance performance, especially, in less trained subjects participating in time to exhaustion tests of longer durations.
... However, a limitation of these methods is the assumption that the rate of endogenous substrate oxidation is identical during the experimental trial and the water trial. This is unlikely to be true since carbohydrate ingestion, as well as exercise, can alter the use of muscle and liver glycogen in a dose-dependent manner (Gonzalez et al., 2015;King et al., 2018;Wallis et al., 2007). Peronnet et al. (1990) showed that exogenous carbohydrate oxidation during exercise can be significantly overestimated when the resting background correction is used (Peronnet et al., 1990), and that this is further exacerbated when using high natural abundance carbohydrate sources as the tracer. ...
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Isotopic tracers can reveal insights into the temporal nature of metabolism and track the fate of ingested substrates. A common use of tracers is to assess aspects of human carbohydrate metabolism during exercise under various established models. The dilution model is used alongside intravenous infusion of tracers to assess carbohydrate appearance and disappearance rates in the circulation, which can be further delineated into exogenous and endogenous sources. The incorporation model can be used to estimate exogenous carbohydrate oxidation rates. Combining methods can provide insight into key factors regulating health and performance, such as muscle and liver glycogen utilization, and the underlying regulation of blood glucose homeostasis before, during, and after exercise. Obtaining accurate, quantifiable data from tracers, however, requires careful consideration of key methodological principles. These include appropriate standardization of pretrial diet, specific tracer choice, whether a background trial is necessary to correct expired breath CO 2 enrichments, and if so, what the appropriate background trial should consist of. Researchers must also consider the intensity and pattern of exercise, and the type, amount, and frequency of feeding (if any). The rationale for these considerations is discussed, along with an experimental design checklist and equation list which aims to assist researchers in performing high-quality research on carbohydrate metabolism during exercise using isotopic tracer methods.
... Carbohydrate ingestion during exercise maintains stable blood glucose levels over long exercise sessions [52] and maintains carbohydrate oxidation rates despite declining muscle glycogen stores so that ingested carbohydrates substitute endogenous carbohydrate stores [53,54]. In addition to this, exogenous carbohydrates can spare or even completely suppress liver glycogen breakdown [55,56]. While some studies have found sparing of muscle glycogen with carbohydrate supplementation during exercise [57,58], most of the studies assessing whole muscle glycogen utilization did not see this effect [28] and a recently published study that evaluated different carbohydrate ingestion rates during cycling exercise did not observe sparing of muscle glycogen in a muscle fiber type-specific manner either [59]. ...
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The importance of carbohydrate as a fuel source for exercise and athletic performance is well established. Equally well developed are dietary carbohydrate intake guidelines for endurance athletes seeking to optimize their performance. This narrative review provides a contemporary perspective on research into the role of, and application of, carbohydrate in the diet of endurance athletes. The review discusses how recommendations could become increasingly refined and what future research would further our understanding of how to optimize dietary carbohydrate intake to positively impact endurance performance. High carbohydrate availability for prolonged intense exercise and competition performance remains a priority. Recent advances have been made on the recommended type and quantity of carbohydrates to be ingested before, during and after intense exercise bouts. Whilst reducing carbohydrate availability around selected exercise bouts to augment metabolic adaptations to training is now widely recommended, a contemporary view of the so-called train-low approach based on the totality of the current evidence suggests limited utility for enhancing performance benefits from training. Nonetheless, such studies have focused importance on periodizing carbohydrate intake based on, among other factors, the goal and demand of training or competition. This calls for a much more personalized approach to carbohydrate recommendations that could be further supported through future research and technological innovation (e.g., continuous glucose monitoring). Despite more than a century of investigations into carbohydrate nutrition, exercise metabolism and endurance performance, there are numerous new important discoveries, both from an applied and mechanistic perspective, on the horizon.
... Most of the common types of carbohydrates, such as glucose, sucrose, and glucose polymers, are effective in maintaining blood glucose concentration, due to their high glycemic index 10 . • Prevents liver but not muscle glycogen depletion 11,12 . In addition, it can contribute to the restoration of endogenous glycogen stores after exercise 13 . ...
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When practicing intense physical exercise for more than an hour, it is recommended to counteract the loss of water and electrolytes and provide an energy substrate by drinking isotonic sports beverages. These contain carbohydrates and mineral salts at the same osmotic pressure as blood, facilitating the rapid absorption of their constituents. Its consumption before, during, and after prolonged physical exercise is more effective than water in preventing dehydration, helping to maintain performance during exercise, delaying the onset of fatigue, and accelerating recovery. Based on the demand for more natural foods, there is an interest from the food industry to produce isotonic drinks from ingredients such as fruits, cereals, among others. In this sense, this review describes some aspects of the formulation and physiological effects of consuming this type of drink.
... From a practical standpoint, professional cyclists are adept at fuelling during competition (29) and are meeting current carbohydrate ingestion recommendations of up to 120g.hr -1 (30). However, there is evidence to suggest that exogenous carbohydrate intake, during exercise, only replenishes liver glycogen without sparing muscle glycogen (31). Thus, even in populations adept at incompetition fuelling, it is somewhat logical that lower CarbOx levels would lead to glycogen sparing and increased durability. ...
Article
Purpose: To determine if durability can be predicted from laboratory measures in a professional cycling population. Methods: Data were collected from 10 professional cyclists (age, 19.2 ± 0.8 years, body mass 70.4 ± 5.52 kg, height 182.9 ± 4.0 cm, BMI 21.0 ± 1.3 kg·m 2 , V O 2max 74.4 ± 4.8 mL·kg-1 ·min-1 , Critical Power 5.57 ± 0.58 W·kg-1 , W´ 23.69 ± 5.42 kJ). Participants completed a laboratory test and a critical power (CP) test on two occasions. The second occasion was preceded by a novel fatiguing protocol which consisted of 5 bouts of 8 mins of exercise at 105-110% of CP. CP in a fatigued state was expressed as a percentage of the fresh CP and coined Delta CP (∆CP). Pearson product correlation analysis was conducted to determine the relationship between laboratory-based measures and (∆CP). Results: Significant positive relationships were found between ∆CP and, relative peak power output (r = 0.891, p <0.001), relative maximum oxygen uptake (r = 0.835, p = 0.003), relative power output at the second ventilatory threshold (r = 0.738, p=.0147), power output at the first ventilatory threshold (r = 0.748, p = .0128) and relative power output at the first ventilatory threshold (r = 0.826, p =0.003) gross efficiency at 300 W (r = 0.869, p = 0.001), and at 200 W (r = 0.792, p = 0.006). Significant negative relationships were found between ∆CP and carbohydrate oxidation at 200W (r =-0.702, p = 0.024). A multiple linear regression demonstrated that ∆CP can be predicted from laboratory measures (R 2 = 0.96-0.98, p<0.001). Conclusions: These findings demonstrate the physiological determinants of durability in a professional cycling population.
... Most of the common types of carbohydrates, such as glucose, sucrose, and glucose polymers, are effective in maintaining blood glucose concentration, due to their high glycemic index 10 . • Prevents liver but not muscle glycogen depletion 11,12 . In addition, it can contribute to the restoration of endogenous glycogen stores after exercise 13 . ...
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Al practicar ejercicio físico intenso durante más de una hora, se recomienda contrarrestar la pérdida de agua y electrolitos y proporcionar un sustrato energético mediante el consumo de bebidas deportivas isotónicas. Estas contienen carbohidratos y sales minerales a la misma presión osmótica que la sangre, facilitando la rápida absorción de sus constituyentes. Se ha demostrado que su consumo antes, durante y después del ejercicio físico duradero es más efectivo que el agua en la prevención de la deshidratación, ayuda a mantener el rendimiento durante el ejercicio, retrasa la aparición de la fatiga y aceleran la recuperación. A partir de la demanda de alimentos más naturales, existe un interés de la industria alimentaria por producir bebidas isotónicas a partir de ingredientes como frutas, cereales, entre otros. En este sentido, en la presente revisión se describen algunos aspectos sobre la formulación y efectos fisiológicos del consumo de este tipo de bebida.
... This is associated with the maintenance of plasma glucose concentration and CHO oxidation during the latter stages of prolonged exercise (2). CHO ingestion can also prevent the depletion or attenuate the use of liver glycogen (3,4) and in some instances muscle glycogen (5,6). The American College of Sports Medicine guidelines recommend consuming up to 90 g·h −1 of CHO during exercise lasting >2.5 h or where endogenous CHO stores will be depleted (7). ...
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Purpose: Beneficial effects of carbohydrate (CHO) ingestion on exogenous CHO oxidation and endurance performance require a well-functioning gastrointestinal (GI) tract. However, GI complaints are common during endurance running. This study investigated the effect of a CHO solution-containing sodium alginate and pectin (hydrogel) on endurance running performance, exogenous and endogenous CHO oxidation and GI symptoms. Methods: Eleven trained male runners, using a randomised, double-blind design, completed three 120-minute steady state runs at 68% V[Combining Dot Above]O2max, followed by a 5-km time-trial. Participants ingested 90 g·h-1 of 2:1 glucose:fructose (13C enriched) either as a CHO hydrogel, a standard CHO solution (non-hydrogel), or a CHO-free placebo during the 120 minutes. Fat oxidation, total and exogenous CHO oxidation, plasma glucose oxidation and endogenous glucose oxidation from liver and muscle glycogen were calculated using indirect calorimetry and isotope ratio mass spectrometry. GI symptoms were recorded throughout the trial.RESULTS: Time-trial performance was 7.6% and 5.6% faster after hydrogel ([minutes:seconds]19:29 ± 2:24; p < 0.001) and non-hydrogel (19:54 ± 2:23, p = 0.002), respectively, versus placebo (21:05 ± 2:34). Time-trial performance after hydrogel was 2.1% faster (p = 0.033) than non-hydrogel. Absolute and relative exogenous CHO oxidation was greater with hydrogel (68.6 ± 10.8 g, 31.9 ± 2.7%; p = 0.01) versus non-hydrogel (63.4 ± 8.1 g, 29.3 ± 2.0%; p = 0.003). Absolute and relative endogenous CHO oxidation were lower in both CHO conditions compared with placebo (p < 0.001), with no difference between CHO conditions. Absolute and relative liver glucose and muscle glycogen oxidation were not different between CHO conditions. Total GI symptoms were not different between hydrogel and placebo, but GI symptoms was higher in non-hydrogel compared with placebo and hydrogel (p < 0.001). Conclusion: Ingestion of glucose and fructose in hydrogel form during running benefited endurance performance, exogenous CHO oxidation and GI symptoms, compared with a standard CHO solution.
... Saturation of the SGLT1 transport pathway occurs at an ingestion rate of 1.2 g/min. Thus, the combination of fructose with glucose has the dual benefit of increasing total exogenous carbohydrate oxidation during exercise (126,127) and reducing the risk of GI complaints with high rates of carbohydrate ingestion (110,128). Conversely, diets rich in glucose and fructose (65% of daily caloric intake) reduced microbial diversity and epithelial TJ function and induced endotoxemia in mice (129). The diet was found to impact the structure of the TJ by reducing the plaque proteins spanning the TJ (zonula occludens) and in the plasma membrane (occludin). ...
Article
Intestinal barrier integrity and function are compromised during exertional heat stress (EHS) potentially leading to consequences that range from minor gastrointestinal (GI) disturbances to fatal outcomes in exertional heat stroke or septic shock. This mini-review provides a concise discussion of nutritional interventions that may protect against intestinal permeability during EHS and suggests physiological mechanisms responsible for this protection. Although diverse nutritional interventions have been suggested to be protective against EHS-induced GI permeability, the ingestion of certain amino acids, carbohydrates, and fluid per se are potentially effective strategies, whereas evidence for various polyphenols and pre/probiotics is developing. Plausible physiological mechanisms of protection include increased blood flow, epithelial cell proliferation, upregulation of intracellular heat shock proteins, modulation of inflammatory signaling, alteration of the GI microbiota, and increased expression of tight junction (TJ) proteins. Further clinical research is needed to propose specific nutritional candidates and recommendations for their application to prevent intestinal barrier disruption and elucidate mechanisms during EHS.
... Gastrointestinal distress is a common occurrence when attempting to adhere to novel or high-dose nutrition strategies, however when ingested in large quantities, glucosefructose mixtures tend to produce less gastrointestinal distress than equivalent quantities of glucose-based carbohydrates Gonzalez et al., 2015). This is likely due to fructose being absorbed by different intestinal transport proteins than glucose, thereby increasing the total capacity for intestinal carbohydrate absorption (Daniel & Zietek, 2015). ...
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This study assessed the effects of glucose-fructose co-ingestion during recovery from high-intensity rugby training on subsequent performance. Nine professional, senior academy Rugby Union players performed two trials in a double-blind, randomized, crossover design. Identical rugby training sessions were separated by a 3-hour recovery period, during which participants ingested protein (0.3 g×kg BM×h⁻¹) and carbohydrate-containing (0.8 g×kg BM×h⁻¹) recovery drinks, comprised of glucose polymers (GLUCOSE ONLY) or a glucose-fructose mixture (GLUCOSE+FRUCTOSE). Performance outcomes were determined from global positioning systems combined with accelerometry and heart rate monitoring. Mean speed during sessions 1 (am) and 2 (pm) of GLUCOSE ONLY was (mean±SD) 118±6 and 117±4 m×min⁻¹, respectively. During GLUCOSE+FRUCTOSE, mean speed during session 1 and 2 was 117±4 and 116±5 m×min⁻¹, respectively (time x trial interaction, p = 0.61). Blood lactate concentrations were higher throughout recovery in GLUCOSE+FRUCTOSE (mean ±SD: 1-h 3.2 ±2.0 mmol×L⁻¹; 3-h 2.1 ±1.2 mmol×L⁻¹) compared to GLUCOSE ONLY (1-h 2.0 ±1.0 mmol×L⁻¹; 3-h 1.4 ±1.0 mmol×L⁻¹; trial effect p = 0.05). Gastrointestinal discomfort low in both conditions. These data suggest glucose-fructose mixtures consumed as protein-carbohydrate recovery drinks following rugby training do not enhance subsequent performance compared to glucose-based recovery drinks.
... The importance of carbohydrate intake for optimal endurance performance, particularly in events lasting more than 90 min, is well-established ( Vandenbogaerde & Hopkins, 2011). The intake of carbohydrate during exercise provides an exogenous fuel source, sparing hepatic (Gonzalez et al., 2015) and sometimes muscle (Tsintzas, Williams, Boobis, & Greenhaff, 1995) glycogen stores in addition to maintaining euglycaemia (Karelis, Smith, Passe, & Péronnet, 2010) and high carbohydrate oxidation rates (Coyle, Coggan, Hemmert, & Ivy, 1986). It is thought that exogenous carbohydrate availability during exercise is limited by intestinal absorption (Gonzalez, Fuchs, Betts, & van Loon, 2017;Jeukendrup & Jentjens, 2000). ...
Article
The benefits of high exogenous glucose availability for endurance exercise performance are well-established. Exogenous glucose oxidation rates are thought to be limited by intestinal glucose transport. Extracellular calcium in rodent intestine increases the translocation of the intestinal glucose transporter GLUT2 which, if translated to humans, could increase the capacity for exogenous glucose availability during exercise. Therefore, this pilot study aimed to explore the effect of calcium co-ingestion during endurance exercise on exogenous glucose oxidation in healthy men. Eight healthy men cycled for 2 h at 50% peak power output, ingesting either 1.2 g·min⁻¹ dextrose alone (GLU) or with the addition of 2000 mg calcium (GLU+CAL), in a randomised crossover design. Expired breath samples were collected to determine whole-body and exogenous glucose oxidation. Peak exogenous glucose oxidation during GLU was 0.83±0.15 g·min⁻¹, and was not enhanced during GLU+CAL (0.88±0.11 g·min⁻¹, p = 0.541). The relative contributions of exogenous carbohydrate (19±3% vs. 20±2%, p = 0.434), endogenous carbohydrate (65±3% vs. 65±3%, p = 0.822) and fat (16±3% vs. 15±3%, p = 0.677) to total substrate utilisation did not differ between trials. These results suggest the addition of calcium to glucose ingestion, at saturating glucose ingestion rates, does not appear to alter exogenous glucose oxidation during endurance exercise in healthy men.
... As described previously, the glycolytic pathway is directly regulated by glycogen content and glucose availability and, thus, lower glycogen and glucose disposal might limit this capacity [67]. It has been demonstrated that CHO intake during exercise could delay glycogen content and maintain blood glucose [54,68] and, hence, maintain greater high intensity run capacity. Therefore, the results obtained in this study could be explained by these mechanisms. ...
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Background: Current carbohydrate (CHO) intake recommendations for ultra-trail activities lasting more than 2.5 h is 90 g/h. However, the benefits of ingesting 120 g/h during a mountain marathon in terms of post-exercise muscle damage have been recently demonstrated. Therefore, the aim of this study was to analyze and compare the effects of 120 g/h CHO intake with the recommendations (90 g/h) and the usual intake for ultra-endurance athletes (60 g/h) during a mountain marathon on internal exercise load, and post-exercise neuromuscular function and recovery of high intensity run capacity. Methods: Twenty-six elite trail-runners were randomly distributed into three groups: LOW (60 g/h), MED (90 g/h) and HIGH (120 g/h), according to CHO intake during a 4000-m cumulative slope mountain marathon. Runners were measured using the Abalakov Jump test, a maximum a half-squat test and an aerobic power-capacity test at baseline (T1) and 24 h after completing the race (T2). Results: Changes in Abalakov jump time (ABKJT), Abalakov jump height (ABKH), half-squat test 1 repetition maximum (HST1RM) between T1 and T2 showed significant differences by Wilcoxon signed rank test only in LOW and MED (p < 0.05), but not in the HIGH group (p > 0.05). Internal load was significantly lower in the HIGH group (p = 0.017) regarding LOW and MED by Mann Whitney u test. A significantly lower change during the study in ABKJT (p = 0.038), ABKH (p = 0.038) HST1RM (p = 0.041) and in terms of fatigue (p = 0.018) and lactate (p = 0.012) within the aerobic power-capacity test was presented in HIGH relative to LOW and MED. Conclusions: 120 g/h CHO intake during a mountain marathon might limit neuromuscular fatigue and improve recovery of high intensity run capacity 24 h after a physiologically challenging event when compared to 90 g/h and 60 g/h.
... 1,9 Theoretically, the endurance performance benefit of glycogen sparing would be more pronounced in prolonged endurance events with race times above 4 hours, where a substantial depletion of muscle glycogen stores is expected. 10 Recently, the relationship between prolonged endurance performance and the peak fat oxidation rate (PFO) was investigated in male and female ironman triathletes. 11,12 PFO was measured during a graded exercise protocol, and race time in the Copenhagen Ironman was used as measure of performance. ...
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The peak fat oxidation rate (PFO) and the exercise intensity that elicits PFO (Fatmax) is associated with endurance performance during exercise primarily involving lower body musculature, but it remains elusive whether these associations are present during predominant upper body exercise. The aim was to investigate the relationship between PFO and Fatmax determined during a graded exercise test on a ski‐ergometer using double poling (GET‐DP) and performance in the long‐distance cross‐country skiing race, Vasaloppet. Forty‐three healthy men completed GET‐DP and Vasaloppet and were divided into two subgroups; recreational (RS, n=35) and elite skiers (ES, n=8). Additionally, RS completed a cycle‐ergometer GET (GET‐Cycling) to elucidate whether the potential relationships were specific to exercise modality. PFO (r²=0.10, p=0.044) and Fatmax (r²=0.26, p<0.001) were correlated with performance, however, V̇O2peak was the only independent predictor of performance (adj. R²=0.36) across all participants. In ES, Fatmax was the only variable associated with performance (r²=0.54, p=0.038). Within RS, DP V̇O2peak (r²=0.11, p=0.047) and ski specific training background (r²=0.30, p=0.001) were associated with performance. Between the two GETs Fatmax (r²=0.20, p=0.006) but not PFO (r²=0.07, p=0.135) was correlated. Independent of exercise mode, neither PFO nor Fatmax were associated with performance in RS (p>0.05). These findings suggest that prolonged endurance performance is related to PFO and Fatmax but foremost to V̇O2peak during predominant upper body exercise. Interestingly, Fatmax may be an important determinant of performance among ES. Among RS, DP V̇O2peak and skiing experience appeared as performance predictors. Additionally, whole‐body fat oxidation seemed specifically coupled to exercise modality.
... S ince the introduction of the muscle biopsy technique in the late 1960s (1), the importance of muscle glycogen for augmenting exercise capacity and performance in endurance events has been well documented. In addition to high endogenous carbohydrate (CHO) availability, augmenting exogenous CHO availability (typically via gels, drinks, or bars) is also ergogenic to exercise performance (2), an effect likely mediated by liver (3) and muscle glycogen sparing (4), maintaining plasma glucose and CHO oxidation rates (5,6) and/or via direct effects on the central nervous system (7). When taken together, nutritional guidelines for competitive endurance events recommend sufficient CHO loading (e.g., 7-12 g·kg −1 body mass depending on event duration) to ensure elevated muscle and liver glycogen stores, as well as to consume exogenous CHO when exercise duration is >1 h (8,9). ...
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Purpose: To quantify net glycogen utilisation in the vastus lateralis (VL) and gastrocnemius (G) of male (n=11) and female (n=10) recreationally active runners during three outdoor training sessions. Methods: After 2 days standardisation of carbohydrate (CHO) intakes (6 g.kg body mass per day), glycogen was assessed before and after 1) a 10-mile road run (10-mile) at lactate threshold, 2) 8 x 800 m track intervals (8 x 800 m) at velocity at V[Combining Dot Above]O2max and 3) 3 x 10 minute track intervals (3 x 10 min) at lactate turnpoint. Results: Resting glycogen concentration was lower in the G of females compared with males (P<0.001) though no sex differences were apparent in the VL (P=0.40). Within the G and VL of males, net glycogen utilisation differed between training sessions where 10-mile was greater than both track sessions (all comparisons, P<0.05). In contrast, net glycogen utilisation in females was not different between training sessions in either muscle (all comparisons, P>0.05). Net glycogen utilisation was greater in males than females in both VL (P=0.02) and G (P=0.07) during the 10-mile road run. With the exception of males during the 3 x 10 min protocol (P=0.28), greater absolute glycogen utilisation was observed in the G versus the VL muscle in both males and females and during all training protocols (all comparisons, P<0.05). Conclusion: Data demonstrate 1) prolonged steady state running necessitates a greater glycogen requirement than shorter but higher intensity track running sessions, 2) females display evidence of reduced resting muscle glycogen concentration and net muscle glycogen utilisation when compared with males and 3), net glycogen utilisation is higher in the gastrocnemius muscle compared with the vastus lateralis.
... Specifically, the ingestion of mixed carbohydrate solutions throughout prolonged running results in lower rates of muscle glycogen degradation than a noncaloric placebo (Tsintzas et al., 1995;1996a, 2001. In contrast, during prolonged exercise cycling carbohydrate ingestion does not result in muscle glycogen sparing; rather, the ergogenic effects operate via reduced hepatic glycogenolysis and maintenance of euglycemia and carbohydrate oxidation (Claassen et al., 2005;Coyle et al., 1986;Gonzalez et al., 2015). Prolonged running and cycling may therefore differ subtly in the mechanisms through which fatigue is initiated and/or the mechanisms through which carbohydrate ingestion can offset fatigue. ...
Article
The timing of carbohydrate ingestion and how this influences net muscle glycogen utilization and fatigue has only been investigated in prolonged cycling. Past findings may not translate to running because each exercise mode is distinct both in the metabolic response to carbohydrate ingestion and in the practicalities of carbohydrate ingestion. To this end, a randomized, cross-over design was employed to contrast ingestion of the same sucrose dose either at frequent intervals (15 × 5 g every 5 min) or at a late bolus (1 × 75 g after 75 min) during prolonged treadmill running to exhaustion in six well-trained runners ( 61 ± 4 ml·kg ⁻¹ ·min ⁻¹ ). The muscle glycogen utilization rate was lower in every participant over the first 75 min of running (Δ 0.51 mmol·kg dm ⁻¹ ·min ⁻¹ ; 95% confidence interval [−0.02, 1.04] mmol·kg dm ⁻¹ ·min ⁻¹ ) and, subsequently, all were able to run for longer when carbohydrate had been ingested frequently from the start of exercise compared with when carbohydrate was ingested as a single bolus toward the end of exercise (105.6 ± 3.0 vs. 96.4 ± 5.0 min, respectively; Δ 9.3 min, 95% confidence interval [2.8, 15.8] min). A moderate positive correlation was apparent between the magnitude of glycogen sparing over the first 75 min and the improvement in running capacity ( r = .58), with no significant difference in muscle glycogen concentrations at the point of exhaustion. This study indicates that failure to ingest carbohydrates from the outset of prolonged running increases reliance on limited endogenous muscle glycogen stores—the ergolytic effects of which cannot be rectified by subsequent carbohydrate ingestion late in exercise.
... Low muscle and liver glycogen concentrations are strongly associated with fatigue during prolonged, moderate-to-high intensity exercise (1,2). The ingestion of carbohydrate during exercise provides an additional (exogenous) source of carbohydrate, which can prevent or attenuate the decline in liver (3), and sometimes muscle (4,5), glycogen contents. Increasing exogenous carbohydrate oxidation via altering the dose or type of carbohydrates ingested can improve endurance performance (6)(7)(8)(9). ...
Article
Purpose: Maximizing carbohydrate availability is important for many endurance events. Combining pectin and sodium alginate with ingested maltodextrin-fructose (MAL+FRU+PEC+ALG) has been suggested to enhance carbohydrate delivery via hydrogel formation but the influence on exogenous carbohydrate oxidation remains unknown. The primary aim of this study was to assess the effects of MAL+FRU+PEC+ALG on exogenous carbohydrate oxidation during exercise compared to a maltodextrin-fructose mixture (MAL+FRU). MAL+FRU has been well established to increase exogenous carbohydrate oxidation during cycling, compared to glucose-based carbohydrates (MAL+GLU). However, much evidence focuses on cycling, and direct evidence in running is lacking. Therefore, a secondary aim was to compare exogenous carbohydrate oxidation rates with MAL+FRU versus MAL+GLU during running. Methods: Nine trained runners completed two trials (MAL+FRU and MAL+FRU+PEC+ALG) in a double-blind, randomised crossover design. A subset (n=7) also completed a MAL+GLU trial to address the secondary aim, and a water trial to establish background expired CO2 enrichment. Participants ran at 60% V˙O2peak for 120 min while ingesting either water only, or carbohydrate solutions at a rate of 1.5 g carbohydrate·min. Results: At the end of 120 min of exercise, exogenous carbohydrate oxidation rates were 0.9 (SD 0.5) g·min with MAL+GLU ingestion. MAL+FRU ingestion increased exogenous carbohydrate oxidation rates to 1.1 (SD 0.3) g·min (p=0.038), with no further increase with MAL+FRU+PEC+ALG ingestion (1.1 (SD 0.3) g·min; p=1.0). No time x treatment interaction effects were observed for plasma glucose, lactate, insulin or non-esterified fatty acids, nor for ratings of perceived exertion or gastrointestinal symptoms (all p>0.05). Conclusion: To maximise exogenous carbohydrate oxidation during moderate-intensity running, athletes may benefit from consuming glucose(polymer)-fructose mixtures over glucose-based carbohydrates alone, but the addition of pectin and sodium alginate offers no further benefit.
... Studies investigating keto-adaptation in endurance athletes for at least 3 weeks demonstrate alterations to substrate availability and metabolism (Table 1), faecal [18] and oral [19] microbiome, and iron regulation [20,21]; whereas, acid-base balance [22] and mucosal immunity [23] remain unaltered. Depleted skeletal muscle [24] and hepatic [25] glycogen stores and an inability to maintain CHO oxidation rates [24,26] may precede metabolic exhaustion during exercise. Therefore, the primary rationale for keto-adaptation is to reduce CHO-utilisation rates, whilst sustaining energy production, for a given exercise intensity. ...
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Ketone bodies (KB) provide an alternative energy source and uniquely modulate substrate metabolism during endurance exercise. Nutritional ketosis (blood KBs > 0.5 mM) can be achieved within minutes via exogenous ketone supplementation or days-to-weeks via conforming to a very low-carbohydrate, ketogenic diet (KD). In contrast to short-term (< 2 weeks) KD ingestion, chronic adherence (> 3 weeks) leads to a state of keto-adaptation. However, despite elevating blood KBs to similar concentrations, exogenous ketone supplementation and keto-adaptation are not similar metabolic states as they elicit diverse and distinct effects on substrate availability and metabolism during exercise; meaning that their influence on endurance exercise performance is different. In contrast to contemporary, high(er)-carbohydrate fuelling strategies, inducing nutritional ketosis is rarely ergogenic irrespective of origin and, in fact, can impair endurance performance. Nonetheless, exogenous ketone supplementation and keto-adaptation possess utility for select endurance events and individuals, thus warranting further research into their performance effects and potential strategies for their optimisation. It is critical, however, that future research considers the limitations of measuring blood KB concentrations and their utilisation, and assess the effect of nutritional ketosis on performance using exercise protocols reflective of real-world competition. Furthermore, to reliably assess the effects of keto-adaptation, rigorous dietary-training controls of sufficient duration should be prioritised.
... For the Training Study, they recorded the composition of their evening meal on the day before a pre-intervention trial and replicated this meal for the post-intervention trial, in line with guidelines for testing postprandial glycemic control (33). This protocol produces fasting muscle and liver glycogen and fasting intramuscular lipid concentrations that are consistent across trial days (34). ...
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Context Pre-exercise nutrient availability alters acute metabolic responses to exercise, which could modulate training responsiveness. Objective To assess acute and chronic effects of exercise performed before versus after nutrient ingestion on whole-body and intramuscular lipid utilization, and postprandial glucose metabolism. Design 1) Acute, randomised, crossover design (Acute Study); 2) 6-week, randomised, controlled design (Training Study). Setting General community. Participants Men with overweight/obesity (mean±SD, BMI: 30.2±3.5 kg.m-2 for Acute Study, 30.9±4.5 kg.m-2 for Training Study). Interventions Moderate-intensity cycling performed before versus after mixed-macronutrient breakfast (Acute Study) or carbohydrate (Training Study) ingestion. Results Acute Study - exercise before versus after breakfast consumption increased net intramuscular lipid utilization in type I (net change: -3.44±2.63% versus 1.44±4.18% area lipid staining, p < 0.01) and type II fibres (-1.89±2.48% versus 1.83±1.92% area lipid staining, p < 0.05). Training Study - postprandial glycemia was not differentially affected by 6-weeks of exercise training performed before versus after carbohydrate intake (p>0.05). However, postprandial insulinemia was reduced with exercise training performed before, but not after carbohydrate ingestion (p=0.03). This resulted in increased oral glucose insulin sensitivity (25±38 vs -21±32 mL.min-1.m-2; p=0.01), associated with increased lipid utilization during exercise (r=0.50, p=0.02). Regular exercise before nutrient provision also augmented remodelling of skeletal muscle phospholipids and protein content of the glucose transport protein GLUT4 (p<0.05). Conclusions Experiments investigating exercise training and metabolic health should consider nutrient-exercise timing, and exercise performed before versus after nutrient intake (i.e., in the fasted state) may exert beneficial effects on lipid utilisation and reduce postprandial insulinemia.
... higher in the high-CHO trials compared with keto-adapted trial. This was likely underpinned by a combination of 1) elevated muscle (31) and hepatic glycogen content (32), 2) elevated blood glucose uptake into the muscle (33), 3) maintenance of blood glucose concentration (3), and 4) reduction in hepatic glycogen utilization (2). However, if CHO was acutely ingested in the post-KD trial, this would oppose adaptations to the KD and suppress hepatic ketogenesis, thus compromising rates of fat oxidation and ketolysis (8). ...
Article
Purpose: We investigated the effect of a 31-d ketogenic diet (KD) on submaximal exercise capacity and efficiency. Methods: A repeated-measures, crossover study with preintervention and postintervention outcomes was conducted in eight trained male endurance athletes (maximal oxygen uptake (V[Combining Dot Above]O2max), 59.4 ± 5.2 mL⋅kg⋅min). Participants ingested their habitual diet (HD) (43% ± 8% carbohydrate and 38% ± 7% fat) or an isoenergetic KD (4% ± 1% carbohydrate and 78% ± 4% fat) from days 0 to 31 (P < 0.001). On days -2 and 29, participants undertook a fasted graded metabolic test (~25 min), and on days 0 and 31, participants completed a run-to-exhaustion trial at 70% of their V[Combining Dot Above]O2max (~12.9 km⋅h) after the ingestion of a high-carbohydrate meal (2 g⋅kg) or an isoenergetic low-carbohydrate, high-fat meal, with carbohydrate (~55 g⋅h) or isoenergetic fat (coconut oil) supplementation during exercise. Results: Training load did not differ between trials, and there was no effect of diet on V[Combining Dot Above]O2max (all, P > 0.05). The KD impaired exercise efficiency, particularly at >70% V[Combining Dot Above]O2max, as evident by oxygen uptake that could not be explained by shifts in RER and increased energy expenditure (all, P < 0.05). However, exercise efficiency was maintained on a KD when exercising at <60% V[Combining Dot Above]O2max (all, P > 0.05). There was no effect of diet on time-to-exhaustion (237 ± 44 min (pre-HD) vs 231 ± 35 min (post-HD), P = 0.44; 239 ± 27 min (pre-KD) vs 219 ± 53 min (post-KD), P = 0.36). Conclusion: A 31-d KD can preserve submaximal exercise capacity in trained endurance athletes; however, endurance variability increases.
... The liver tissue can accommodate approximately 85 kg wet weight −1 , or approximately 100 g o glycogen or the average liver weighing 1.2 kg [24]. In recent years, the use o 13 C magnetic resonance spectroscopy studies has shown that liver glycogen content does not di er between trained and untrained individuals and declines signi cantly during submaximal endurance exercise o about 60-70% VO 2 max [27,29,30]. Conversely, muscle glycogen levels are usually 20-66% higher in endurance trained compared with untrained individuals. ...
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Maintaining metabolic homeostasis is of paramount importance for the human organism. Accordingly, adenosine triphosphate (ATP) levels, the energy currency of the human body, are adequately maintained in skeletal and heart muscle by the continuous formation of ATP aerobically and anaerobically. The main substrates used for ATP formation are phosphocreatine, carbohydrates, and free fatty acids, while branched-chain amino acids contribute to a smaller extent. The main factor dictating the dominant metabolic pathway and the type of substrate used is exercise intensity, whereas other factors such as exercise duration, fitness status, gender, diet, and environmental conditions may also influence exercise metabolism. The metabolic pathways do not function independently. Rather, they interact via extracellular and intracellular signals from the exercising muscles and communicate with distant organs such as the liver, heart, and brain. Moreover, hormones secreted by cells of the endocrine system regulate activity of cells in other parts of the body, they can be released in response to exercise-induced stress, and, among other multiple functions, they modulate metabolism during exercise. Several clinical implications for health benefits of special populations rely on exercise metabolism alterations.
... Además, se ha visto que los aminoácidos actúan en sinergia con los CHO para estimular la producción de insulina. Esta acción insulinotrópica de las proteínas parece deberse, a dos factores: por un lado, al aumento de la liberación de incretinas por parte de las células enteroendocrinas del intestino, y en segundo lugar, por la estimulación directa de las células beta pancreáticas por las concentraciones de aminoácidos (Gonzalez et al., 2015;Mace, Schindler, & Patel, 2012). El tipo de proteína parece influir en la secreción de insulina. ...
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Resumen: Las estrategias nutricionales durante la fase de recuperación del deportista son fundamentales. Uno de los principales objetivos de la recu-peración es la reposición del glucógeno muscular. Este aspecto se hace más importante cuando los deportistas se enfrentan a entrenamientos intensos o eventos competitivos con cortos periodos de recuperación. Además, la manipulación deliberada de su disponibilidad puede mejorar las adaptacio-nes moleculares al entrenamiento. La presente revisión tiene por objetivo informar sobre los aspectos fisiológicos básicos de esta situación, así como conocer el momento del consumo, la cantidad, el tipo y la interacción de diferentes nutrientes con los hidratos de carbono, para poder maximizar o jugar con la reposición del mismo en función de las necesidades y/o las estrategias planteadas. El glucógeno ya no debe ser visto como un simple almacén de energía sino como una molécula que puede desencadenar numerosos procesos celulares importantes para el deportista. Abstract: Nutritional interventions play a fundamental role during the post-exercise recovery phase. One of the main goals of recovery is restoring muscle glycogen stores. This becomes more important when athletes are subjected to intense training or competition with short recovery periods between bouts. Furthermore, manipulating muscle glycogen availability can improve molecular adaptations to training. The objective of this review is thus to present the basic physiological aspects of this phenomenon, and to discuss carbohydrate consumption, timing, type, and amount, as well as its interaction with different nutrients, in order to maximize or play with the restoration of muscle glycogen depending on the needs and/ or the strategies proposed. Glycogen should no longer be seen as a simple form of energy storage, but as a molecule that can trigger numerous cellular processes important for athletic performance .
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Viewing metabolism through the lens of exercise biology has proven an accessible and practical strategy to gain new insights into local and systemic metabolic regulation. Recent methodological developments have advanced understanding of the central role of skeletal muscle in many exercise-associated health benefits and have uncovered the molecular underpinnings driving adaptive responses to training regimens. In this Review, we provide a contemporary view of the metabolic flexibility and functional plasticity of skeletal muscle in response to exercise. First, we provide background on the macrostructure and ultrastructure of skeletal muscle fibres, highlighting the current understanding of sarcomeric networks and mitochondrial subpopulations. Next, we discuss acute exercise skeletal muscle metabolism and the signalling, transcriptional and epigenetic regulation of adaptations to exercise training. We address knowledge gaps throughout and propose future directions for the field. This Review contextualizes recent research of skeletal muscle exercise metabolism, framing further advances and translation into practice.
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Purpose This study aimed to investigate whether carbohydrate ingestion during 3 h long endurance exercise in highly trained cyclists at a rate of 120 g h ⁻¹ in 0.8:1 ratio between fructose and glucose-based carbohydrates would result in higher exogenous and lower endogenous carbohydrate oxidation rates as compared to ingestion of 90 g h ⁻¹ in 1:2 ratio, which is the currently recommended approach for exercise of this duration. Methods Eleven male participants (V̇O 2peak 62.6 ± 7 mL kg ⁻¹ min ⁻¹ , gas exchange threshold (GET) 270 ± 17 W and Respiratory compensation point 328 ± 32 W) completed the study involving 4 experimental visits consisting of 3 h cycling commencing after an overnight fast at an intensity equivalent to 95% GET. During the trials they received carbohydrates at an average rate of 120 or 90 g h ⁻¹ in 0.8:1 or 1:2 fructose-maltodextrin ratio, respectively. Carbohydrates were naturally high or low in ¹³ C stable isotopes enabling subsequent calculations of exogenous and endogenous carbohydrate oxidation rates. Results Exogenous carbohydrate oxidation rates were higher in the 120 g h ⁻¹ condition (120–180 min: 1.51 ± 0.22 g min ⁻¹ ) as compared to the 90 g h ⁻¹ condition (1.29 ± 0.16 g min ⁻¹ ; p = 0.026). Endogenous carbohydrate oxidation rates did not differ between conditions (2.15 ± 0.30 and 2.20 ± 0.33 g min ⁻¹ for 120 and 90 g h ⁻¹ conditions, respectively; p = 0.786). Conclusions The results suggest that carbohydrate ingestion at 120 g h ⁻¹ in 0.8:1 fructose-maltodextrin ratio as compared with 90 g h ⁻¹ in 1:2 ratio offers higher exogenous carbohydrate oxidation rates but no additional sparing of endogenous carbohydrates. Further studies should investigate potential performance effects of such carbohydrate ingestion strategies.
Article
We examined the effects of carbohydrate (CHO) delivery form on exogenous CHO oxidation, gastrointestinal discomfort, and exercise capacity. In a randomised repeated measures design (after 24 h of high CHO intake (8 g·kg-1) and pre-exercise meal (2 g·kg-1)), nine trained males ingested 120 g CHO·h-1 from fluid (DRINK), semi-solid gel (GEL), solid jelly chew (CHEW), or a co-ingestion approach (MIX). Participants cycled for 180 min at 95% lactate threshold followed by an exercise capacity test (150% lactate threshold). Peak rates of exogenous CHO oxidation (DRINK, 1.56 ± 0.16; GEL, 1.58 ± 0.13; CHEW, 1.59 ± 0.08; MIX, 1.66 ± 0.02 g·min-1) and oxidation efficiency (DRINK, 72 ± 8; GEL, 72 ± 5; CHEW, 75 ± 5; MIX, 75 ± 6%) were not different between trials (all P > 0.05). Despite ingesting 120 g·h-1, participants reported minimal symptoms of gastrointestinal distress across all trials. Exercise capacity was also not significantly different (all P < 0.05) between conditions (DRINK, 446 ± 350; GEL, 529 ± 396; CHEW, 596 ± 416; MIX, 469 ± 395 sec). Data represent the first time that rates of exogenous CHO oxidation (via stable isotope methodology) have been simultaneously assessed using feeding strategies (i.e., pre-exercise CHO feeding and the different forms and combinations of CHO during exercise) commonly adopted by elite endurance athletes. We conclude 120 g·h-1 CHO (in a 1:0.8 ratio of maltodextrin or glucose:fructose) is a practically tolerable strategy to promote high CHO availability and oxidation during exercise.
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Optimal carbohydrate and protein intakes are vital for modulating training adaptation, recovery, and exercise performance. However, the research base underpinning contemporary sport nutrition guidelines has largely been conducted in male populations with a lack of consensus on whether the menstrual phase and associated changes in sex hormones allow broad application of these principles to female athletes. The present review will summarise our current understanding of carbohydrate and protein requirements in female athletes across the menstrual cycle and provide a critical analysis on how they compare to male athletes. On the basis of current evidence, we consider it premature to conclude that female athletes require sex specific guidelines in relation to CHO or protein requirements provided energy needs are met. However, there is a need for further research using sport-specific competition and training related exercise protocols that rigorously control for prior exercise, CHO/energy intake, contraceptive use and phase of menstrual cycle. Our overarching recommendation is to use current recommendations as a basis for adopting an individualised approach that takes into account athlete specific training and competition goals whilst also considering personal symptoms associated with the menstrual cycle.
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Key points Muscle glycogen and intramuscular triglycerides (IMTG, stored in lipid droplets) are important energy substrates during prolonged exercise. Exercise‐induced changes in lipid droplet (LD) morphology (i.e. LD size and number) have not yet been studied under nutritional conditions typically adopted by elite endurance athletes, that is, after carbohydrate (CHO) loading and CHO feeding during exercise. We report for the first time that exercise reduces IMTG content in both central and peripheral regions of type I and IIa fibres, reflective of decreased LD number in both fibre types whereas reductions in LD size were exclusive to type I fibres. Additionally, CHO feeding does not alter subcellular IMTG utilisation, LD morphology or muscle glycogen utilisation in type I or IIa/II fibres. In the absence of alterations to muscle fuel selection, CHO feeding does not attenuate cell signalling pathways with regulatory roles in mitochondrial biogenesis. Abstract We examined the effects of carbohydrate (CHO) feeding on lipid droplet (LD) morphology, muscle glycogen utilisation and exercise‐induced skeletal muscle cell signalling. After a 36 h CHO loading protocol and pre‐exercise meal (12 and 2 g kg–1, respectively), eight trained males ingested 0, 45 or 90 g CHO h–1 during 180 min cycling at lactate threshold followed by an exercise capacity test (150% lactate threshold). Muscle biopsies were obtained pre‐ and post‐completion of submaximal exercise. Exercise decreased (P < 0.01) glycogen concentration to comparable levels (∼700 to 250 mmol kg–1 DW), though utilisation was greater in type I (∼40%) versus type II fibres (∼10%) (P < 0.01). LD content decreased in type I (∼50%) and type IIa fibres (∼30%) (P < 0.01), with greater utilisation in type I fibres (P < 0.01). CHO feeding did not affect glycogen or IMTG utilisation in type I or II fibres (all P > 0.05). Exercise decreased LD number within central and peripheral regions of both type I and IIa fibres, though reduced LD size was exclusive to type I fibres. Exercise induced (all P < 0.05) comparable AMPKThr172 (∼4‐fold), p53Ser15 (∼2‐fold) and CaMKIIThr268 phosphorylation (∼2‐fold) with no effects of CHO feeding (all P > 0.05). CHO increased exercise capacity where 90 g h–1 (233 ± 133 s) > 45 g h–1 (156 ± 66 s; P = 0.06) > 0 g h–1 (108 ± 54 s; P = 0.03). In conditions of high pre‐exercise CHO availability, we conclude CHO feeding does not influence exercise‐induced changes in LD morphology, glycogen utilisation or cell signalling pathways with regulatory roles in mitochondrial biogenesis.
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Nutrition and exercise metabolism are vibrant physiological fields, yet at times it feels as if greater progress could be made by better integrating these disciplines. Exercise is advocated for improving metabolic health, in part by increasing peripheral insulin sensitivity and glycaemic control. However, when a modest‐to‐high carbohydrate load is consumed before and/or during each exercise bout within a training programme, increases in oral glucose insulin sensitivity can be blunted in both men of a healthy weight and those with overweight/obesity. Exercise training‐induced adaptation in the energy sensing AMP‐activated protein kinase (AMPK) and the insulin‐sensitive glucose transporter GLUT4 protein levels are sensitive to pre‐exercise feeding status in both healthy individuals and individuals classified as overweight or obese. Increased lipid oxidation may, in part, explain the enhanced adaptive responses to exercise training performed before (i.e. fasted‐state exercise) versus after nutrient ingestion. Evidence in individuals with type 2 diabetes currently shows no effect of altering nutrient–exercise timing for measured markers of metabolic health, or greater reductions in glycated haemoglobin (HbA1c) concentrations with exercise performed after versus before nutrient provision. Since the metabolic inflexibility associated with type 2 diabetes diminishes differences in lipid oxidation between the fasted and fed states, it is plausible that pre‐exercise feeding status does not alter adaptations to exercise when metabolic flexibility is already compromised. Current evidence suggests restricting carbohydrate intake before and during exercise can enhance some health benefits of exercise, but in order to establish clinical guidelines, further research is needed with hard outcomes and different populations. image
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Professional rugby league (RL) football is a contact sport involving repeated collisions and high-intensity efforts; both training and competition involve high energy expenditure. The present review summarizes and critiques the available literature relating the physiological demands of RL to nutritional requirements and considers potential ergogenic supplements that could improve players’ physical capacity, health, and recovery during the preparatory and competition phases of a season. Although there may not be enough data to provide RL-specific recommendations, the available data suggest that players may require approximately 6–8 g·kg−1·day−1 carbohydrate, 1.6–2.6 g·kg−1·day−1 protein, and 0.7–2.2 g·kg−1·day−1 fat, provided that the latter also falls within 20–35% of total energy intake. Competition nutrition should maximize glycogen availability by consuming 1–4 g/kg carbohydrate (∼80–320 g) plus 0.25 g/kg (∼20–30 g) protein, 1–4 hr preexercise for 80–120 kg players. Carbohydrate intakes of approximately 80–180 g (1.0–1.5 g/kg) plus 20–67 g protein (0.25–0.55 g/kg) 0–2 hr postexercise will optimize glycogen resynthesis and muscle protein synthesis. Supplements that potentially improve performance, recovery, and adaptation include low to moderate dosages of caffeine (3–6 mg/kg) and ∼300 mg polyphenols consumed ∼1 hr preexercise, creatine monohydrate “loading” (0.3 g·kg−1·day−1) and/or maintenance (3–5 g/day), and beta-alanine (65–80 mg·kg−1·day−1). Future research should quantify energy expenditures in young, professional male RL players before constructing recommendations.
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Purpose This study investigated the effect of carbohydrate supplementation on substrate oxidation during exercise in hypoxia after pre-exercise breakfast consumption and omission. Methods Eleven men walked in normobaric hypoxia (FiO2 ~11.7%) for 90-min at 50% of hypoxic V[Combining Dot Above]O2max. Participants were supplemented with a carbohydrate beverage (1.2g·min-1 glucose) and a placebo beverage (both enriched with U-13C6 D-glucose) after breakfast consumption and after omission. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate carbohydrate (exogenous and endogenous (muscle and liver)) and fat oxidation. Results In the first 60-min of exercise, there was no significant change in relative substrate oxidation in the carbohydrate compared with placebo trial after breakfast consumption or omission (both p = 0.99). In the last 30-min of exercise, increased relative carbohydrate oxidation occurred in the carbohydrate compared with placebo trial after breakfast omission (44.0 ± 8.8 vs. 28.0 ± 12.3, p < 0.01) but not consumption (51.7 ± 12.3 vs. 44.2 ± 10.4, p = 0.38). In the same period, a reduction in relative liver (but not muscle) glucose oxidation was observed in the carbohydrate compared with placebo trials after breakfast consumption (liver: 7.7 ± 1.6% vs. 14.8 ± 2.3%, p < 0.01; muscle: 25.4 ± 9.4% vs. 29.4 ± 11.1%, p = 0.99) and omission (liver: 3.8 ± 0.8% vs. 8.7 ± 2.8%, p < 0.01; muscle: 19.4 ± 7.5% vs. 19.2 ± 12.2%, p = 0.99). No significant difference in relative exogenous carbohydrate oxidation was observed between breakfast consumption and omission trials (p = 0.14). Conclusion In acute normobaric hypoxia, carbohydrate supplementation increased relative carbohydrate oxidation during exercise (> 60 min) after breakfast omission, but not consumption.
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The benefits of ingesting exogenous carbohydrate (CHO) during prolonged exercise performance are well established. A recent food technology innovation has seen sodium alginate and pectin included in solutions of multiple transportable CHO, to encapsulate them at pH levels found in the stomach. Marketing claims include enhanced gastric emptying and delivery of CHO to the muscle with less gastrointestinal distress, leading to better sports performance. Emerging literature around such claims was identified by searching electronic databases; inclusion criteria were randomized controlled trials investigating metabolic and/or exercise performance parameters during endurance exercise >1 hr, with CHO hydrogels versus traditional CHO fluids and/or noncaloric hydrogels. Limitations associated with the heterogeneity of exercise protocols and control comparisons are noted. To date, improvements in exercise performance/capacity have not been clearly demonstrated with ingestion of CHO hydrogels above traditional CHO fluids. Studies utilizing isotopic tracers demonstrate similar rates of exogenous CHO oxidation, and subjective ratings of gastrointestinal distress do not appear to be different. Overall, data do not support any metabolic or performance advantages to exogenous CHO delivery in hydrogel form over traditional CHO preparations; although, one study demonstrates a possible glycogen sparing effect. The authors note that the current literature has largely failed to investigate the conditions under which maximal CHO availability is needed; high-performance athletes undertaking prolonged events at high relative and absolute exercise intensities. Although investigations are needed to better target the testimonials provided about CHO hydrogels, current evidence suggests that they are similar in outcome and a benefit to traditional CHO sources.
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Carbohydrate (CHO) ingestion is an established strategy to improve endurance performance. Race fuels should not only sustain performance, but also be readily digested and absorbed. Potatoes are a whole-food based option that fulfills these criteria yet their impact on performance remains unexamined. We investigated the effects of potato purée ingestion during prolonged cycling on subsequent performance versus commercial CHO gel or a water-only condition. Twelve cyclists (70.7 ± 7.7 kg, 173 ± 8 cm, 31± 9 years, 22 ± 5.1 % body fat; mean ± SD) with average peak oxygen consumption (VO 2PEAK ) of 60.7 ± 9.0 mL/kg/min performed a 2 h cycling challenge (60-85%VO 2PEAK ) followed by a time trial (TT, 6kJ/kg body mass) while consuming potato, gel, or water in a randomized-crossover design. The race fuels were administered with U-[ ¹³ C 6 ]glucose for an indirect estimate of gastric emptying rate. Blood samples were collected throughout the trials. Blood glucose concentrations were higher ( P<0.001) in potato and gel conditions when compared to water condition. Blood lactate concentrations were higher ( P=0.001) after the TT completion in both CHO conditions when compared to water condition. TT performance was improved ( P=0.032) in both potato (33.0 ± 4.5 min) and gel (33.0 ± 4.2 min) conditions when compared to the water condition (39.5 ± 7.9 min). Moreover, no difference was observed in TT performance between CHO conditions ( P=1.00). In conclusion, potato and gel ingestion equally sustained blood glucose concentrations and TT performance. Our results support the effective use of potatoes to support race performance for trained cyclists.
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In health, food carbohydrate is stored as glycogen in muscle and liver, preventing a deleterious rise in osmotically active plasma glucose after eating. Glycogen concentrations increase sequentially after each meal to peak in the evening, and fall to fasting levels thereafter. Skeletal muscle accounts for the larger part of this diurnal buffering capacity with liver also contributing.The effectiveness of this diurnal mechanism has not been previously studied in type 2 diabetes. We have quantified the changes in muscle and liver glycogen concentration with 13C magnetic resonance spectroscopy at 3.0 Tesla before and after 3 meals consumed at 4 hours intervals. We studied 40 (25M;15F) well controlled type 2 diabetes subjects on metformin only (HbA1c 6.4±0.07% or 47±0.8 mmol/mol) and 14 (8M;6F) glucose tolerant controls matched for age, weight and BMI. Muscle glycogen concentration increased by 17% after day-long eating in the control group (68.1±4.8 to 79.7±4.2mmol/l; P=0.006), and this change inversely correlated with Homeostatic Model Assessment of Insulin Resistance [HOMA-IR] (r=-0.56; P=0.02). There was no change in muscle glycogen in the type 2 diabetes group after day-long eating (68.3±2.6 to 67.1±2.0mmol/mol; P=0.62). Liver glycogen rose similarly in normal control (325.9±25.0 to 388.1±30.3mmol/L; P=0.005) and type 2 diabetes groups (296.1±16.0 to 350.5 ± 6.7mmol/l; P<0.0001). In early type 2 diabetes, the major physiological mechanism for skeletal muscle postprandial glycogen storage is completely inactive. This is directly related to insulin resistance, although liver glycogen storage is normal.
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The consumption of carbohydrate before, during and after exercise is a central feature of the athlete's diet, particularly those competing in endurance sports. Sucrose is a carbohydrate present within the diets of athletes. Whether sucrose, by virtue of its component monosaccharide's glucose and fructose, exerts a meaningful advantage for athletes over other carbohydrate types or blends is unclear. This narrative reviews the literature on the influence of sucrose, relative to other carbohydrate types, on exercise performance or the metabolic factors that may underpin exercise performance. Inference from the research to date suggests that sucrose appears to be as effective as other highly metabolizable carbohydrates (e.g., glucose, glucose polymers) in providing an exogenous fuel source during endurance exercise, stimulating the synthesis of liver and muscle glycogen during exercise recovery and improving endurance exercise performance. Nonetheless, gaps exist in our understanding of the metabolic and performance consequences of sucrose ingestion before, during and after exercise relative to other carbohydrate types or blends, particularly when more aggressive carbohydrate intake strategies are adopted. While further research is recommended and discussed in this review, based on the currently available scientific literature it would seem that sucrose should continue to be regarded as one of a variety of options available to help athletes achieve their specific carbohydrate intake goals.
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• Contemporary stable isotope methodology was applied in combination with muscle biopsy sampling to accurately quantify substrate utilisation and study the regulation of muscle fuel selection during exercise. • Eight cyclists were studied at rest and during three consecutive 30 min stages of exercise at intensities of 40, 55 and 75 % maximal workload (Wmax). A continuous infusion of [U-13C]palmitate and [6,6-2H2]glucose was administered to determine plasma free fatty acid (FFA) oxidation and estimate plasma glucose oxidation, respectively. Biopsy samples were collected before and after each exercise stage. • Muscle glycogen and plasma glucose oxidation rates increased with every increment in exercise intensity. Whole-body fat oxidation increased to 32 ± 2 kJ min−1 at 55 % Wmax, but declined at 75 % Wmax (19 ± 2 kJ min−1). This decline involved a decrease in the oxidation rate of both plasma FFA and triacylglycerol fat sources (sum of intramuscular plus lipoprotein-derived triacylglycerol), and was accompanied by increases in muscle pyruvate dehydrogenase complex activation and acetylation of the carnitine pool, resulting in a decline in muscle free carnitine concentration. • We conclude that the most likely mechanism for the reduction in fat oxidation during high-intensity exercise is a downregulation of carnitine palmitoyltransferase I, either by this marked decline in free carnitine availability or by a decrease in intracellular pH.
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Lactic acid is a well known metabolic by-product of intense exercise, particularly under anaerobic conditions. Lactate is also a key source of energy and an important metabolic substrate, and it has also been hypothesized to be a signaling molecule directing metabolic activity. Here we show that GPR81, an orphan G-protein-coupled receptor highly expressed in fat, is in fact a sensor for lactate. Lactate activates GPR81 in its physiological concentration range of 1-20 mM and suppresses lipolysis in mouse, rat, and human adipocytes as well as in differentiated 3T3-L1 cells. Adipocytes from GPR81-deficient mice lack an antilipolytic response to lactate but are responsive to other antilipolytic agents. Lactate specifically induces internalization of GPR81 after receptor activation. Site-directed mutagenesis of GPR81 coupled with homology modeling demonstrates that classically conserved key residues in the transmembrane binding domains are responsible for interacting with lactate. Our results indicate that lactate suppresses lipolysis in adipose tissue through a direct activation of GPR81. GPR81 may thus be an attractive target for the treatment of dyslipidemia and other metabolic disorders.
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This study examined effects of ingesting a 10% carbohydrate (CHO) drink (CI) or placebo (PI) at 500 ml/h on total (splanchnic) glucose appearance (endogenous+exogenous; Ra), blood glucose oxidation, and muscle glycogen utilization in 14 male endurance-trained cyclists who rode for 180 min at 70% of maximal O2 uptake after CHO loading [starting muscle glycogen 203 +/- 7 (SE) mmol/kg wet wt]. Total CHO oxidation was similar in CI and PI, but Ra increased significantly during the trial in both groups with CI reaching a plateau after 75 min. Ra was significantly greater in CI than in PI at the end of exercise. Blood glucose oxidation also increased significantly during the trial to a plateau in CI and was significantly higher in CI than in PI at the end of exercise. However, mean endogenous Ra was significantly lower in CI than in PI throughout exercise, as was oxidation of endogenous blood glucose, which remained almost constant in CI and reached 43 +/- 8 and 73 +/- 13 mumol.min-1.kg fat-free mass-1 in CI and PI, respectively, at the end of exercise. At 0.83 g/min of CHO ingestion, 0.77 +/- 0.03 g/min was oxidized. Muscle glycogen utilization was identical in both groups and was higher during the 1st h of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)
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The purposes of this study were 1) to investigate the effect of carbohydrate (CHO) ingestion on endogenous glucose production (EGP) during prolonged exercise, 2) to study whether glucose appearance in the circulation could be a limiting factor for exogenous CHO oxidation, and 3) to investigate whether large CHO feedings can reduce muscle glycogen oxidation during exercise. Six well-trained subjects exercised three times for 120 min at 50% maximum workload while ingesting water (FAST), a 4% glucose solution (LO-Glc), or a 22% glucose solution (HI-Glc). A primed continuous intravenous [6, 6-2H2]glucose infusion was given, and the ingested glucose was enriched with [U-13C]glucose. Glucose ingestion significantly elevated CHO oxidation as well as the rates of appearance (Ra) and disappearance. Ra glucose equaled Ra of glucose in gut (Ra gut) during HI-Glc, whereas EGP was completely suppressed. During LO-Glc, EGP was partially suppressed, whereas Ra gut provided most of the total glucose Ra. We conclude that 1) high rates of CHO ingestion can completely block EGP, 2) Ra gut may be a limiting factor for exogenous CHO oxidation, and 3) muscle glycogen oxidation was not reduced by large glucose feedings.
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This study investigated the effect of carbohydrate (CHO) ingestion on postexercise glycogen resynthesis, measured simultaneously in liver and muscle (n = 6) by (13)C magnetic resonance spectroscopy, and subsequent exercise capacity (n = 10). Subjects cycled at 70% maximal oxygen uptake for 83 +/- 8 min on six separate occasions. At the end of exercise, subjects ingested 1 g/kg body mass (BM) glucose, sucrose, or placebo (control). Resynthesis of glycogen over a 4-h period after treatment ingestion was measured on the first three occasions, and subsequent exercise capacity was measured on occasions four through six. No glycogen was resynthesized during the control trial. Liver glycogen resynthesis was evident after glucose (13 +/- 8 g) and sucrose (25 +/- 5 g) ingestion, both of which were different from control (P < 0.01). No significant differences in muscle glycogen resynthesis were found among trials. A relationship between the CHO load (g) and change in liver glycogen content (g) was evident after 30, 90, 150, and 210 min of recovery (r = 0.59-0. 79, P < 0.05). Furthermore, a modest relationship existed between change in liver glycogen content (g) and subsequent exercise capacity (r = 0.53, P < 0.05). However, no significant difference in mean exercise time was found (control: 35 +/- 5, glucose: 40 +/- 5, and sucrose: 46 +/- 6 min). Therefore, 1 g/kg BM glucose or sucrose is sufficient to initiate postexercise liver glycogen resynthesis, which contributes to subsequent exercise capacity, but not muscle glycogen resynthesis.
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1. The aim of this study was to examine the effect of carbohydrate (CHO) ingestion on changes in ATP and phosphocreatine (PCr) concentrations in different muscle fibre types during prolonged running and relate those changes to the degree of glycogen depletion. 2. Five male subjects performed two runs at 70 % maximum oxygen uptake (.V(O2,max)), 1 week apart. Each subject ingested 8 ml (kg body mass (BM))(-1) of either a placebo (Con trial) or a 5.5 % CHO solution (CHO trial) immediately before each run and 2 ml (kg BM)(-1) every 20 min thereafter. In the Con trial, the subjects ran to exhaustion (97.0 +/- 6.7 min). In the CHO trial, the run was terminated at the time coinciding with exhaustion in the Con trial. Muscle samples were obtained from the vastus lateralis before and after each trial. 3. Carbohydrate ingestion did not affect ATP concentrations. However, it attenuated the decline in PCr concentration by 46 % in type I fibres (CHO: 20 +/- 8 mmol (kg dry matter (DM))(-1); Con: 34 +/- 6 mmol (kg DM)(-1); P < 0.05) and by 36 % in type II fibres (CHO: 30 +/- 5 mmol (kg DM)(-1); Con: 48 +/- 6 mmol (kg DM)(-1); P < 0.05). 4. A 56 % reduction in glycogen utilisation in type I fibres was observed in CHO compared with Con (117 +/- 39 vs. 240 +/- 32 mmol glucosyl units (kg DM)(-1), respectively; P < 0.01), but no difference was observed in type II fibres. 5. It is proposed that CHO ingestion during exhaustive running attenuates the decline in oxidative ATP resynthesis in type I fibres, as indicated by sparing of both PCr and glycogen breakdown. The CHO-induced sparing of PCr, but not glycogen, in type II fibres may reflect differential recruitment and/or role of PCr between fibre types.
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A recent study from our laboratory has shown that a mixture of glucose and fructose ingested at a rate of 1.8 g/min leads to peak oxidation rates of approximately 1.3 g/min and results in approximately 55% higher exogenous carbohydrate (CHO) oxidation rates compared with the ingestion of an isocaloric amount of glucose. The aim of the present study was to investigate whether a mixture of glucose and fructose when ingested at a high rate (2.4 g/min) would lead to even higher exogenous CHO oxidation rates (>1.3 g/min). Eight trained male cyclists (VO2max: 68+/-1 ml/kg per min) cycled on three different occasions for 150 min at 50% of maximal power output (60+/-1% VO2max) and consumed either water (WAT) or a CHO solution providing 1.2 g/min glucose (GLU) or 1.2 g/min glucose+1.2 g/min fructose (GLU+FRUC). Peak exogenous CHO oxidation rates were higher (P<0.01) in the GLU+FRUC trial compared with the GLU trial (1.75 (SE 0.11) and 1.06 (SE 0.05) g/min, respectively). Furthermore, exogenous CHO oxidation rates during the last 90 min of exercise were approximately 50% higher (P<0.05) in GLU+FRUC compared with GLU (1.49 (SE 0.08) and 0.99 (SE 0.06) g/min, respectively). The results demonstrate that when a mixture of glucose and fructose is ingested at high rates (2.4 g/min) during 150 min of cycling exercise, exogenous CHO oxidation rates reach peak values of approximately 1.75 g/min.
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Using contemporary stable-isotope methodology and fluorescence microscopy, we assessed the impact of carbohydrate supplementation on whole-body and fiber-type-specific intramyocellular triacylglycerol (IMTG) and glycogen use during prolonged endurance exercise. Ten endurance-trained male subjects were studied twice during 3 h of cycling at 63 ± 4% of maximal O2 uptake with either glucose ingestion (CHO trial; 0.7 g CHO kg−1 h−1) or without (CON placebo trial; water only). Continuous infusions with [U-13C] palmitate and [6,6-2H2] glucose were applied to quantify plasma free fatty acids (FFA) and glucose oxidation rates and to estimate intramyocellular lipid and glycogen use. Before and after exercise, muscle biopsy samples were taken to quantify fiber-type-specific IMTG and glycogen content. Plasma glucose rate of appearance (R a) and carbohydrate oxidation rates were substantially greater in the CHO vs CON trial. Carbohydrate supplementation resulted in a lower muscle glycogen use during the first hour of exercise in the CHO vs CON trial, resulting in a 38 ± 19 and 57 ± 22% decreased utilization in type I and II muscle-fiber glycogen content, respectively. In the CHO trial, both plasma FFA R a and subsequent plasma FFA concentrations were lower, resulting in a 34 ± 12% reduction in plasma FFA oxidation rates during exercise (P < 0.05). Carbohydrate intake did not augment IMTG utilization, as fluorescence microscopy revealed a 76 ± 21 and 78 ± 22% reduction in type I muscle-fiber lipid content in the CHO and CON trial, respectively. We conclude that carbohydrate supplementation during prolonged cycling exercise does not modulate IMTG use but spares muscle glycogen use during the initial stages of exercise in endurance-trained men.
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Carbohydrate (CHO) ingestion during exercise, in the form of CHO-electrolyte beverages, leads to performance benefits during prolonged submaximal and variable intensity exercise. However, the mechanism underlying this ergogenic effect is less clear. Euglycaemia and oxidation of blood glucose at high rates late in exercise and a decreased rate of muscle glycogen utilisation (i.e. glycogen ‘sparing’) have been proposed as possible mechanisms underlying the ergogenic effect of CHO ingestion. The prevalence of one or the other mechanism depends on factors such as the type and intensity of exercise, amount, type and timing of CHO ingestion, and pre-exercise nutritional and training status of study participants. The type and intensity of exercise and the effect of these on blood glucose, plasma insulin and catecholamine levels, may play a major role in determining the rate of muscle glycogen utilisation when CHO is ingested during exercise. The ingestion of CHO (except fructose) at a rate of >45 g/h, accompanied by a significant increase in plasma insulin levels, could lead to decreased muscle glycogen utilisation (particularly in type I fibres) during exercise. Endurance training and alterations in pre-exercise muscle glycogen levels do not seem to affect exogenous glucose oxidation during submaximal exercise. Thus, at least during low intensity or intermittent exercise, CHO ingestion could result in reduced muscle glycogen utilisation in well trained individuals with high resting muscle glycogen levels. Further research needs to concentrate on factors that regulate glucose uptake and energy metabolism in different types of muscle fibres during exercise with and without CHO ingestion.
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G*Power (Erdfelder, Faul, & Buchner, 1996) was designed as a general stand-alone power analysis program for statistical tests commonly used in social and behavioral research. G*Power 3 is a major extension of, and improvement over, the previous versions. It runs on widely used computer platforms (i.e., Windows XP, Windows Vista, and Mac OS X 10.4) and covers many different statistical tests of the t, F, and chi2 test families. In addition, it includes power analyses for z tests and some exact tests. G*Power 3 provides improved effect size calculators and graphic options, supports both distribution-based and design-based input modes, and offers all types of power analyses in which users might be interested. Like its predecessors, G*Power 3 is free.