Influence of carbohydrate ingestion on counterregulatory hormones during prolonged exercise

This study was undertaken to determine the effects of ingesting 5.0 (CHO-5), 6.0 (CHO-6), and 7.5 g/100 ml (CHO-7.5) carbohydrate (CHO) solutions on blood glucose and counterregulatory hormonal responses during prolonged intermittent exercise. Eight well-trained cyclists performed four trials consisting of seven 12-min cycling bouts at 70% of VO2max with 3 min rest between each ride. A final 12 min ride was an all-out self-paced performance ride. During the rest interval the subjects ingested either a water placebo (WP) or one of the CHO solutions at a rate of 8.5 mg/kg/h (approx. 150 ml). Blood samples were taken at 0, 25, 55, 85, and 115 min of exercise and were assayed for glucose, glucagon (GG), cortisol (CT), insulin (IN), epinephrine (EP), and norepinephrine (NE). Blood glucose levels were significantly lower in the WP trial compared to the CHO trials at 25 (4.6 +/- 0.2 vs 5.7 +/- 0.5 mmol/l) and 55 min (4.4 +/- 0.3 vs 5.0 +/- 0.8 mmol/l). At 85 min blood glucose was significantly lower in the WP compared to the CHO-6 and CHO-7.5 trials. GG and IN levels were not significantly different between trials; however, the GG:IN molar ratio was significantly higher in the WP than in the CHO-7.5 trial. CT was significantly elevated in the WP trial compared to the CHO-7.5 trial. EP and NE levels were not affected by CHO ingestion. These data suggest that CHO feedings prevent the typical hormonal responses which are responsible for hepatic glucose release, thus eliciting a possible hepatic glycogen sparing.     

Pre-exercise ingestion of carbohydrate and transient hypoglycemia during exercise.

To study the occurrence and contributing factors of transient hypoglycemia after pre-exercise ingestion of glucose after a 4-hour fast, 19 well-trained cyclists ingested 50 grams of glucose dissolved in water around noon after having a normal breakfast. The ingestion of the glucose solution was followed by 30 minutes rest after which the subjects cycled for 40 minutes at 60% of the predetermined maximal power output. Every 10 minutes blood was sampled for determination of glucose, catecholamines, and insulin concentrations. In 6 subjects (hypo-group) plasma glucose levels dropped transiently below 3.0 mmol/l, while in the other 13 subjects (non-hypo group) plasma glucose level remained above this level. Although at the onset of exercise the plasma glucose levels were lower in the hypo-group, insulin levels were similar in both groups, suggesting a higher insulin sensitivity in the hypo-group. During exercise, norepinephrine was lower in the hypo-group, indicating a lower sympathetic activity in the hypo-group. The lowest plasma glucose levels in both groups were observed after 20 minutes of exercise, after which plasma glucose concentration returned to normal levels. It is concluded that pre-exercise carbohydrate ingestion after a 4-hour fast is sufficient to induce a transient hypoglycemia. The data suggest that the occurrence of hypoglycemia is determined by a combination of a high insulin sensitivity, a small amount of ingested glucose, and a low sympathetic activity.     

Metabolism and performance following carbohydrate ingestion late in exercise

To determine whether a single carbohydrate feeding could rapidly restore and maintain plasma glucose availability late in exercise, six trained cyclists were studied on two occasions during exercise to fatigue at 70 +/- 1% of VO2max. After 135 min of exercise, the men were fed either an artificially sweetened placebo or glucose polymers (3 g.kg-1 in a 50% solution). Prolonged exercise led to a decline in plasma glucose from 4.6 +/- 0.1 mM at rest to 3.9 +/- 0.2 mM after 135 min (P less than 0.05). Plasma glucose decreased further (P less than 0.05) to 3.2 +/- 2.0 mM at fatigue following placebo ingestion but increased (P less than 0.05) and was then maintained at 4.5-4.9 mM following carbohydrate ingestion. Respiratory exchange ratio (R) fell gradually during the placebo trial from 0.88 +/- 0.01 after 10 min of exercise to 0.81 +/- 0.01 at fatigue (P less than 0.01). R also reached a minimum of 0.81-0.82 in each subject after 135-180 min of exercise during the carbohydrate feeding trial but increased again to 0.84-0.86 as plasma glucose rose following the carbohydrate feeding. Exercise time to fatigue was 21% longer (205 +/- 17 vs 169 +/- 12 min; P less than 0.01) during the carbohydrate ingestion trial. Plasma insulin did not increase significantly, whereas plasma free fatty acids and blood glycerol plateaued following carbohydrate ingestion. These data indicate that a single carbohydrate feeding late in exercise can supply sufficient carbohydrate to restore euglycemia and increase carbohydrate oxidation, thereby delaying fatigue.     

Effects of carbohydrate ingestion on gastric emptying and exercise performance

In an effort to determine the effects of 5 (CHO-5), 6 (CHO-6), and 7.5 (CHO-7.5) percent carbohydrate solutions on gastric emptying and performance, 8 trained male cyclists performed 4 trials of intermittent (7- x ;12-min bout) cycling at 70% VO2max. A final 12-min self-paced "performance" ride was performed on an isokinetic ergometer (Fitron) interfaced with a computer to provide total work output. A water placebo (WP) was used as a control. Each 12-min ride was followed by 3-min rest, during which a drink was consumed (8.5 ml.kg-1; mean total = 1,336 ml.2 h-1). Blood samples were taken at 0, 25, 55, 85, and 115 min for blood glucose analysis. Following the performance ride, gastric residue was obtained by intubation and aspiration. Of the original 1,336 ml ingested during each trial, the volumes emptied by the stomach for the four trials were 1,306 +/- 76, 1,262 +/- 82, 1,288 +/- 75, and 1,278 +/- 77 ml (+/- SE) for WP, CHO-5, CHO-6, and CHO-7.5, respectively. Only the volume in the CHO-5 trial was significantly different from WP. The performance data showed that in all of the CHO trials, significantly more work was produced compared to the WP trial (CHO-5 = 1.98 +/- 0.09 x 10(5) Nm vs WP = 1.83 +/- 0.11 x 10(5) Nm). There were no significant differences in performance between any of the CHO trials.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of supplemental carbohydrate ingestion on intermittent isokinetic leg exercise.

BACKGROUND: The purpose of this investigation was to examine the effects of carbohydrate (CHO) supplementation on isokinetic leg extension/flexion exercise performance, blood glucose responses, blood free fatty acid (FFA) responses, and blood lactate (La) responses. METHODS: Eight resistance trained males (mean+/-SEM, age: 23.7+/-1.3 yrs, height: 180.0+/-3.5 cm, bodymass: 94.9+/-4.9 kg) participated in a randomized, double blind protocol with testing sessions separated by 7-d. Subjects were given CHO or placebo (P) while performing 16 sets of 10 repetitions at 120 degrees x s(-1) on a Cybex isokinetic dynamometer. Performance variables measured were; total work (TW), average work (AW), peak torque (PT) and average torque (AT). Plasma glucose (PG), FFA, and La were measured prior to testing (PRE), after set 8 (MID), and 16 (POST). RESULTS: Results indicated that the CHO treatment elicited significantly (p<0.05) more TW (CHO: 41.1+/-3.9 kJ; P: 38.1+/-3.9 kJ) and AW (CHO: 2.6+/-0.2 kJ; P: 2.4+/-0.2 kJ). There were no differences (p<0.05) between treatments for PT of the hamstrings (CHO: 91.6+/-6.5 Nm; P: 87.4+/-8.5 Nm) and quadriceps (CHO: 129.7+/-9.5 Nm; P: 123.0+/-10.6 Nm). The AT of the hamstrings (CHO: 77.8+/-5.2 Nm; P: 75.7+/-8.7 Nm) and quadriceps (CHO: 116.9+/-8.9 Nm; P: 110.0+/-8.5 Nm) were not statistically different (p>0.05) between the treatments. PG was significantly higher at the POST blood draw in the CHO treatment. No significant differences (p>0.05) were observed between the treatments for FFA and La concentrations. CONCLUSIONS: The data from this investigation indicate that the use of CHO supplementation during isokinetic leg exercise allows for the performance of more work.     

Effects of carbohydrate beverage ingestion on the salivary IgA response to intermittent exercise in the heat

The purpose of this study was to establish if provision of CHO altered the mucosal immune and salivary cortisol responses to intermittent exercise in the heat. In a double-blind design, 10 males undertook soccer-specific intermittent exercise on a motorized treadmill on 2 occasions, each over 90 min and separated by 1 week. During CHO and placebo trials, subjects were given either a carbohydrate solution (3 ml · kg (-1) body weight) or placebo drink, 5 min before the commencement of exercise, at 15, 30 min, at half time, 60 and 75 min into exercise. Salivary flow rate increased throughout the placebo trial and decreased throughout the CHO treatment; the difference between conditions neared statistical significance (P=0.055). Neither s-IgA concentration nor s-IgA to osmolality ratio was affected by 2 conditions or differed at any time-point post-exercise (P>0.05). The s-IgA secretion rate increased, s-IgA to protein ratio decreased post-exercise and salivary cortisol decreased 24 h post-exercise (P<0.05) compared to pre-exercise. Carbohydrate supplementation whilst exercising in the heat, does not influence rating of perceived exertion, thermal sensation, salivary flow rate, s-IgA concentration, s-IgA secretion rate, s-IgA to osmolality ratio or s-IgA to protein ratio and salivary cortisol but heart rate was increased.     

Double blind carbohydrate ingestion does not improve exercise duration in warm humid conditions

The positive effects of carbohydrate (CHO) supplementation on endurance exercise are well documented but the placebo (PLA(c)) effect can make the ergogenic qualities of substances more difficult to determine. Therefore, this study tested the effect of double blind ingestion of PLA(c) and CHO(c) in capsules versus known capsule (CHO(k)) ingestion on prolonged exercise heat stress. Nine well trained male volunteers (mean+/-S.D.: 23+/-3 years; 62.4+/-6.5 kg and 65.8+/-5.2 mL kg(-1) min(-1) peak oxygen consumption) exercised at 60% of maximum power output until volitional exhaustion (TTE) in the three different conditions. Capsules were ingested with 252+/-39 mL of water. Blood glucose in CHO(c) and CHO(k) was similar but higher (p<0.05) than PLA(c) from 45 min to end of exercise. There were no differences in TTE between PLA(c) (125.2+/-37.1 min) or CHO(c) (138.8+/-47.0 min) or between CHO(c) and CHO(k) (155.8+/-54.2 min). Time to volitional exhaustion was different between PLA(c) and CHO(k) (p<0.05). Increased TTE resulted when participants and researchers knew the capsule content, but not in the double blind condition. The difference could be related to a combined effect of CHO ingestion and knowledge of what was ingested possibly acting as a potent psychological motivator.     

Effects of acute ingestion of salbutamol during submaximal exercise

To assess the eventual effects of acute oral salbutamol intake on performance and metabolism during submaximal exercise, nine healthy volunteers completed two cycling trials at a power corresponding to 80-85% VO2max, after either placebo (Pla) or salbutamol (Sal, 6 mg) treatment, according to a double-blind randomized protocol. Blood samples were collected both at rest and during exercise (5 min-, 10 min-, 15 min-exhaustion) for C-peptide, FFA, lactate and blood glucose measurements. Cycling performance was significantly improved in the Sal vs. Pla trials (p < 0.05). After Sal intake, resting C-peptide, lactate, FFA and blood glucose values were higher whereas exercise lactate and free fatty acid concentrations were greater during and at the conclusion of the exercise period (p < 0.05). These results suggest that acute salbutamol ingestion improved performance during submaximal exercise probably through an enhancement of the overall contribution to energy production from both aerobic and anaerobic metabolisms.     

Effect of carbohydrate ingestion on exercise endurance and metabolism after a 1-day fast

Fasting before an exercise event has been demonstrated to decrease endurance. The purpose of this study was to investigate whether this decrement in performance after fasting could be reversed by ingestion of a carbohydrate solution before and during exercise. Nine fit male subjects ran to exhaustion at approximately 70% VO2max in two counterbalanced trials. The subjects were fasted for 21 h before both trials, and the trials were arranged so that the subjects ingested either a carbohydrate (CHO) or placebo (PL) solution. Although ratings of perceived exertion were significantly lower in the CHO trial, there were no differences in endurance time to exhaustion in the two trials (102 +/- 8 min in the PL trial and 106 +/- 8 min in the CHO trial). There were no differences between trials for the VO2, heart rate, and blood lactate concentrations. As expected, the blood glucose and insulin concentrations were higher in the CHO trial. The respiratory exchange ratio was significantly higher in the CHO trial at 40 min of exercise and tended to be higher at all other times, suggesting a greater reliance on carbohydrate and less on fat as an energy source. This seemed to be confirmed by the significantly lower plasma glycerol concentration, which suggested less fat mobilization in the CHO trial. Ingestion of a glucose polymer solution increased carbohydrate utilization in fasted subjects, but exercise performance was not improved.     

Oxidation of carbohydrate ingested during prolonged endurance exercise.

Classic studies conducted in the 1920s and 1930s established that the consumption of a high carbohydrate (CHO) diet before exercise and the ingestion of glucose during exercise delayed the onset of fatigue, in part by preventing the development of hypoglycaemia. For the next 30 to 40 years, however, interest in CHO ingestion during exercise waned. Indeed, it was not until the reintroduction of the muscle biopsy technique into exercise physiology in the 1960s that a series of studies on CHO utilisation during exercise appeared. Investigations by Scandinavian physiologists showed that muscle glycogen depletion during prolonged exercise coincided with the development of fatigue. Despite this finding, attempts to delay fatigue during prolonged exercise focused principally on techniques that would increase muscle glycogen storage before exercise. The possibility that CHO ingestion during exercise might also delay the development of muscle glycogen depletion and hence, at least potentially, fatigue, was not extensively investigated. This, in part, can be explained by the popular belief that water replacement to prevent dehydration and hyperthermia was of greater importance than CHO replacement during prolonged exercise. This position was strengthened by studies in the early 1970s which showed that the ingestion of CHO solutions delayed gastric emptying compared with water, and might therefore exacerbate dehydration. As a result, athletes were actively discouraged from ingesting even mildly concentrated (greater than 5 g/100ml) CHO solutions during exercise. Only in the early 1980s, when commercial interest in the sale of CHO products to athletes was aroused, did exercise physiologists again begin to study the effects of CHO ingestion during exercise. These studies soon established that CHO ingestion during prolonged exercise could delay fatigue; this finding added urgency to the search for the optimum CHO type for ingestion during exercise. Whereas in the earlier studies, estimates of CHO oxidation were made using respiratory gas exchange measurements, investigations since the early 1970s have employed stable 13C and radioactive 14C isotope techniques to determine the amount of ingested CHO that is oxidised during exercise. Most of the early interest was in glucose ingestion during exercise. These studies showed that significant quantities of ingested glucose can be oxidised during exercise. Peak rates of glucose oxidation occur approximately 75 to 90 minutes after ingestion and are unaffected by the time of glucose ingestion during exercise. Rates of oxidation also appear not to be influenced to a major extent by the use of different feeding schedules.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of carbohydrate ingestion on ratings of perceived exertion during a marathon

PURPOSE: The purpose of this study was to investigate the effects of carbohydrate substrate availability on ratings of perceived exertion (RPE) and hormonal regulation during a competitive marathon. METHODS: A randomized, double-blind study design was used in which subjects ran the marathon, and every 3.2 km, RPE and heart rate were measured. The marathoners were randomly assigned to receive carbohydrate (C) (N = 48) or placebo (P) (N = 50) beverages at a rate of 1 L x h(-1) during the race. RESULTS: Heart rate (%(HRMAX) ) was lower in P (82.0% +/- 0.6) than C (84.2% +/- 0.6) (P < 0.01), especially during the final 10 km: (78.7% +/- 1.0) and (84.5% +/- 0.7), respectively (P < 0.001). RPE was not significantly different between P and C throughout the marathon (P = 0.08) or during the final 10 km: (16.8 +/- 0.3) and (16.1 +/- 0.3), respectively (P = 0.06). Postrace plasma glucose (P < 0.001), insulin (P < 0.001), and lactate (P < 0.05) levels were significantly lower in P than C, and postrace cortisol (P < 0.05) significantly higher in P compared with C. CONCLUSIONS: Marathoners ingesting carbohydrate compared with placebo beverages were able to run at a higher intensity while reporting a nonsignificant difference in RPEs during a competitive race.     

Oxidation of combined ingestion of maltodextrins and fructose during exercise

PURPOSE: To determine whether combined ingestion of maltodextrin and fructose during 150 min of cycling exercise would lead to exogenous carbohydrate oxidation rates higher than 1.1 g.min. METHODS: Eight trained cyclists VO2max: 64.1 +/- 3.1 mL.kg.min) performed three exercise trials in a random order. Each trial consisted of 150 min cycling at 55% maximum power output (64.2+/-3.5% VO2max) while subjects received a solution providing either 1.8 g.min of maltodextrin (MD), 1.2 g.min of maltodextrin + 0.6 g.min of fructose (MD+F), or plain water. To quantify exogenous carbohydrate oxidation, corn-derived MD and F were used, which have a high natural abundance of C. RESULTS: Peak exogenous carbohydrate oxidation (last 30 min of exercise) rates were approximately 40% higher with combined MD+F ingestion compared with MD only ingestion (1.50+/-0.07 and 1.06+/-0.08 g.min, respectively, P<0.05). Furthermore, the average exogenous carbohydrate oxidation rate during the last 90 min of exercise was higher with combined MD+F ingestion compared with MD alone (1.38+/-0.06 and 0.96+/-0.07 g.min, respectively, P<0.05). CONCLUSIONS: The present study demonstrates that with ingestion of large amounts of maltodextrin and fructose during cycling exercise, exogenous carbohydrate oxidation can reach peak values of approximately 1.5 g.min, and this is markedly higher than oxidation rates from ingesting maltodextrin alone.     

Effect of fructose ingestion on muscle glycogen usage during exercise

Eight healthy males were studied to compare the effects of preexercise fructose and glucose ingestion on muscle glycogen usage during exercise. Subjects performed three randomly assigned trials, each involving 30 min of cycling exercise at 75% VO2max. Forty-five min prior to commencing each trial, subjects ingested either 50 g of glucose (G), 50 g of fructose (F), or sweet placebo (C). No differences in VO2 or respiratory exchange ratio were observed between the trials. Blood glucose was elevated (P less than 0.05) as a result of the glucose feeding. With the onset of exercise, blood glucose declined rapidly during G, reaching a nadir of 3.18 +/- 0.15 (SE) mmol X 1(-1) at 20 min of exercise. This value was lower (P less than 0.05) than the corresponding values in F (3.79 +/- 0.20) and C (3.99 +/- 0.18). No differences in exercise blood glucose levels were observed between F and C. Muscle glycogen utilization was greater (P less than 0.05) during G (55.4 +/- 3.3 mmol X kg-1 w.w.) than C (42.8 +/- 4.2). No difference was observed between F (45.6 +/- 4.3) and C. There was a trend (P = 0.07) for muscle glycogen usage to be lower during F than G. These results suggest that the adverse effects of preexercise glucose ingestion are, in general, not observed with either fructose or sweet placebo.     

Dose-related effects of prolonged NaHCO3 ingestion during high-intensity exercise

PURPOSE: Sodium bicarbonate (NaHCO3) ingestion may prevent exercise-induced perturbations in acid-base balance, thus resulting in performance enhancement. This study aimed to determine whether different levels of NaHCO3 intake influences acid-base balance and performance during high-intensity exercise after 5 d of supplementation. METHODS: Twenty-four men (22 +/- 1.7 yr) were randomly assigned to one of three groups (eight subjects per group): control (C, placebo), moderate NaHCO3 intake (MI, 0.3 g x kg(-1) x d(-1)), and high NaHCO3 intake (HI, 0.5 g x kg(-1) x d(-1)). Arterial pH, HCO3(-), PO2, PCO2, K+, Na, base excess (BE), lactate, and mean power (MP) were measured before and after a Wingate test pre- and postsupplementation. RESULTS: HCO3(-) increased proportionately to the dosage level. No differences were detected in C. Supplementation increased MP (W x kg(-)) in MI (7.36 +/- 0.7 vs 6.73 +/- 1.0) and HI (7.72 +/- 0.9 vs 6.69 +/- 0.6), with HI being more effective than MI. NaHCO3 ingestion resulted postexercise in increased lactate (mmol x L(-1)) (12.3 +/- 1.8 vs 10.3 +/- 1.9 and 12.4 +/- 1.2 vs 10.4 +/- 1.5 in MI and HI, respectively), reduced exercise-induced drop of pH (7.305 +/- 0.04 vs 7.198 +/- 0.02 and 7.343 +/- 0.05 vs 7.2 +/- 0.01 in MI and HI, respectively) and HCO3(-) (mmol x L(-1)) (13.1 +/- 2.4 vs 17.5 +/- 2.8 and 13.2 +/- 2.7 vs 19.8 +/- 3.2 for HCO3 in MI and HI, respectively), and reduced K (3.875 +/- 0.2 vs 3.625 +/- 0.3 mmol x L(-1) in MI and HI, respectively). CONCLUSION: NaHCO3 administration for 5 d may prevent acid-base balance disturbances and improve performance during anaerobic exercise in a dose-dependent manner.     

Caffeine ingestion during exercise to exhaustion in elite distance runners. Revision.

Caffeine is currently being used as an ergogenic aid by many athletes. The aim of this research was to determine whether a large dose of caffeine (10 mg.kg-1) taken immediately prior to the start of endurance exercise would have the desired effect of increasing endurance performance. Six males, who were not habitual caffeine users and who had performed at least two marathons, served as subjects in this experiment. They ran on a treadmill at a speed which had been calculated would elicit 75% of their VO2max for 45 minutes, after which time the speed was increased by two miles per hour till exhaustion. During the caffeine trial the athletes ran further than either the control or placebo conditions (p less than 0.05). Blood lactate values did not change across condition except for the final collection period which was significantly higher in the caffeine trial (p less than 0.05). As expected there was a significant time effect in all conditions (p less than 0.0001). Blood triglycerides after the start of the test were always higher in the caffeine condition but this was only significant at the 45 minute and end of exercise collection periods (p less than 0.05). The results suggest that endurance athletes can use caffeine just prior to exercise rather than one to three hours prior to exercise.     

Short-term recovery from prolonged exercise: exploring the potential for protein ingestion to accentuate the benefits of carbohydrate supplements

This review considers aspects of the optimal nutritional strategy for recovery from prolonged moderate to high intensity exercise. Dietary carbohydrate represents a central component of post-exercise nutrition. Therefore, carbohydrate should be ingested as early as possible in the post-exercise period and at frequent (i.e. 15- to 30-minute) intervals throughout recovery to maximize the rate of muscle glycogen resynthesis. Solid and liquid carbohydrate supplements or whole foods can achieve this aim with equal effect but should be of high glycaemic index and ingested following the feeding schedule described above at a rate of at least 1 g/kg/h in order to rapidly and sufficiently increase both blood glucose and insulin concentrations throughout recovery. Adding ≥0.3 g/kg/h of protein to a carbohydrate supplement results in a synergistic increase in insulin secretion that can, in some circumstances, accelerate muscle glycogen resynthesis. Specifically, if carbohydrate has not been ingested in quantities sufficient to maximize the rate of muscle glycogen resynthesis, the inclusion of protein may at least partially compensate for the limited availability of ingested carbohydrate. Some studies have reported improved physical performance with ingestion of carbohydrate-protein mixtures, both during exercise and during recovery prior to a subsequent exercise test. While not all of the evidence supports these ergogenic benefits, there is clearly the potential for improved performance under certain conditions, e.g. if the additional protein increases the energy content of a supplement and/or the carbohydrate fraction is ingested at below the recommended rate. The underlying mechanism for such effects may be partly due to increased muscle glycogen resynthesis during recovery, although there is varied support for other factors such as an increased central drive to exercise, a blunting of exercise-induced muscle damage, altered metabolism during exercise subsequent to recovery, or a combination of these mechanisms.     

Fat and carbohydrate metabolism during submaximal exercise in children

During exercise, the contribution of fat and carbohydrate to energy expenditure is largely modulated by the intensity of exercise. Age, a short- or long-term diet enriched in carbohydrate or fat substrate stores, training and gender are other factors that have also been found to affect this balance. These factors have been extensively studied in adults from the perspective of improving performance in athletes, or from a health perspective in people with diseases. During the last decade, lifestyle changes associated with high-energy diets rich in lipid and reduced physical activity have contributed to the increase in childhood obesity. This lifestyle change has emerged as a serious health problem favouring the early development of cardiovascular diseases, insulin resistance or type 2 diabetes mellitus. Increasing physical activity levels in young people is important to increase energy expenditure and promote muscle oxidative capacity. Therefore, it is surprising that the regulation of balance between carbohydrate and lipid use during exercise has received much less attention in children than in adults. In this review, we have focused on the factors that affect carbohydrate and lipid metabolism during exercise and have identified areas that may be relevant in explaining the higher contribution of lipid to energy expenditure in children when compared with adults. Low muscle glycogen content is possibly associated with a low activity of glycolytic enzymes and high oxidative capacity, while lower levels of sympathoadrenal hormones are likely to favour lipid metabolism in children. Changes in energetic metabolism occurring during adolescence are also dependent on pubertal events with an increase in testosterone in boys and estrogen and progesterone in girls. The profound effects of ovarian hormones on carbohydrate and fat metabolism along with their effects on oxidative enzymes could explain that differences in substrate metabolism have not always been observed between girls and women. Finally, although the regulatory mechanisms of fat and carbohydrate balance during exercise are quite well identified, there are a lack of data specific to children and most of the evidences reported in this review were drawn from studies in adults. Isotope tracer techniques and nuclear magnetic resonance will allow non-invasive investigation of the metabolism of the different substrate sources in skeletal muscle.