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Does Cardio After an
Overnight Fast Maximize
Fat Loss?
Brad Schoenfeld, MS, CSCS
Global Fitness Services, Scarsdale, New York
Acommon fat burning strategy
employed by bodybuilders, ath-
letes, and fitness enthusiasts is to
perform cardiovascular exercise early in
the morning on an empty stomach. This
strategy was popularized by Bill Phillips
in his book, ‘‘Body for Life’’ (23).
According to Phillips, performing 20
minutes of intense aerobic exercise after
an overnight fast has greater effects on
fat loss than performing an entire hour
of cardio in the postprandial state. The
rationale for the theory is that low
glycogen levels cause your body to shift
energy utilization away from carbohy-
drates, thereby allowing greater mobili-
zation of stored fat for fuel. However,
although the prospect of reducing the
body fat by training in a fasted state may
sound enticing, science does not support
its efficacy.
First and foremost, it is shortsighted to
look solely at how much fat is burned
during an exercise session. The human
body is very dynamic and continually
adjusts its use of fat for fuel. Substrate
utilization is governed by a host of
factors (i.e., hormonal secretions, en-
zyme activity, transcription factors,
etc), and these factors can change by
the moment (27). Thus, fat burning
must be considered over the course of
days—not on an hour-to-hour basis—to
get a meaningful perspective on its
impact on body composition (13). As
a general rule, if you burn more
carbohydrate during a workout, you
inevitably burn more fat in the post-
exercise period and vice versa.
It should be noted that high-intensity
interval training (HIIT) has proven to
be a superior method for maximizing
fat loss compared with a moderate-
intensity steady-state training
(10,26,29). Interestingly, studies show
that blood flow to adipose tissue
diminishes at higher levels of in-
tensity (24). This is believed to entrap
free fatty acids within fat cells,
impeding their ability to be oxidized
while training. Yet, despite lower fat
oxidation rates during exercise, fat
loss is nevertheless greater over time
in those who engage in HIIT versus
training in the ‘‘fat burning zone’’
(29), providing further evidence that
24-hour energy balance is the most
important determinant in reducing
body fat.
The concept of performing cardiovas-
cular exercise on an empty stomach to
enhance fat loss is flawed even when
examining its impact on the amount of
fat burned in the exercise session alone.
True, multiple studies show that con-
sumption of carbohydrate before low-
intensity aerobic exercise (up to
approximately 60%
max) in un-
trained subjects reduces the entry of
long-chain fatty acids in the mitochon-
dria, thereby blunting fat oxidation
(1,14,18,28). This is attributed to an
insulin-mediated attenuation of adi-
pose tissue lipolysis, an increased
glycolytic flux, and a decreased expres-
sion of genes involved in fatty acid
transport and oxidation (3,6,15). How-
ever, both training status and aerobic
exercise intensity have been shown to
mitigate the effects of a pre-exercise
meal on fat oxidation (4,5,24). Recent
research has shed light on the com-
plexities of the subject.
Horowitz et al. (14) studied the fat
burning response of 6 moderately trained
individuals in a fed versus fasted state to
different training intensities. Subjects
cycled for 2 hours at varying intensities
on 4 separate occasions. During 2 of the
trials, they consumed a high-glycemic
carbohydrate meal at 30, 60, and
90 minutes of training, once at a low
intensity (25% peak oxygen consump-
tion) and once at a moderate intensity
(68% peak oxygen consumption). During
the other 2 trials, subjects were kept
fat burning; fat oxidation; lipolysis;
aerobic exercise; cardiovascular
exercise; interval training
Copyright ÓNational Strength and Conditioning Association Strength and Conditioning Journal | 23
fasted for 12–14 hours before exercise
and for the duration of training. Results in
the low-intensity trials showed that
although lipolysis was suppressed by
22% in the fed state compared with the
fasted state, fat oxidation remained
similar between groups until 80–90
minutes of cycling. Only after this point
was a greater fat oxidation rate observed
in fasted subjects. Conversely, during
moderate-intensity cycling, fat oxidation
was not different between trials at any
time—this is despite a 20–25% reduction
in lipolysis and plasma Free fatty acid
More recently, Febbraio et al. (9)
evaluated the effect of pre-exercise
and during exercise carbohydrate con-
sumption on fat oxidation. Using
a crossover design, 7 endurance-
trained subjects cycled for 120 minutes
at approximately 63% of peak power
output, followed by a ‘‘performance
cycle’’ where subjects expended 7
kJ/(kg body weight) by pedaling as
fast as possible. Trials were conducted
on 4 separate occasions, with subjects
given (a) a placebo before and during
training, (b) a placebo 30 minutes
before training and then a carbohydrate
beverage every 15 minutes throughout
exercise, (c) a carbohydrate beverage
30 minutes before training and then
a placebo during exercise, or (d)
a carbohydrate beverage both before
and every 15 minutes during exercise.
The study was carried out in a double-
blind fashion with trials performed in
random order. Consistent with previous
research, results showed no evidence of
impaired fat oxidation associated with
consumption of carbohydrate either
before or during exercise.
Taken together, these studies show
that during moderate-to-high intensity
cardiovascular exercise in a fasted
state—and for endurance-trained indi-
viduals regardless of training intensity—
significantly more fat is broken down
than that the body can use for fuel. Free
fatty acids that are not oxidized
ultimately become re-esterified in ad-
ipose tissue, nullifying any lipolytic
benefits afforded by pre-exercise
It should also be noted that consump-
tion of food before training increases
the thermic effect of exercise. Lee et al.
(19) compared the lipolytic effects of
an exercise bout in either a fasted state
or after consumption of a glucose/milk
(GM) beverage. In a crossover design,
4 experimental conditions were stud-
ied: low-intensity long duration exer-
cise with GM, low-intensity long
duration exercise without GM, high-
intensity short duration exercise with
GM, and high-intensity short duration
exercise without GM. Subjects were
10 male college students who per-
formed all 4 exercise bouts in random
order on the same day. Results showed
that ingestion of the GM beverage
resulted in a significantly greater excess
postexercise oxygen consumption
compared with exercise performed in
a fasted state in both high- and low-
intensity bouts. Other studies have pro-
duced similar findings, indicating a clear
thermogenic advantage associated with
pre-exercise food intake (7,11).
The location of adipose tissue mobi-
lized during training must also be taken
into account here. During low-to-
moderate intensity training performed
at a steady state, the contribution of fat
as a fuel source equates to approxi-
mately 40–60% of total energy expen-
diture (30). However, in untrained
subjects, only about 50–70% of this
fat is derived from plasma Free fatty
acids; the balance comes from intra-
muscular triglycerides (IMTG) (30).
IMTG are stored as lipid droplets in
the sarcoplasm near the mitochondria
(2), with the potential to provide
approximately two-thirds the available
energy of muscle glycogen (32). Similar
to muscle glycogen, IMTG can only be
oxidized locally within the muscle. It is
estimated that IMTG stores are ap-
proximately 3 times greater in type I
versus type II muscle fibers (8,21,31),
and lipolysis of these stores are max-
imally stimulated when exercising at
max (24).
The body increases IMTG stores with
consistent endurance training, which
results in a greater IMTG utilization for
more experienced trainees (12,16,22,31).
It is estimated that nonplasma fatty acid
utilization during endurance exercise is
approximately twice that for trained
versus untrained individuals (24,32).
Hurley et al. (17) reported that the
contribution of IMTG stores in trained
individuals equated to approximately
80% of the total body fat utilization
during 120 minutes of moderate-
intensity endurance training.
The important point here is that IMTG
stores have no bearing on health and/or
appearance; it is the subcutaneous fat
stored in adipose tissue that influences
body composition. Consequently, the
actual fat burning effects of any fitness
strategy intended to increase fat oxida-
tion must be taken in the context of
the specific adipose deposits providing
energy during exercise.
Another factor that must be considered
when training in a fasted state is its
impact on proteolysis. Lemon and
Mullin (20) found that nitrogen losses
were more than doubled when training
while glycogen depleted compared
with glycogen loaded. This resulted
in a protein loss estimated at 10.4% of
the total caloric cost of exercise after
1 hour of cycling at 61%
max. This
would suggest that performing cardio-
vascular exercise while fasting might
not be advisable for those seeking to
maximize muscle mass.
Finally, the effect of fasting on energy
levels during exercise ultimately has an
effect on fat burning. Training early in
the morning on an empty stomach
makes it very difficult for an individual
to train at even a moderate level of
intensity. Attempting to engage in
a HIIT style routine in a hypoglycemic
state almost certainly will impair
performance (33). Studies show that
a pre-exercise meal allows an individual
to train more intensely compared with
exercise while fasting (25). The net
result is that a greaternumber of calories
are burned both during and after
physical activity, heightening fat loss.
In conclusion, the literature does not
support the efficacy of training early in
the morning on an empty stomach as
Cardio After an Overnight Fast and Fat Loss
a tactic to reduce body fat. At best, the
net effect on fat loss associated with
such an approach will be no better than
training after meal consumption, and
quite possibly, it would produce in-
ferior results. Moreover, given that
training with depleted glycogen levels
has been shown to increase proteolysis,
the strategy has potential detrimental
effects for those concerned with mus-
cle strength and hypertrophy.
Schoenfeld is
president of Global
Fitness Services.
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Strength and Conditioning Journal | 25
... In addition to the aforementioned benefits, high intensity cardio burns primarily carbohydrate during exercise. It has been suggested that if more carbohydrates are burned during exercise, more fat is burned throughout the rest of the day and vice-versa 121 . Indeed, a study comparing 20 weeks of high intensity interval exercise to low intensity exercise found significantly increased activities of many enzymes involved in fat oxidation and significantly increased fat loss with high-intensity interval training compared with low-intensity endurance training (-14 mm vs. -5 mm on a 6 site skin-fold test, respectively) 122 . ...
... Some studies have found that carbohydrate consumption prior to cardio significantly reduces fat oxidation during exercise [124][125][126] while others have shown that pre-exercise carbohydrate consumption has no significant effect on fat oxidation during exercise 127,128 . However, acute changes in fat oxidation during exercise are not as important as the total fat oxidation over the course of the day and, as previously discussed, if more carbohydrates are oxidized during exercise, more fat is oxidized throughout the course of the day 121,129 . Therefore, consumption of carbohydrates prior to exercise resulting in a decreased fat oxidation during exercise may actually result in increased fat oxidation throughout the day 121 . ...
... However, acute changes in fat oxidation during exercise are not as important as the total fat oxidation over the course of the day and, as previously discussed, if more carbohydrates are oxidized during exercise, more fat is oxidized throughout the course of the day 121,129 . Therefore, consumption of carbohydrates prior to exercise resulting in a decreased fat oxidation during exercise may actually result in increased fat oxidation throughout the day 121 . In support of this contention, a recent study by Paoli et al. 130 demonstrated that respiratory exchange ratio was significantly lower at 12 and 24 hours after fed versus fasted cardio, indicating that consuming a meal prior to exercise results in a prolonged shift toward lipid use following the training bout. ...
Full-text available
The anabolic effect of resistance training can mitigate muscle loss during contest preparation. In reviewing relevant literature, we recommend a periodized approach be utilized. Block and undulating models show promise. Muscle groups should be trained 2 times weekly or more, although high volume training may benefit from higher frequencies to keep volume at any one session from becoming excessive. Low to high (~3--15) repetitions can be utilized but most repetitions should occur in the 6--12 range using 70--80% of 1 repetition maximum. Roughly 40--70 reps per muscle group per session should be performed, however higher volume may be appropriate for advanced bodybuilders. Traditional rest intervals of 1--3 minutes are adequate, but longer intervals can be used. Tempo should allow muscular control of the load;; 1-- 2sec concentric and 2--3sec eccentric tempos. Training to failure should be limited when performing heavy loads on taxing exercises, and primarily relegated to single--joint exercises and higher repetitions. A core of multi--joint exercises with some single--joint exercises to address specific muscle groups as needed should be used, emphasizing full range of motion and proper form. Cardiovascular training can be used to enhance fat loss. Interference with strength training adaptations increases concomitantly with frequency and duration of cardiovascular training. Thus, the lowest frequency and duration possible while achieving sufficient fat loss should be used. Full--body modalities or cycling may reduce interference. High intensities may as well;; however, require more recovery. Fasted cardiovascular training may not have benefits over fed--state and could be detrimental.
... Recently some bodybuilding competitors and fitness enthusiasts, based on anecdotal knowledge, have been using a fat loss approach which consists of performing low intensity aerobic exercise (e.g. walking), for 20 to 60 min, after an overnight fast [5,6]. This strategy is based under the premises that: i) low or moderate intensity exercise uses predominantly fat in relation to the carbohydrates as energy substrate [7,8]; and ii) the fasting condition increases fat oxidation rates [7,8]. ...
... Moreover, exercising muscles use FFA as a source of energy [40]. However, when the availability of FFA in blood exceeds the muscles´ability to capture and oxidize this macronutrient, this surplus turns-over to the adipose tissue and is stored as triglyceride [5]. Even so, in the current study, no differences were found between FAST and FED in the ELP and fat used. ...
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The aim of the present study was to observe whether performing a low intensity endurance exercise following an overnight fasted (FAST) or fed (FED) condition promotes different cardiorespiratory, enzymatic and hormonal responses. Nine male physical active subjects, (age 21.89 ± 2.52 years old, height 175.89 ± 5.16 cm, weight 72.10 ± 4.31 kg, estimated body fat 7.25 ± 2.11%), randomly performed two sessions of 45 minutes’ low intensity exercise (individual ventilator threshold) interspersed by seven days, differentiated only in whether they were provided with a standardized meal or not. The oxygen consumption (VO2) and heart rate (HR) were measured continuously at the 30-min rest, the 45-min during and the 30-min post-exercise. The testosterone (T) and cortisol (C) hormones were measured at rest, immediately post-exercise and 15-min post-exercise. The Glucose (GLU), Free fatty acids (FFA) and enzyme lipase activity (ELP) were measured at rest, 15-min and 30-min exercise, immediately, 15-min and 30-min post-exercise. Significantly lower values were observed in FED compared to FAST with: C (nmol/L) from pre (428.87 ± 120.41; 454.62 ± 148.33, respectively) to immediately post-exercise (285.10 ± 85.86; 465.66 ± 137.70, respectively) and 15-min post-exercise (248.00 ± 87.88; 454.31 ± 112.72, respectively) (p<0.05); and GLU at all times, with an exception at 15-min post-exercise. The testosterone/cortisol ratio (T/C) was significantly higher in the FED compared with FAST from pre (0.05 ± 0.02, 0.05 ± 0.01, respectively) to 15-min post-exercise (0.08 ± 0.03, 0.05 ± 0.02, respectively). No other significant differences were observed between conditions. We conclude that fasting prior to low intensity endurance exercise does not seem be advantageous, when it comes to fat loss, compared with the same exercise performed after a meal.
... Schoenfeld et al., verificou que o treino da força praticado em jejum (pós-jejum noturno) pode não ser vantajoso para a perda de gordura e pode mesmo ser prejudicial no que concerne o aumento da massa muscular devido a um potencial aumento da proteólise miocitária (24). ...
The scientific community currently expresses a high level of interest in intermittent fasting - periods of voluntary abstinence from energy intake, ranging from several hours to days. Intermittent fasting is clinically relevant and may represent an effective non- pharmacological strategy to improve physical performance and body composition. It has been studied mainly in athletes during the religious period of Ramadan and in people predisposed to decrease body fat without loss of fat-free mass parallel. The purpose of this review is to provide an overview of the impact of intermittent fasting during Ramadan vs. non-Ramadan intermittent fasting in terms of physical performance and body composition. The literature shows some inconsistencies in terms of the interaction between intermittent fasting and physical performance. However, non-Ramadan intermittent fasting is found to be effective in improving maximal aerobic power. Nevertheless, this intervention reduces performance during the repeated sprints over the first few days of intervention. On the other hand, intermittent fasting during Ramadan being the maximum aerobic power and this is more expressive during the second half of this religious period. However, both interventions are manifestly innocuous in terms of muscle strength and anaerobic capacity. With regard to body composition, there is greater consensus. According to available data, both interventions encourage beneficial adaptations at this level. Still, fat loss is more pronounced with intermittent non-Ramadan fasting.
... constant daily CR) in decreasing adiposity, while maintaining muscle mass and strength, are scarce and conflicting. Schoenfeld contended that exercise while fasting (e.g., after an overnight fast) is not more effective in reducing adiposity than exercising in a fed state, and may possibly be detrimental for muscle and strength gains due to the potentially increased proteolysis [23]. However, three recent randomized controlled trials [2,6,24], including participants undergoing 8 weeks of resistance training combined with either a normal diet or a TRF protocol, indicate that this approach may be beneficial for improving body composition (i.e., preserved fat free mass and reduced fat mass) while exerting no detrimental effects on muscle strength. ...
Full-text available
Intermittent fasting (IF) has been studied in athletes during Ramadan and in those willing to decrease adiposity while maintaining or increasing lean body mass. The purpose of this systematic review was to summarize the effects of IF on performance outcomes. We searched peer-reviewed articles in the following databases: PubMed, Web of Science and Sport Discus (up to December 2019). Studies were selected if they included samples of adults (≥18 years), had an experimental or observational design, investigated IF (Ramadan and time-restricted feeding (TRF)), and included performance outcomes. Meta-analytical procedures were conducted when feasible. Twenty-eight articles met the eligibility criteria. Findings indicated that maximum oxygen uptake is significantly enhanced with TRF protocols (SMD = 1.32, p = 0.001), but reduced with Ramadan intermittent fasting (Ramadan IF; SMD = −2.20, p < 0.001). Additional effects of IF may be observed in body composition (body mass and fat mass). Non-significant effects were observed for muscle strength and anaerobic capacity. While Ramadan IF may lead to impairments in aerobic capacity, TRF may be effective for improving it. As there are few studies per performance outcome, more research is needed to move the field forward.
... Postula-se que a prática de exercícios aeróbios após uma noite de jejum acelera a perda de gordura corporal. De acordo com Phillips, a realização de 20 minutos de exercício aeróbio de alta intensidade, em jejum, leva a utilização de gordura maior do que uma hora de exercício aeróbio em estado pós-prandial 15 . Entretanto, isto não foi comprovado em estudo realizado em mulheres com sobrepeso e submetidas a 20 minutos de exercício intenso, 3 vezes por semana, durante 6 semanas. ...
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Fasting has been practiced for millennia, but only recently we began to understand its physiological effects and why, in fact, this practice can be beneficial in certain situations. Furthermore, it appears to have medical applications, in some cases, as effective as medicines. Studies also suggest that fasting can be an effective strategy for reducing body weight, prevent aging, and improving health and sports performance. The purpose of this article is to review the scientific literature on relevant topics to physiological adaptations of fasting, with emphasis on the sporting context. Both in health and sporting area, specifically related to endurance training responses, the results are promising. The regular practice of fasting seems to act positively on the central nervous system and could induce cognitive benefits and prevent some degenerative diseases. Moreover, energy stress, through exercise while fasting, enhances molecular responses to endurance exercise. On the other hand, although controversial, the evidence does not support the use of this strategy aiming at improving body composition. Further studies are necessary in order to point out safe clinical procedures for their use.
... Heart rate monitors (model F7U, Polar Electro Inc, Lake Success, NY) were used to ensure that exercise remained at the appropriate intensity. A low-to-moderate training intensity was used because it has been shown to maximize lipid oxidation during fasted aerobic exercise as compared to higher-training intensities [19]. All training sessions were supervised by research assistants who were upper level undergraduate students in exercise science. ...
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It has been hypothesized that performing aerobic exercise after an overnight fast accelerates the loss of body fat. The purpose of this study was to investigate changes in fat mass and fat-free mass following four weeks of volume-equated fasted versus fed aerobic exercise in young women adhering to a hypocaloric diet. Twenty healthy young female volunteers were randomly assigned to 1 of 2 experimental groups: a fasted training (FASTED) group that performed exercise after an overnight fast (n = 10) or a post-prandial training (FED) group that consumed a meal prior to exercise (n = 10). Training consisted of 1 hour of steady-state aerobic exercise performed 3 days per week. Subjects were provided with customized dietary plans designed to induce a caloric deficit. Nutritional counseling was provided throughout the study period to help ensure dietary adherence and self-reported food intake was monitored on a regular basis. A meal replacement shake was provided either immediately prior to exercise for the FED group or immediately following exercise for the FASTED group, with this nutritional provision carried out under the supervision of a research assistant. Both groups showed a significant loss of weight (P = 0.0005) and fat mass (P = 0.02) from baseline, but no significant between-group differences were noted in any outcome measure. These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.
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To determine liver enzymes and lipids profile level after daily useof (30g) flaxseeds for 12 weeks as supplement with exercise (cardio-exercise) and diet or without them in 100 moderate obese men. The study applied on 100 men, who suffering from mild and moderate obesity, their weight 104±6.23 kg, BMI 34.26 ±3.67kg/m 2 , age 33.5±6.1. They were divided in to 5 groups: group 1 (18) men: they are not training, not having flaxseed; group 2(21) men: they are training, not having flaxseeds; group 3(16) men: they are training, taking flaxseeds; group 4(23) men: they are not training, taking flaxseeds; group 5 (22) men: they are training, taking flaxseeds. The first four groups are under diet controlwhile the last group werewithout diet control. All groups were compared with the control group (16) healthy men who are not training, not taking any supplementand not following any diet program. ALT, AST, total cholesterol (TC), triglycerides (TG), high density lipoprotein (HDL) and low density lipoprotein (LDL) were measured in all group membersbefore and after the study period. From the static analysis of study results, it was noticed that there was a decrease in level of AST, ALT, cholesterol, TG and LDL in all five group,especially in group 3 and 5. Whereas HDL level more increased in group 3 and 5 in spite of its raising in the other 3 groups. One of the most important risks of obesity is elevated liver enzymes and lipids profile that cause problems and other diseases. The use of flaxseeds with cardio exercise led to a decrease in the level of liver enzymes as well as cholesterol, TG and LDL and elevated level of HDL. Therefore, we must payattention to weight gain, which mostly ends in obesity by arranging the number of meals in a day, andavoid eating fatty food as well as fast food. In addition, the increase in weight can be reduced by focusing on the contents of the daily diet followed,increase physical activity and using flaxseeds in diets to reduce excess weight and avoid the incidence of chronic diseases.
Physical activity has multiple health bene􏰀its and is a critical component in managing overweight and obesity. However, one of the main barriers to achieving regular physical activity in overweight today is lack of time. High- Intensity Interval Training (HIIT) has bridged both these issues systemati- cally. Twenty-four men and women (age 18-60 years) volunteered to partici- pate in 6 weeks of modi􏰀ied HIIT exercises program where whole-body func- tional training exercises was provided. Their body weight, body mass index, waist to hip ratio and skinfold fat were measured at the beginning and the end of the six weeks duration. Statistical signi􏰀icance was found between the variables at p<0.05 The results showed that after modi􏰀ied HIIT exercise inter- ventions, there is a considerable decrease in the level of adiposity up to 77.8%. Obesity and overweight have become complex pandemic disorders wherein physical inactivity and lack of time to exercise plays a signi􏰀icant role leading to various complications. A decrease in adiposity through structured exer- cises protocols will enhance a better lifestyle and thereby increases the overall metabolism of the body. Newer interventions such as HIIT exercises serve as the perfect pathway to address the time factor and enhancement of physical activity as well.
This review aimed to verify the effect of exercise and meal timing on energy metabolism. Many people are exercising and playing sports in their own spare time. Although guidelines for daily exercise for healthy life suggest indications of intensity and frequency, there is no instruction about when exercise should be performed. However, there are some diurnal variations in energy metabolism responses to exercise and food intake. In addition, exercise performed before meals and vice versa are different stimuli to whole body energy metabolism, respectively. Further research is required to optimize that translating the results in laboratory to real life, because with growing diversity in lifestyle.
Athletes who are properly fueled and hydrated before, during, and after exercise can improve training, increase performance, and decrease fatigue. Many commercial products are marketed to athletes and are designed to be consumed at specific periods: before, during, and after exercise. Are these products really necessary? This article reviews the research on nutrition support for the athlete surrounding exercise and discusses the different nutritional needs of the recreational athlete versus the elite athlete. Guidelines for macronutrient intake before, during, and after exercise are provided along with decision trees to help the practitioner guide the athlete to proper fueling strategies.
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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 μU/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 μmol ⋅ kg-1⋅ min-1and fat oxidation was 6.1 ± 0.2, 4.2 ± 0.5, and 3.1 ± 0.3 μmol ⋅ kg-1⋅ min-1during 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 μmol ⋅ kg-1⋅ 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.
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This study investigated the effect of varying exercise intensity on the thermic effect of food (TEF). Sixteen lean male subjects were matched for and randomly assigned to either a high or low intensity group for 30 min of treadmill exercise. Caloric expenditure was measured using indirect calorimetry at rest and at 30-min intervals OYer 3 hrs following each of three conditions: a 750-kcal liquid meal, high or low intensity exercise, and a 750-kcal liquid meal followed by high or low intensity exercise. Low intensity exercise enhanced the TEF during recovery at 60 and 90 min while high intensity enhanced it only at 180 min but depressed it at 30 min. Total metabolic expense for a 3-hr postmeal period was not differently affected by the two exercise intensities. Exercise following a meal had a synergistic effect on metabolism; however, this effect was delayed until 180 min postmeal when exercise intensity was high. The circulatory demands of high intensity exercise may have initially blunted the TEF, but ultimately the TEF measured over the 3-hr period was at least equal to that experienced following low intensity exercise.
Muscle biopsy samples were obtained from healthy subjects in order to evaluate quantitative differences in single fibres of substrate (glycogen and triglyceride) and ion concentrations (Na+ and K+) as well as enzyme activity levels (succinate-dehydrogenase, SDH; phosphofructokinase, PFK; 3-hydroxyacyl-CoA-dehydrogenase, HAD; myosin ATPase) between human skeletal muscle fibre types. After freeze drying of the muscle specimen fragments of single fibres were dissected out and stained for myofibrillar-ATPase with preincubations at pH's of 10.3, 4.6, 4.35. Type I ("red") and II A,B, and C ("white") fibres could then be identified. Glycogen content was the same in different fibres, whereas triglyceride content was highest in Type I fibres (2-3 X Type II). No significant differences were observed for Na+ and K+ between fibre types. The activity for the enzymes studied were quite different in the fibre types (SDH and HAD, Type I is approximately 1.5 X Type II; PFK Type I is approximately 0.5 X Type II, Myosin ATPase Type I is approxiamtely 0.4 X Type II). The subgroups of Type II fibres were distinguished by differences in both SDH and PFK activities (SDH, Type II C is greater than A is greater than B; PFK, Type II B is greater than A is approximately C). It is concluded that contractile and metabolic characteristics of human skeletal fibres are very similar to many other species. One difference, however, appears to be than no Type II fibres have an oxidative potential higher than Type I fibres.
Healthy subjects were studied at rest and during 4 h of exercise at approximately 30% of maximal oxygen uptake. At 90 min of exercise 200 g glucose were ingested. A control group was studied during prolonged exercise without glucose administration. Glucose ingestion was followed by a 35% rise in arterial glucose, a 60-70% fall in arterial FFA and glycerol and a two- to threefold rise in arterial insulin. Plasma glucagon, which rose fourfold in controls, failed to rise in the glucose-fed subjects. Glucose uptake by the exercising legs was twofold greater than in controls, accounting for 60% of leg oxygen consumption. Splanchnic glucose output rose rapidly after glucose ingestion to values twice those observed in controls. However, splanchnic uptake of gluconeogenic precursors (lactate, pyruvate and glycerol) fell by 70-100%. Total splanchnic glucose escape after glucose ingestion was 84 +/- 5 g representing 42% of the ingested load. It is concluded that glucose ingestion during prolonged exercise results in a) augmented uptake and oxidation of glucose by the exercising legs, b) diminished lipolysis, c) augmented splanchnic glucose escape in association with decreased hepatic gluconeogenesis, d) retention of half of the ingested glucose within the splanchnic bed, and e) reversal of exercise-induced stimulation of glucagon secretion.
This study investigated the effect of varying exercise intensity on the thermic effect of food (TEF). Sixteen lean male subjects were matched for VO2 max and randomly assigned to either a high or low intensity group for 30 min of treadmill exercise. Caloric expenditure was measured using indirect calorimetry at rest and at 30-min intervals over 3 hrs following each of three conditions: a 750-kcal liquid meal, high or low intensity exercise, and a 750-kcal liquid meal followed by high or low intensity exercise. Low intensity exercise enhanced the TEF during recovery at 60 and 90 min while high intensity enhanced it only at 180 min but depressed it at 30 min. Total metabolic expense for a 3-hr postmeal period was not differently affected by the two exercise intensities. Exercise following a meal had a synergistic effect on metabolism; however, this effect was delayed until 180 min postmeal when exercise intensity was high. The circulatory demands of high intensity exercise may have initially blunted the TEF, but ultimately the TEF measured over the 3-hr period was at least equal to that experienced following low intensity exercise.
This study examined the effects of no carbohydrate (PP), preexercise carbohydrate feeding (CP), carbohydrate feedings during exercise (PC), and the combination of carbohydrate feedings before and during exercise (CC) on the metabolic responses during exercise and on exercise performance. Nine well-trained cyclists exercised at 70% of maximal O2 uptake until exhaustion. Blood glucose peaked 30 min after the preexercise carbohydrate feeding and at the start of exercise was 25% below the prefeeding concentration (4.76 mM). At exhaustion, glucose had declined to 3.8 (PP), 4.0 (CP), 4.6 (PC), and 5.0 mM (CC). Insulin was 300% above basal (7 microU/ml) at the start of exercise for CC and CP and returned to baseline by 120 min of exercise. When carbohydrates were consumed, the rate of carbohydrate oxidation was significantly higher throughout exercise than during PP. Total work produced during exercise was 19-46% (P less than 0.05) higher when carbohydrates were consumed. Time to exhaustion was 44% (CC), 32% (PC), and 18% (CP) greater than PP (201 min; P less than 0.05). Performance was improved by ingestion of carbohydrates before and/or during exercise; performance was further improved by their combination. This is probably the result of enhanced carbohydrate oxidation, especially during the later stages of exercise.
The thermogenic effects of pre- and postprandial exercise was examined in seven lean active females. Energy expenditure was measured for 3 h via open circuit indirect calorimetry after four separate treatments: Exercise Only (25 min treadmill run at 60% VO2 max), Meal Only (910 kcal mixed meal), Exercise-Meal and Meal-Exercise. The thermogenic response to the Exercise-Meal treatment was similar to the Meal Only treatment. However, the Meal-Exercise treatment resulted in a greater energy expenditure than the Meal Only and Exercise-Meal treatments. The Exercise Only treatment showed the lowest thermogenic response. These data suggest that exercise following a meal would be more beneficial than exercise before a meal in increasing and maintaining an elevated energy expenditure.
The respiratory exchange ratio (RER) is lower during exercise of the same intensity in the trained compared with the untrained state, even though plasma free fatty acids (FFA) and glycerol levels are lower, suggesting reduced availability of plasma FFA. In this context, we evaluated the possibility that lipolysis of muscle triglycerides might be higher in the trained state. Nine adult male subjects performed a prolonged bout of exercise of the same absolute intensity before and after adapting to a strenuous 12-wk program of endurance exercise. The exercise test required 64% of maximum O2 uptake before training. Plasma FFA and glycerol concentrations and RER during the exercise test were lower in the trained than in the untrained state. The proportion of the caloric expenditure derived from fat, calculated from the RER, during the exercise test increased from 35% before training to 57% after training. Muscle glycogen utilization was 41% lower, whereas the decrease in quadriceps muscle triglyceride concentration was roughly twice as great (12.7 +/- 5.5 vs. 26.1 +/- 9.3 mmol/kg dry wt, P less than 0.001) in the trained state. These results suggest that the greater utilization of FFA in the trained state is fueled by increased lipolysis of muscle triglyceride.