Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability.

PURPOSE: Five days of a high-fat diet produce metabolic adaptations that increase the rate of fat oxidation during prolonged exercise. We investigated whether enhanced rates of fat oxidation during submaximal exercise after 5 d of a high-fat diet would persist in the face of increased carbohydrate (CHO) availability before and during exercise. METHODS: Eight well-trained subjects consumed either a high-CHO (9.3 g x kg(-1) x d(-1) CHO, 1.1 g x kg(-1) x d(-1) fat; HCHO) or an isoenergetic high-fat diet (2.5 g x kg(-1) x d(-1) CHO, 4.3 g x kg(-1) x d(-1) fat; FAT-adapt) for 5 d followed by a high-CHO diet and rest on day 6. On day 7, performance testing (2 h steady-state (SS) cycling at 70% peak O(2) uptake [VO(2peak)] + time trial [TT]) of 7 kJ x kg(-1)) was undertaken after a CHO breakfast (CHO 2 g x kg(-1)) and intake of CHO during cycling (0.8 g x kg(-1) x h(-1)). RESULTS: FAT-adapt reduced respiratory exchange ratio (RER) values before and during cycling at 70% VO(2peak); RER was restored by 1 d CHO and CHO intake during cycling (0.90 +/- 0.01, 0.80 +/- 0.01, 0.91 +/- 0.01, for days 1, 6, and 7, respectively). RER values were higher with HCHO (0.90 +/- 0.01, 0.88 +/- 0.01 (HCHO > FAT-adapt, P < 0.05), 0.95 +/- 0.01 (HCHO > FAT-adapt, P < 0.05)). On day 7, fat oxidation remained elevated (73 +/- 4 g vs 45 +/- 3 g, P < 0.05), whereas CHO oxidation was reduced (354 +/- 11 g vs 419 +/- 13 g, P < 0.05) throughout SS in FAT-adapt versus HCHO. TT performance was similar for both trials (25.53 +/- 0.67 min vs 25.45 +/- 0.96 min, NS). CONCLUSION: Adaptations to a short-term high-fat diet persisted in the face of high CHO availability before and during exercise, but failed to confer a performance advantage during a TT lasting approximately 25 min undertaken after 2 h of submaximal cycling.     

The role of dietary macronutrients in optimizing endurance performance

We have evaluated the evidence underlying certain key nutritional recommendations for endurance sporting activities. A relatively high daily carbohydrate (CHO) intake (> 6 g/kg/d) and CHO ingestion (30-60 g/h) during exercise appears to delay the onset of fatigue. However, the mechanisms of this effect are governed in part by an attenuation of effort perception, rather than solely as a consequence of delaying an impending energy crisis. CHO loading and ingestion also impart some neuroprotection from fatigue during prolonged exercise, as well as high-intensity, intermittent exercise. In addition, the individual response to short-term (5- to 6-day) ingestion of a high-fat diet (> 60% of calories from fat) followed by 1-day CHO loading seems to vary widely, and is most likely to be of benefit only during ultra-endurance activities. Furthermore, CHO and protein intake in the postexercise period aid in protein synthesis and restoration of muscle glycogen stores. However, athletes and their advisors must be prepared to try various nutritional strategies in order to optimize both performance and training.     

Effect of short-term fat adaptation on high-intensity training

PURPOSE: To determine the effect of short-term (3-d) fat adaptation on high-intensity exercise training in seven competitive endurance athletes (maximal O2 uptake 5.0 +/- 0.5 L x min(-1), mean +/-SD). METHODS: Subjects consumed a standardized diet on d-0 then, in a randomized cross-over design, either 3-d of high-CHO (11 g x kg(-1)d(-1) CHO, 1 g x kg(-1) x d(-1) fat; HICHO) or an isoenergetic high-fat (2.6 g CHO x kg(-1) x d(-1), 4.6 g FAT x kg(-1) x d(-1); HIFAT) diet separated by an 18-d wash out. On the 1st (d-1) and 4th (d-4) day of each treatment, subjects completed a standardized laboratory training session consisting of a 20-min warm-up at 65% of VO2peak (232 +/- 23W) immediately followed by 8 x 5 min work bouts at 86 +/- 2% of VO2peak (323 +/- 32 W) with 60-s recovery. RESULTS: Respiratory exchange ratio (mean for bouts 1, 4, and 8) was similar on d-1 for HIFAT and HICHO (0.91 +/- 0.04 vs 0.92 +/- 0.03) and on d-4 after HICHO (0.92 +/- 0.03) but fell to 0.85 +/- 0.03 (P < 0.05) on d-4 after HIFAT. Accordingly, the rate of fat oxidation increased from 31 +/- 13 on d-1 to 61 +/- 25 micromol x kg(-1) x min(-1) on d-4 after HIFAT (P < 0.05). Blood lactate concentration was similar on d-1 and d-4 of HICHO and on d-1 of HIFAT (3.5 +/- 0.9 and 3.2 +/- 1.0 vs 3.7 +/- 1.2 mM) but declined to 2.4 +/- 0.5 mM on d-4 after HIFAT (P < 0.05). Ratings of perception of effort (legs) were similar on d-1 for HIFAT and HICHO (14.8 +/- 1.5 vs 14.1 +/- 1.4) and on d-4 after HICHO (13.8 +/- 1.8) but increased to 16.0 +/- 1.3 on d-4 after HIFAT (P < 0.05). CONCLUSIONS: 1) competitive endurance athletes can perform intense interval training during 3-d exposure to a high-fat diet, 2) such exercise elicited high rates of fat oxidation, but 3) compared with a high-carbohydrate diet, training sessions were associated with increased ratings of perceived exertion.     

Effects of acute prednisolone intake on substrate utilization during submaximal exercise

We examined the hypothesis that acute therapeutic glucocorticoid intake could change the contribution of fat and carbohydrate (CHO) in energy production during exercise. Nine healthy recreationally-trained male subjects twice performed submaximal exercise (60 min at 60 % VO2max) after ingestion of placebo (Pla) or 20 mg of prednisolone (Pred), according to a double blind and randomized protocol. Respiratory exchange was monitored during exercise and blood samples were collected at rest, every 10 min during exercise and after 5, 10, and 20 min of passive recovery. Pred intake significantly increased total energy expenditure during exercise, but CHO oxidation was lower and fat oxidation higher after Pred vs. Pla. ACTH and IL-6 concentrations were significantly decreased with Pred during exercise, whereas no variations were found in GH, insulin, blood glucose, and lactate between the 2 treatments. In conclusion, it appears that acute prednisolone systemic administration does reduce total carbohydrate oxidation during submaximal exercise. Further studies are necessary to clarify the mechanisms involved and to determine whether this modification in the substrate oxidation balance under glucocorticoid administration in recreationally-trained male subjects could result in a competitive advantage in elite athletes.     

Symposium: Limits to fat oxidation by skeletal muscle during exercise--introduction

Lipids, in the form of adipose tissue triacylglycerol (TG), intramuscular triglyceride (IMTG), and dietary-derived fatty acids (FA) from plasma TG (chylomicrons), and very low-density lipoproteins (VLDL), represent the largest store of nutrient energy in humans. Yet despite the abundance of endogenous TG, there is limited capacity for FA oxidation during exercise: there are no mechanisms that match the availability and metabolism of FA to the rate of energy expenditure. Because of the body's limited carbohydrate (CHO) stores, and because depletion of muscle and liver glycogen reserves often coincide with exhaustion, there is interest in several nutritional interventions that increase FA availability and rates of fat oxidation during exercise: such strategies have the potential to slow the rate of glycogen utilization and delay the onset of fatigue. The five papers comprising this symposium provide a synopsis of 1) the regulation of fat oxidation in human skeletal muscle during aerobic exercise; 2) selected nutritional techniques that increase fat oxidation, spare endogenous CHO stores, and modify exercise capacity; and 3) dietary manipulations that alter macronutrient availability and muscle gene expression.     

Effects of dietary fat on muscle substrates,metabolism, and performance in athletes

INTRODUCTION: The present investigation aimed at identifying differences in muscle structural composition, substrate selection, and performance capacity in highly trained endurance athletes as a consequence of consuming a high-fat or a low-fat diet. METHODS: Eleven duathletes ingested high-fat (53% fat; HF) or high-carbohydrate diets (17% fat; LF) for 5 wk in a randomized crossover design. RESULTS: In m. vastus lateralis, oxidative capacity estimated as volume of mitochondria per volume of muscle fiber (HF: 9.86 +/- 0.36 vs LF: 9.79 +/- 0.52%, mean +/- SE) was not different after the two diet periods. Intramyocellular lipid (IMCL) was significantly increased after HF compared with LF (1.54 +/- 0.27% vs 0.69 +/- 0.09%, P = 0.0076). Glycogen content was lower after HF than after LF, but this difference was not statistically significant (487.8 +/- 38.2 vs 534.4 +/- 32.6 mmol x kg-1 dry weight, P = 0.2454). Maximal power and [OV0312]O(2max) (63.6 +/- 0.9 vs 63.9 +/- 1.2 mL O(2) x min-1 x kg-1 on HF and LF) during an incremental exercise test to exhaustion were not different between the two diet periods. Total work output during a 20-min all-out time trial (298 +/- 6 vs 297 +/- 7 W) on a bicycle ergometer as well as half-marathon running time (80 min 12 s +/- 86 s vs 80 min 24 s +/- 82 s) were not different between HF and LF. Blood lactate concentrations and respiratory exchange ratios (RER) were significantly lower after HF than after LF at rest and during all submaximal exercise loads. CONCLUSIONS: Muscle glycogen stores were maintained after a 5-wk high-fat diet period whereas IMCL content was more than doubled. Endurance performance capacity was maintained at moderate to high-exercise intensities with a significantly larger contribution of lipids to total energy turnover.     

Increased fat availability enhances the capacity of trained individuals to perform prolonged exercise

METHODS: After a familiarization period, six well-trained males participated in a diet and exercise regimen lasting 9 d and comprising three cycling tests to exhaustion. A work rate was selected during the familiarization period that would result in fatigue after approximately 90-100 min at an ambient temperature of 10 degrees C (i.e., approximately 75% of VO2max). The first exercise test was a depletion trial and was preceded by a period during which the subjects' normal diet was consumed. A prescribed 70% carbohydrate (CHO) diet was then consumed for 3.5 d. After this diet, a second exercise test was performed; one of two isoenergetic experimental meals was consumed 4 h before this test (70% CHO meal, CHO trial; or 90% fat meal, fat trial). The second exercise test was followed by a further 3.5-d period on the high CHO diet. Four hours before the third test, subjects consumed the other meal. Heparin was administered intravenously 30 min (1000 U), 15 min (500 U), and 0 min (500 U) before exercise on the fat trial. Subjects were assigned to the two meals in randomized order. RESULTS: Time to exhaustion increased from 118.2 (12.4) min on the CHO trial to 127.9 (12.1) min on the fat trial (P = 0.001). Although no difference in VO2, RER, HR or RPE was found between trials, there was an earlier reduction in RER and an earlier rise in RPE on the fat trial. No difference in total CHO oxidation was found between trials (383 +/- 70 g on the CHO trial and 362 +/- 59 g on the fat trial). CONCLUSIONS: These results suggest that increasing fat availability immediately before exercise by acute fat feeding and heparin infusion can improve endurance exercise in a cool environment in well-trained individuals. This study was not intended to have immediate application to the sports performance field but rather to contribute to our understanding of the factors that may limit endurance performance. Heparin injection to elevate plasma fatty acid concentration would not represent sound medical practice.     

Long-term fat diet adaptation effects on performance,training capacity,and fat utilization

It is well known that adaptation to a fat-rich carbohydrate-poor diet results in lower resting muscle glycogen content and a higher rate of fat oxidation during exercise when compared with a carbohydrate-rich diet. The net effect of such an adaptation could potentially be a sparing of muscle glycogen, and because muscle glycogen storage is coupled to endurance performance, it is possible that adaptation to a high-fat diet potentially could enhance endurance performance. Therefore, the first issue in this review is to critically evaluate the available evidence for a potential endurance performance enhancement after long-term fat-rich diet adaptation. Attainment of optimal performance is among other factors dependent also on the quality and quantity of the training performed. When exercise intensity is increased, there is an increased need for carbohydrates. On the other hand, consumption of a fat-rich diet decreases the storage of glycogen in both muscle and liver. Therefore, training intensity may be compromised in individuals while consuming a fat-rich diet. During submaximal exercise, fat for oxidation in muscle is recruited from plasma fatty acids, plasma triacylglycerol, and muscle triacylglycerol: the final question addressed in this review is which of these source(s) of fat contributes to the increased oxidation of fat during submaximal exercise after long-term fat diet adaptation.     

Substrate oxidation and GH responses to exercise are independent of menstrual phase and status.

The purpose of this study was to determine the extent to which growth hormone (GH) and energy substrate utilization are influenced by basal sex steroid levels during prolonged submaximal exercise across menstrual phase and status. Also the 17 beta-estradiol (E2) and progesterone responses during prolonged exercise were compared according to menstrual phase and menstrual status. Six amenorrheic (AMc) athletes and seven eumenorrheic (EUc) athletes ran at 60% VO2max for 90 min and serial blood samples were taken at rest, every 10 min throughout exercise, and 5 and 15 min post-exercise. The EUc athletes were tested in the early follicular phase (EF) (days 3-5), the late follicular phase (LF) (days 14-16) and the mid-luteal phase (ML) (days 22-25). The incremental GH response to exercise, measured by area under the curve, was consistent with previous reposts and was not altered according to menstrual phase or status (EF-37.5 +/- 11.5, LF-61.9 +/- 11.5, ML-48.1 +/- 12.8 micrograms.1-1.90 min-1). Furthermore, carbohydrate and fat utilization during exercise were not influenced by basal sex steroid levels associated with menstrual phase or status. The incremental E2 response to exercise in AMc athletes was significantly smaller than seen in EUc athletes (AMc-208.1 +/- 44.0, EF-383.0 +/- 116.4, LF-204.7 +/- 84.1, ML-45.1 +/- 18.4 pmol.1(-1).90 min-1), although the pattern of release is similar between groups. In conclusion, GH levels and substrate utilization are independent of both menstrual phase and status; hence, menstrual phase has no negative ramifications on metabolism during exercise. Amenorrhea does not result in metabolic consequences during prolonged exercise by influencing substrate utilization.     

Effect of exercise intensity on fat utilization in males and females

This study was conducted to assess fat utilization across different exercise intensities and between males and females. Eleven males and 11 females completed a maximal test and four submaximal trials at 40%, 50%, 60%, and 70% VO(2) peak. The VO(2) peak and ventilatory threshold (VT) were assessed during the maximal test. Caloric expenditure (Cal), carbohydrate (COX), and fat oxidation (FOX) were measured during each submaximal trial. Maximal fat oxidation (Fmax) was determined as the intensity where the highest FOX was observed. There was no main effect of intensity on FOX. FOX was higher in (p < 0.05) in women than in men at 40% VO(2)peak. The Fmax occurred most frequently at 60% VO(2)peak and correlated with VT (r = 0.62) and VO(2) peak (r = 0.64). In conclusion, despite the fact that Fmax is most frequently observed at 60% VO(2) peak, rates of fat oxidation remained statistically similar across intensities from 40% to 70% VO(2)peak. Women oxidized more fat than men at 40% VO(2)peak, but this gender difference did not occur at higher intensities. It appears that exercising within the moderate intensity domain will produce similar rates of fat utilization.     

Effects of resistance exercise on lipolysis during subsequent submaximal exercise

PURPOSE: This study examined effects of prior resistance exercise on fat metabolism during subsequent submaximal exercise with different recovery periods between exercise bouts. METHODS: Ten male subjects performed three types of exercise regimens: 1) submaximal endurance exercise only (E), 2) submaximal endurance exercise with prior resistance exercise and 20 min of rest (RE20), and 3) submaximal endurance exercise with prior resistance exercise and 120 min of rest (RE120). Resistance exercise consisted of six exercises, each with three to four sets at 10-repetition maximum. Subjects performed cycle ergometer exercise at 50% of the maximal oxygen uptake for 60 min. RESULTS: Prior resistance exercise caused increases in blood lactate, plasma norepinephrine, serum growth hormone (GH), insulin, and glycerol concentrations (P < 0.01). Before the submaximal exercise, serum free fatty acid (FFA) concentration was higher in the RE120 than in the RE20 and E trials (P < 0.01), although concentrations of plasma norepinephrine, serum GH, insulin, and glycerol were higher in the RE20 than in the RE120 and E trials (P < 0.05). Concentrations of FFA and glycerol during the 60-min submaximal exercise were higher in the RE120 and RE20 trials than in the E trial (P < 0.05). No significant difference was observed in the acetoacetate and 3-hydroxybutyrate responses. In the RE20 trial, fat oxidation throughout the 60-min submaximal exercise (mean value) was greater than in the E trial (P < 0.05), but no significant difference was found between the RE120 and E trials. CONCLUSION: Fat availability during the submaximal exercise was enhanced by prior resistance exercise. However, augmentation of fat oxidation was observed only in the trial with shorter rest between resistance exercise and submaximal exercise bouts (RE20 trial).     

Effects of ingesting a sports bar versus glucose polymer on substrate utilisation and ultra-endurance performance.

The purpose of this study was to determine whether the ingestion of a sports bar (BAR) containing a mixture of fat (7 g), protein (14 ) and carbohydrate (CHO; 19 ) improved ulta-endurance cycling performance compared to when an equicaloric amount of CHO was consumed. On two occasions separated by a minimum of 7 days, six highly trained (peak power output [PPO] 414 +/- 8 W) endurance cyclists rode for 330 min at approximately 50% of PPO (203 +/- 8 W) while ingesting either the BAR or just CHO, before performing a 400 k] time trial as fast as possible. Rates of fat oxidation were significantly greater at the end of the submaximal ride when subjects ingested the BAR compared to CHO (1.09 +/- 0.08 vs 0.73 +/- 0.08g x min(-1); P<0.05), and accordingly total fat oxidation was significantly higher (280 +/- 24 vs 203 +/- 25 g, P < 0.05). However, two subjects failed to complete the time trial after they consumed the BAR during the prolonged, submaximal ride, whereas all subjects managed to finish the time trial when ingesting CHO. In conclusion, ingestion of the sports bar enhanced fat metabolism during prolonged, submaximal exercise, but impaired subsequent high-intensity time-trial performance.     

Exercise and green tea extract stimulate fat oxidation and prevent obesity in mice

INTRODUCTION/PURPOSE: The purpose of the present study was to explore the combined effects of dietary supplementation with green tea extract (GTE) and regular exercise on the development of obesity in high fat-fed C57BL/6J mice. METHODS: Weight and age-matched male mice were divided into 5 groups of 10 mice each. Groups were treated as follows: a low-fat diet and not exercised (LF), a high-fat diet and not exercised (HF), a high-fat diet supplemented with GTE and not exercised (GTE-HF), a high-fat diet and exercised regularly (EX-HF), or a high-fat diet supplemented with GTE and exercised regularly (GTEEX-HF). The exercise modality was treadmill running. RESULTS: After 15 wk, GTE alone and regular exercise alone caused a 47 and 24% reduction in body weight gain induced by the high-fat diet, respectively, and when combined, resulted in an 89% reduction. In visceral fat accumulation, GTE alone, exercise alone, and their combination caused a 58, 37, and 87% reduction, respectively. Indirect calorimetry showed that the GTEEX-HF group had the highest energy expenditure and fat utilization in the sedentary condition after 4 wk. Furthermore, the GTEEX-HF group utilized more fat than the EX-HF group during exercise. GTE supplementation increased hepatic fatty acid oxidation both in the exercised and nonexercised groups. In addition, when combined with regular exercise, GTE supplementation also stimulated skeletal muscle fatty acid oxidation. CONCLUSION: In conclusion, dietary GTE and regular exercise, if combined, stimulate fat catabolism not only in the liver but also in skeletal muscle, and attenuate high-fat diet-induced obesity more effectively than each alone in C57BL/6J mice.     

Comparison of sympatho-adrenal activity during endurance exercise performed under high- and low-carbohydrate diet conditions.

Effects of varied carbohydrate (CHO) content in the diet on sympatho-adrenal activity to endurance exercise during which blood sugar was kept over a preexercise level were studied in five male physical education students. The CHO loading was used and consisted of a 7-day low CHO diet (30% CHO, 50% fat, 20% protein) followed by a 7-day high CHO diet (70% CHO, 20% fat, 10% protein). The results obtained from the present study were as follows: (1) plasma epinephrine (E) was almost the same between the low and the high CHO diets before and at 30 min of the exercise, while plasma norepinephrine (NE) level at 30 min of the exercise was significantly higher in the low (959 +/- 98 pg/ml) than in the high CHO diet (679 +/- 64 pg/ml) (p less than 0.05); (2) serum free fatty acid (FFA) level was significantly higher in the low than in the high CHO diet before (p less than 0.05) and at 30 min of the exercise (p less than 0.01); (3) a negative correlation was found between muscle glycogen and plasma NE (p less than 0.05). In all the subjects, increase in serum FFA accompanied by increase in plasma NE was detected in the low CHO diet. In conclusion, sympathetic activity to endurance exercise during which blood sugar was kept over a preexercise level was elevated more in the low than in the high CHO diet. It was suggested that the more elevated sympathetic nervous activity would have resulted from glycogen depletion in the working muscle due to the low CHO diet and would have increased FFA mobilization from the adipose tissue.     

Influence of high and low glycemic index meals on endurance running capacity

PURPOSE: The purpose of this study was to examine the effect of high and low glycemic index (GI) carbohydrate (CHO) pre-exercise meals on endurance running capacity. METHODS: Eight active subjects (five male and three female) ran on a treadmill at approximately 70% VO2max to exhaustion on two occasions separated by 7 d. Three hours before the run after an overnight fast, each subject was given in a single-blind, random order, isoenergetic meal of 850+/-21 kcal (mean+/-SEM; 67% carbohydrate, 30% protein, and 3% fat) containing either high (HGI) or low (LGI) GI carbohydrate foods providing 2.0 g CHO.kg(-1) body weight. RESULTS: Ingestion of the HGI meal resulted in a 580% and 330% greater incremental area under the 3-h blood glucose and serum insulin response curves, respectively. Performance times were not different between the HGI and LGI trials (113+/-4 min and 111+/-5 min, respectively). During the first 80 min of exercise in the LGI trial, CHO oxidation was 12% lower and fat oxidation was 118% higher than in the HGI trial. Although serum insulin concentrations did not differ between trials, blood glucose at 20 min into exercise in the HGI trial was lower than that during the LGI trial at the same time (3.6+/-0.3 mmol.L(-1) vs 4.3+/-0.3 mmol.L(-1); P < 0.05). During exercise, plasma glycerol and serum free fatty acid concentrations were lower in the HGI trial than in the LGI trial. CONCLUSIONS: This results demonstrate that although there is a relative shift in substrate utilization from CHO to fat when a low GI meal is ingested before exercise compared with that for a high GI meal, there is no difference in endurance running capacity.     

Regulation of substrate use during the marathon

The energy required to run a marathon is mainly provided through oxidative phosphorylation in the mitochondria of the active muscles. Small amounts of energy from substrate phosphorylation are also required during transitions and short periods when running speed is increased. The three inputs for adenosine triphosphate production in the mitochondria include oxygen, free adenosine diphosphate and inorganic phosphate, and reducing equivalents. The reducing equivalents are derived from the metabolism of fat and carbohydrate (CHO), which are mobilised from intramuscular stores and also delivered from adipose tissue and liver, respectively. The metabolism of fat and CHO is tightly controlled at several regulatory sites during marathon running. Slower, recreational runners run at 60-65% maximal oxygen uptake (VO(2max)) for approximately 3:45:00 and faster athletes run at 70-75% for approximately 2:45:00. Both groups rely heavily on fat and CHO fuels. However, elite athletes run marathons at speeds requiring between 80% and 90% VO(2max), and finish in times between 2:05:00 and 2:20:00. They are highly adapted to oxidise fat and must do so during training. However, they compete at such high running speeds, that CHO oxidation (also highly adapted) may be the exclusive source of energy while racing. Further work with elite athletes is needed to examine this possibility.     

Role of fat metabolism in exercise

Fat and carbohydrate are the two major energy sources used during exercise. Either source can predominate, depending upon the duration and intensity of exercise, degree of prior physical conditioning, and the composition of the diet consumed in the days prior to a bout of exercise. Fatty acid oxidation can contribute 50 to 60 per cent of the energy expenditure during a bout of low intensity exercise of long duration. Strenuous submaximal exercise requiring 65 to 80 per cent of VO2 max will utilize less fat (10 to 45 per cent of the energy expended). Exercise training is accompanied by metabolic adaptations that occur in skeletal muscle and adipose tissue and that facilitate a greater delivery and oxidation of fatty acids during exercise. The trained state is characterized by an increased flux of fatty acids through smaller pools of adipose tissue energy. This is reflected by smaller, more metabolically active adipose cells in smaller adipose tissue depots. Peak blood concentrations of free fatty acids and ketone bodies are lower during and following exercise in trained individuals, probably due to increased capacity of the skeletal musculature to oxidize these energy sources. Trained individuals oxidize more fat and less carbohydrate than untrained subjects when performing submaximal work of the same absolute intensity. This increased capacity to utilize energy from fat conserves crucial muscle and liver glycogen stores and can contribute to increased endurance. Further benefits of the enhanced lipid metabolism accompanying chronic aerobic exercise training are decreased cardiac risk factors. Exercise training results in lower blood cholesterol and triglycerides and increased high density lipoprotein cholesterol. High-fat diets are not recommended because of their association with atherosclerotic heart disease. Recent evidence suggests that low-fat high-carbohydrate diets may increase blood triglycerides and reduce high density lipoproteins. This suggests that the chronic ingestion of diets that are extreme in their composition of either fat or carbohydrate should be approached with caution in health-conscious athletes, as well as in sedentary individuals.     

Carbohydrate intake and recovery from exercise

Prolonged heavy exercise, whether as part or training or competition, can only be continued when there is an adequate amount of carbohydrate available to fuel muscles and the brain. Fatigue is closely associated with depletion of the limited stores of carbohydrate in the muscle and in the liver. Therefore, it is not surprising that strategies have been developed to ensure that not only are the carbohydrate stores well stocked before exercise but that they are also restored as soon as possible after exercise. Consuming carbohydrate immediately after exercise increases the rate of muscle glycogen resynthesis and also results in greater endurance capacity during subsequent exercise. A recovery diet that is high in carbohydrate (~10 g kg^–1 body mass/day) will allow athletes to restore their exercise capacity on the following day, which is not the case when they eat a mixed diet with matching energy content. The type of carbohydrate in the recovery diet also has an influence on endurance capacity the following day. A recovery diet that contains low glycaemic index carbohydrates result in a higher rates of fat oxidation and greater endurance running capacity than diets that are contain mainly high glycaemic index carbohydrate foods. Although consuming carbohydrate–protein mixtures during recovery from exercise increases the insulin response, and possibly glycogen resynthesis rate, there appears to be no greater recovery of endurance capacity than following the consumption of carbohydrate alone.     

Antioxidant restriction and oxidative stress in short-duration exhaustive exercise

PURPOSE: To determine the effect of dietary antioxidant restriction on oxidative stress, antioxidant defenses, and exercise performance in athletes. Oxidative stress has been shown to increase during exercise. To alleviate oxidative stress, a high intake of antioxidant rich foods or supplements may be required in trained athletes. METHODS: Plasma oxidative stress and antioxidant defenses were examined in 17 trained athletes who underwent two separate exercise tests. Before the initial exercise test participants followed their habitual (high) antioxidant (H-AO) diets. Then they followed a 2-wk restricted-antioxidant (R-AO) diet before the second exercise test. Blood was taken at rest, after submaximal and high-intensity exhaustive exercise, and after 1 h of recovery. RESULTS: The R-AO diet induced a threefold reduction in antioxidant intake when compared with habitual-antioxidant (H-AO) diets. F(2)-isoprostane concentration (marker of oxidative stress) was significantly higher after submaximal exercise (38%), exhaustion (45%), and 1 h of recovery (31%) when following the R-AO diet compared with the H-AO diet. Rate of perceived exertion was increased on the R-AO diet whilst exercise time to exhaustion was not affected. Total antioxidant capacity and circulating antioxidant concentrations, although not significantly different, tended to be lower when following the R-AO diet. CONCLUSION: Athletes regularly participating in up to 40 min of acute high-intensity exercise may require higher intakes of exogenous antioxidants to defend against increased oxidative stress during exercise, which can be met through an adequate intake of high-antioxidant foods. Thus, there seems no valid reason to recommend antioxidant supplements to athletes participating in acute high-intensity exercise events up to 40 min in duration, except in those known to be consuming a low-antioxidant diet for prolonged periods.