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Glycemic index testing of cherry juice, a potentially beneficial beverage for endurance athletes
Abstract
Tart cherry consumption may have a favorable impact on exercise performance in endurance athletes through its anti-inflammatory roles and low glycemic index (GI). Low GI foods may be of benefit for endurance athletes because of the slow release of glucose into the blood after digestion. However, no study has measured the GI of tart cherry juice to see if the low GI of tart cherries is affected by processing. The purpose of this study was to evaluate the GI of raw tart cherries and juice from tart cherries to see if the low GI is preserved when raw cherries are processed into juice. Seven participants (6 females, 1 male, 28±5 y, 62.6±13.2 kg, 165±6 cm) visited our lab on four different occasions with each visit separated by a week and preceded by a 10 hour fast. Participants consumed a glucose standard (250 mL water with 25 g glucose) during the first two visits and then 167 g raw cherries+250 mL water or 189 mL of cherry juice+89 mL of water for the last two conditions. Blood glucose was measured at baseline and at 15, 30, 45, 60, 90, and 120 minutes following consumption. Glycemic index was determined by dividing the incremental area under the curve (iAUC) of the blood glucose response of the cherries or juice by the iAUC for the glucose standard. The GI of the raw cherries (46±39) was not significantly different from the juice (45±27), p>0.05, with both being classified as “low” GI (i.e. GI≤55). The GI of raw cherries is low and a low GI can be maintained in cherry juice form. (Supported by the Canadian Cherry Producers Incorporated.)
Objective: Tart cherry concentrate has been shown to improve muscle function, and reduce muscle damage, oxidative stress/inflammation, and muscle soreness in athletes; however, evidence for acute endurance performance benefits is scarce. The purpose of this review was to evaluate the effect of tart cherry juice on endurance exercise performance with a meta-analysis.
Method: Data sources included Medline, Embase, Web of Science, and Google Scholar. Eligibility criteria were randomized controlled trials with endurance exercise performance tests. Participants were healthy individuals. Interventions included tart cherry supplementation and placebo ingested before, and/or on the day of exercise. Ten studies were included (totaling 127 males and 20 females). Standardized mean differences (SMD) with 95% confidence intervals were calculated for each study and pooled effects were assessed.
Results: Tart cherry concentrate in juice or powdered form, ingested for 7 days to 1.5 hours before exercise performance testing significantly improved endurance exercise performance (SMD: 0.36; 95% CI: 0.07 to 0.64; p = 0.01; I² = 0%) upon pooling of the ten studies.
Conclusions: Tart cherry concentrate has a significant benefit for endurance exercise performance.
• Key teaching points
• Tart cherry concentrate has a significant benefit for endurance exercise performance.
• Tart cherry concentrate may enhance endurance exercise performance via its low glycemic index, anti-inflammatory and anti-oxidative capacity, and blood flow enhancing effects.
We compared the effects of a low-glycemic index pulse-based diet, containing lentils, beans, split peas, and chickpeas, to the Therapeutic Lifestyle Changes (TLC) diet on cardio-metabolic measures in women with polycystic ovary syndrome (PCOS). Ninety-five women (18⁻35 years) enrolled in a 16-week intervention; 30 women in the pulse-based and 31 in the TLC groups completed the study. Women participated in aerobic exercise training (minimum 5 days/week for 45 min/day) and were counselled (monthly) about PCOS and lifestyle modification. Women underwent longitudinal follow-up post-intervention. The pulse-based group had a greater reduction in total area under the curve for insulin response to a 75-g oral glucose tolerance test (mean change ± SD: -121.0 ± 229.9 vs. -27.4 ± 110.2 µIU/mL × min; p = 0.05); diastolic blood pressure (-3.6 ± 6.7 vs. -0.2 ± 6.7 mmHg, p = 0.05); triglyceride (-0.2 ± 0.6 vs. 0.0 ± 0.5 mmol/L, p = 0.04); low-density lipoprotein cholesterol (-0.2 ± 0.4 vs. -0.1 ± 0.4 mmol/L, p = 0.05); total cholesterol/high-density lipoprotein cholesterol (TC/HDL-C; -0.4 ± 0.4 vs. 0.1 ± 0.4, p < 0.001); and a greater increase in HDL-C (0.1 ± 0.2 vs. -0.1 ± 0.2 mmol/L, p < 0.01) than the TLC group. Decreased TC/HDL-C (p = 0.02) at six-month and increased HDL-C and decreased TC/HDL-C (p ≤ 0.02) at 12-month post-intervention were maintained in the pulse-based group. A pulse-based diet may be more effective than the TLC diet at improving cardio-metabolic disease risk factors in women with PCOS.
Trial registration:
CinicalTrials.gov identifier, NCT01288638.
Background
The purpose of this study was to determine whether short-term supplementation of a powdered tart cherry supplement prior to and following stressful endurance exercise would affect markers of muscle damage, inflammation, oxidative stress, and/or muscle soreness.
Methods
27 endurance-trained runners or triathlete (21.8 ± 3.9 years, 15.0 ± 6.0 % body fat, 67.4 ± 11.8 kg) men (n = 18) and women (n = 9) were matched based on average reported race pace, age, body mass, and fat free mass. Subjects were randomly assigned to ingest, in a double-blind manner, capsules containing 480 mg of a rice flour placebo (P, n = 16) or powdered tart cherries [CherryPURE®] (TC, n = 11). Subjects supplemented one time daily (480 mg/day) for 10-d, including race day, up to 48-hr post-run. Subjects completed a half-marathon run (21.1 km) under 2-hr (111.98 ± 11.9 min). Fasting blood samples and quadriceps muscle soreness ratings using an algometer with a graphic pain rating scale were taken pre-run, 60-min, 24 and 48-h post-run and analyzed by MANOVA with repeated measures.
Results
Subjects in the TC group averaged 13 % faster half-marathon race finish times (p = 0.001) and tended to have smaller deviations from predicted race pace (p = 0.091) compared to P. Attenuations in TC muscle catabolic markers were reported over time for creatinine (p = 0.047), urea/blood urea nitrogen (p = 0.048), total protein (p = 0.081), and cortisol (p = 0.016) compared to P. Despite lower antioxidant activity pre-run in TC compared to P, changes from pre-run levels revealed a linear increase in antioxidant activity at 24 and 48-h of recovery in TC that was statistically different (16–39 %) from P and pre-run levels. Inflammatory markers were 47 % lower in TC compared to P over time (p = 0.053) coupled with a significant difference between groups (p = 0.017). Soreness perception between the groups was different over time in the medial quadriceps (p = 0.035) with 34 % lower pre-run soreness in TC compared to P. Over the 48-h recovery period, P changes in medial quadriceps soreness from pre-run measures were smaller compared to TC.
Conclusion
Results revealed that short-term supplementation of Montmorency powdered tart cherries surrounding an endurance challenge attenuated markers of muscle catabolism, reduced immune and inflammatory stress, better maintained redox balance, and increased performance in aerobically trained individuals.
We reviewed the effects of glycaemic index (GI) of carbohydrates, consumed before exercise, on metabolism and performance. A carbohydrate with a high GI results in rapid absorption of glucose and a large and fast rise in blood insulin. A carbohydrate with a low GI results in a slower and smaller rise in blood glucose and insulin levels. Increased insulin levels inhibit fat oxidation and promote carbohydrate oxidation, which may result in faster glycogen depletion when high-GI carbohydrates are consumed before exercise. High-GI carbohydrates may also result in rebound hypoglycaemia, which would leave one with lower than normal blood glucose levels and hinder exercise performance. Most studies of low-versus high-GI carbohydrates consumed before exercise indicate that low-GI carbohydrates confer a better metabolic response (i.e. lower insulin release, increased fat oxidation and decreased glucose oxidation compared with high-GI carbohydrates). Despite this improved metabolic response, intake of low- and high-GI carbohydrates result in similar improvements in exercise performance. In the 20 studies we reviewed, low-GI carbohydrates were more beneficial for improvement in performance in seven studies, high-GI carbohydrates were more beneficial in one study, and there was no difference in performance improvement between carbohydrates of differing GI in 12 studies. We conclude that low-GI carbohydrates result in a more favourable metabolic response compared with high-GI carbohydrates when consumed before exercise, but this does not always result in a greater improvement in exercise performance.
The metabolic and performance benefits of prior consumption of low-glycaemic index (GI) meals v. high-GI meals were determined in extended high-intensity intermittent exercise. Participants (ten males and four females, aged 25·8 (sd 7·3) years) completed two testing days (each consisting of back-to-back 90-min intermittent high-intensity treadmill running protocols separated by 3 h) spaced by at least 7 d. Using a randomised counterbalanced cross-over design, low-GI, lentil-based meals (GI about 42) or high-GI, potato-based meals (GI about 78) matched for energy value were consumed 2 h before, and within 1 h after, the first exercise session. Performance was measured by the distance covered during five 1-min sprints (separated by 2·5 min walking) at the end of each exercise session. Peak postprandial blood glucose was higher by 30·8 % in the high-GI trial compared with the low-GI trial, as was insulin (P = 0·039 and P = 0·003, respectively). Carbohydrate oxidation was lower by 5·5 % during the low-GI trials compared with the high-GI trials at the start of the first exercise session (P < 0·05). Blood lactate was significantly higher (6·1 v. 2·6 mmol/l; P = 0·019) and blood glucose significantly lower (4·8 v. 5·4 mmol/l; P = 0·039) at the end of the second exercise session during the high-GI trial compared with the low-GI trial. Sprint distance was not significantly different between conditions. A low-GI meal improved the metabolic profile before and during extended high-intensity intermittent exercise, but did not affect performance. Improvements in metabolic responses when consuming low-GI meals before exercise may be beneficial to the long-term health of athletes.
Dietary non-oil-seed pulses (chickpeas, beans, peas, lentils, etc.) are a good source of slowly digestible carbohydrate, fibre and vegetable protein and a valuable means of lowering the glycaemic-index (GI) of the diet. To assess the evidence that dietary pulses may benefit glycaemic control, we conducted a systematic review and meta-analysis of randomised controlled experimental trials investigating the effect of pulses, alone or as part of low-GI or high-fibre diets, on markers of glycaemic control in people with and without diabetes.
We searched MEDLINE, EMBASE, CINAHL, and the Cochrane Library for relevant controlled trials of >or=7 days. Two independent reviewers (A. Esfahani and J. M. W. Wong) extracted information on study design, participants, treatments and outcomes. Data were pooled using the generic inverse variance method and expressed as standardised mean differences (SMD) with 95% CIs. Heterogeneity was assessed by chi (2) and quantified by I (2). Meta-regression models identified independent predictors of effects.
A total of 41 trials (39 reports) were included. Pulses alone (11 trials) lowered fasting blood glucose (FBG) (-0.82, 95% CI -1.36 to -0.27) and insulin (-0.49, 95% CI -0.93 to -0.04). Pulses in low-GI diets (19 trials) lowered glycosylated blood proteins (GP), measured as HbA(1c) or fructosamine (-0.28, 95% CI -0.42 to -0.14). Finally, pulses in high-fibre diets (11 trials) lowered FBG (-0.32, 95% CI -0.49 to -0.15) and GP (-0.27, 95% CI -0.45 to -0.09). Inter-study heterogeneity was high and unexplained for most outcomes, with benefits modified or predicted by diabetes status, pulse type, dose, physical form, duration of follow-up, study quality, macronutrient profile of background diets, feeding control and design.
Pooled analyses demonstrated that pulses, alone or in low-GI or high-fibre diets, improve markers of longer term glycaemic control in humans, with the extent of the improvements subject to significant inter-study heterogeneity. There is a need for further large, well-designed trials.
The glycemic index (GI) characterizes foods by using the incremental area under the glycemic response curve relative to a similar amount of oral glucose. Its ability to differentiate between curves of different shapes, the peak response, and other aspects of the glycemic response is debatable.
The objective was to explore the association between a food's GI and the shape of the curve in healthy individuals.
A large database of 1,126 foods tested by standardized GI methodology in 8-12 healthy subjects was analyzed systematically. Each food's absolute and incremental blood glucose concentrations were compared at individual time points with the GI. The average curve was generated for low-GI (< or = 55), medium-GI (56-69), and high-GI (> or = 70) foods within major food categories.
The GI of individual foods was found to correlate strongly with the incremental and actual peak (Spearman's correlations of r = 0.76 and r = 0.73, respectively), incremental and actual glucose concentration at 60 min (r = 0.70 and r = 0.66, respectively), and maximum amplitude of glucose excursion (r = 0.68) (all P < 0.001). In contrast, there was only a weak correlation between the food's GI and the 120-min glucose concentration (incremental r = 0.20, P < 0.001; absolute r = 0.16, P < 0.001). Within food groups, the mean GI, 30- and 60-min glucose concentrations, and maximum amplitude of glucose excursion varied significantly for foods classified as having a low, medium, or high GI (P < 0.001).
The GI provides a good summary of postprandial glycemia. It predicts the peak (or near peak) response, the maximum glucose fluctuation, and other attributes of the response curve.
To systematically tabulate published and unpublished sources of reliable glycemic index (GI) values.
A literature search identified 205 articles published between 1981 and 2007. Unpublished data were also included where the data quality could be verified. The data were separated into two lists: the first representing more precise data derived from testing healthy subjects and the second primarily from individuals with impaired glucose metabolism.
The tables, which are available in the online-only appendix, list the GI of over 2,480 individual food items. Dairy products, legumes, and fruits were found to have a low GI. Breads, breakfast cereals, and rice, including whole grain, were available in both high and low GI versions. The correlation coefficient for 20 staple foods tested in both healthy and diabetic subjects was r = 0.94 (P < 0.001).
These tables improve the quality and quantity of GI data available for research and clinical practice.
Context:
: Although pre-exercise consumption of a low-glycemic-index (LGI) carbohydrate meal is generally recommended, the findings regarding subsequent exercise performance are inconsistent. This meta-analytic study was conducted to determine whether a pre-exercise LGI carbohydrate meal leads to greater endurance performance than a pre-exercise high-glycemic-index (HGI) meal. The following electronic databases were searched until April 2016: MEDLINE, SPORTDiscus, Web of Science, and Cochrane Central Register of Controlled Trials. The reference lists of selected articles were searched manually. Randomized controlled or crossover trials comparing the effects of LGI and HGI pre-exercise carbohydrate meals on subsequent exercise performance of healthy participants were included. The Jadad scale was used to assess the methodological quality of the included studies. A fixed-effects model was used to evaluate overall and subgroup estimates. In total, 15 eligible studies from 727 articles were included in this meta-analysis. All included studies were of low research quality. The synthesized effect size ( d = 0.42, z = 3.40, P = 0.001) indicated that the endurance performance following an LGI meal was superior to that following an HGI meal. Subgroup analyses demonstrated that the treatment effect did not vary across outcome measures (exercise to exhaustion, time trial, and work output) or athletic status (trained or recreational participants). Weak evidence supports the claim that endurance performance following a pre-exercise LGI meal is superior to that following a pre-exercise HGI meal. Further high-quality research in this area is warranted.
Many athletes use dietary supplements as part of their regular training or competition routine, including about 85% of elite track and field athletes. Supplements commonly used include vitamins, minerals, protein, creatine, and various "ergogenic" compounds. These supplements are often used without a full understanding or evaluation of the potential benefits and risks associated with their use, and without consultation with a sports nutrition professional. A few supplements may be helpful to athletes in specific circumstances, especially where food intake or food choice is restricted. Vitamin and mineral supplements should be used only when a food-based solution is not available. Sports drinks, energy bars, and protein-carbohydrate shakes may all be useful and convenient at specific times. There are well-documented roles for creatine, caffeine, and alkalinizing agents in enhancing performance in high-intensity exercise, although much of the evidence does not relate to specific athletic events. There are potential costs associated with all dietary supplements, including the risk of a positive doping result as a consequence of the presence of prohibited substances that are not declared on the label.
Consuming carbohydrate-rich meals before continuous endurance exercise improves performance, yet few studies have evaluated the ideal preexercise meal for high-intensity intermittent exercise, which is characteristic of many team sports. The authors' purpose was to investigate the effects of low- and high-glycemic-index (GI) meals on metabolism and performance during high-intensity, intermittent exercise. Sixteen male participants completed three 90-min high-intensity intermittent running trials in a single-blinded random order, separated by ~7 d, while fasted (control) and 2 hr after ingesting an isoenergetic low-GI (lentil), or high-GI (potato and egg white) preexercise meal. Serum free fatty acids were higher and insulin lower throughout exercise in the fasted condition (p < .05), but there were no differences in blood glucose during exercise between conditions. Distance covered on a repeated-sprint test at the end of exercise was significantly greater in the low-GI and high-GI conditions than in the control (p < .05). Rating of perceived exertion was lower in the low-GI condition than in the control (p = .01). In a subsample of 5 participants, muscle glycogen availability was greater in the low-and high-GI conditions versus fasted control before the repeated-sprint test (p < .05), with no differences between low and high GI. When exogenous carbohydrates are not provided during exercise both low- and high-GI preexercise meals improve high-intensity, intermittent exercise performance, probably by increasing the availability of muscle glycogen. However, the GI does not influence markers of substrate oxidation during high-intensity, intermittent exercise.
The glycemic index (GI) of a preexercise meal may affect substrate utilization and performance during continuous exercise.
To examine the effects of low- and high-GI foods on metabolism and performance during high-intensity, intermittent exercise.
Seven male athletes participated in three experimental trials (low-GI, high-GI, and fasted control) separated by approximately 7 days. Foods were consumed 3 h before (approximately 1.3 g x kg(-1) carbohydrate) and halfway through (approximately 0.2 g x kg(-1) carbohydrate) 90 min of intermittent treadmill running designed to simulate the activity pattern of soccer. Expired gas was collected during exercise to estimate substrate oxidation. Performance was assessed by the distance covered on five 1-min sprints during the last 15 min of exercise.
Respiratory exchange ratio was higher and fat oxidation lower during exercise in the high-GI condition compared with fasting (P < .05). The mean difference in total distance covered on the repeated sprint test between low GI and fasting (247 m; 90% confidence limits +/-352 m) represented an 81% (likely, probable) chance that the low-GI condition improved performance over fasting. The mean difference between high GI and fasted control (223 m; +/- 385 m) represented a 76% (likely, probable) chance of improved performance. There were no differences between low and high GI.
When compared with fasting, both low- and high-GI foods consumed 3 h before and halfway through prolonged, high-intensity intermittent exercise improved repeated sprint performance. High-GI foods impaired fat oxidation during exercise but the GI did not appear to influence high-intensity, intermittent exercise performance.
The determine the effect of different foods on the blood glucose, 62 commonly eaten foods and sugars were fed individually to groups of 5 to 10 healthy fasting volunteers. Blood glucose levels were measured over 2 h, and expressed as a percentage of the area under the glucose response curve when the same amount of carbohydrate was taken as glucose. The largest rises were seen with vegetables (70 +/- 5%), followed by breakfast cereals (65 +/- 5%), cereals and biscuits (60 +/- 3%), fruit (50 +/- 5%), dairy products (35 +/- 1%), and dried legumes (31 +/- 3%). A significant negative relationship was seen between fat (p less than 0.01) and protein (p less than 0.001) and postprandial glucose rise but not with fiber or sugar content.
Physical training affects carbohydrate metabolism and results in an increased insulin-stimulated glucose disposal. To investigate if carbohydrate and lipid metabolism would be affected by nutritional factors in optimally trained elite athletes, during a 1-year period we studies elite ice-hockey players on two Swedish top-performance teams. Players on one team were subjected to extensive dietary monitoring and intervention, whereas players on the second team continued their ordinary diet. Blood levels of insulin, C-peptide, glucose, hemoglobin A1C (HbA1c), lipids, and lipoproteins were measured repeatedly. Basal insulin levels and insulin resistance (IR) were significantly lower among athletes on both teams compared with a sedentary group, and muscle weight and body mass index were significantly higher. During the course of the study in the intervention group, insulin levels decreased (3.6 +/- 0.3 v 6.2 +/- 0.6 [mean +/- SEM], P <.05) in conjunction with a decreased relative fat energy content, but returned toward baseline levels when relative fat energy content increased. IR decreased in parallel (0.59 +/- 0.05 v l.12 +/- 0.12, P <.05) and followed a similar pattern, reverting toward baseline levels. Also, levels of HbA1c changed during dietary manipulation. No changes in these parameters were observed among the elite players from the team not participating in the diet regimen. In contrast to the parameters for glucose homeostasis, no significant changes were found in serum lipid or lipoprotein levels in either team during the course of the study. The results verify the presence of an improved carbohydrate metabolism in elite athletes. The observed changes in glycemic control and glucose homeostasis as a consequence of dietary modification demonstrate further that nutritional factors may affect carbohydrate metabolism also in well-trained athletes.
This study determined if the suppression of lipolysis after preexercise carbohydrate ingestion reduces fat oxidation during exercise. Six healthy, active men cycled 60 min at 44 +/- 2% peak oxygen consumption, exactly 1 h after ingesting 0.8 g/kg of glucose (Glc) or fructose (Fru) or after an overnight fast (Fast). The mean plasma insulin concentration during the 50 min before exercise was different among Fast, Fru, and Glc (8 +/- 1, 17 +/- 1, and 38 +/- 5 microU/ml, respectively; P < 0.05). After 25 min of exercise, whole body lipolysis was 6.9 +/- 0.2, 4.3 +/- 0.3, and 3.2 +/- 0.5 micromol x kg(-1) x min(-1) and fat oxidation was 6.1 +/- 0.2, 4.2 +/- 0.5, and 3.1 +/- 0.3 micromol x kg(-1) x min(-1) during Fast, Fru, and Glc, respectively (all P < 0.05). During Fast, fat oxidation was less than lipolysis (P < 0.05), whereas fat oxidation approximately equaled lipolysis during Fru and Glc. In an additional trial, the same subjects ingested glucose (0.8 g/kg) 1 h before exercise and lipolysis was simultaneously increased by infusing Intralipid and heparin throughout the resting and exercise periods (Glc+Lipid). This elevation of lipolysis during Glc+Lipid increased fat oxidation 30% above Glc (4.0 +/- 0.4 vs. 3.1 +/- 0.3 micromol x kg(-1) x min(-1); P < 0.05), confirming that lipolysis limited fat oxidation. In summary, small elevations in plasma insulin before exercise suppressed lipolysis during exercise to the point at which it equaled and appeared to limit fat oxidation.