Carbohydrate intake during prolonged cycling minimizes effect of glycemic index of preexercise meal

We studied the effects of the glycemic index (GI) of preexercise meals on metabolism and performance when carbohydrate (CHO) was ingested throughout exercise. Six well-trained cyclists performed three counterbalanced trials of 2-h cycling at approximately 70% of maximal oxygen uptake, followed by a performance ride of 300 kJ. Meals consumed 2 h before exercise consisted of 2 g CHO/kg body mass of either high-GI potato (HGI trial) or low-GI pasta (LGI trial), or of a low-energy jelly (Con trial). Immediately before and throughout exercise, subjects ingested a 10 g/100 ml [U-14C]glucose solution for a total of 24 ml/kg body mass. Despite differences in preexercise glucose, insulin, and free fatty acids concentrations among trials, both total CHO oxidation for HGI, LGI, and Con trials, respectively, during steady-state exercise [403 +/- 16, 376 +/- 29, and 373 +/- 24 (SE) g/2 h] and oxidation of the ingested CHO (65 +/- 6, 57 +/- 6, and 63 +/- 5 g/2 h) were similar. There was no difference in time to complete the subsequent performance ride (946 +/- 23, 954 +/- 35, and 970 +/- 26 s for HGI, LGI, and Con trials, respectively). When CHO is ingested during exercise in amounts presently recommended by sports nutrition guidelines, preexercise CHO intake has little effect on metabolism or on subsequent performance during prolonged cycling (approximately 2.5 h).     

Carbohydrate drinks delay fatigue during intermittent, high-intensity cycling in active men and women

The effects of ingesting carbohydrate drinks on fatigue during intermittent, high-intensity cycling in men and women were determined. Physically active but untrained women (n = 7) and men (n = 9) completed one practice trial and two experimental sessions separated by 1 week. Sessions consisted of repeated 1-min cycling bouts on a bicycle ergometer at 120-130% VO2max separated by 3 min rest until fatigue. Carbohydrate (CHO) or placebo (P) beverages (4 ml.kg body weight-1) were ingested immediately before exercise (18% CHO) and every 20 min during exercise (6% CHO). Plasma glucose and insulin were higher, RPE for the legs was lower, and time to fatigue was longer in CHO than P. Men's and women's responses were not different for any variable measured. These data suggest a beneficial role of CHO drinks on performance of intermittent, high-intensity exercise in men and women.     

Oxidation of exogenous [13C]galactose and [13C]glucose during exercise

The present study examined the oxidation of exogenous galactose or glucose during prolonged submaximal cycling exercise. Eight highly trained volunteers exercised on two occasions on a cycle ergometer at 65% of maximal workload for 120 min, followed by a 60-min rest period and a second exercise bout of 30 min at 60% maximal workload. At random, subjects ingested a 8% galactose solution to which an [1-13C]galactose tracer was added or a 8% glucose solution to which an [U-13C]glucose tracer was added. Drinks were provided at the end of the warm-up period (8 ml/kg) and every 15 min (2 ml/kg) during the first 120 min of the test. Blood and breath samples were collected every 30 and 15 min, respectively, during the test. The exogenous carbohydrate (CHO) oxidation was calculated from the 13CO2/12CO2 ratio and CO2 production of the expired air. Peak exogenous CHO oxidation during exercise for galactose and glucose was 0.41 +/- 0.03 and 0.85 +/- 0.04 g/min, respectively. Total CHO and fat oxidation were not significantly different between the treatments. Forty-six percent of the ingested glucose was oxidized, whereas only 21% of the ingested galactose was oxidized. As a consequence, more endogenous CHO was utilized with galactose than with glucose (124.4 +/- 6.7 and 100.1 +/- 3.6 g, respectively). These results indicate that the oxidation rate of orally ingested galactose is maximally approximately 50% of the oxidation rate of a comparable amount of orally ingested glucose during 120 min of exercise.     

Influence of carbohydrate supplementation early in exercise on endurance running capacity

The aim of this study is to examine the effect of carbohydrate (CHO) ingestion during the first hour of treadmill running on endurance capacity. Eleven male subjects ran at 70% VO2max to exhaustion on three occasions one week apart. On two occasions two CHO-electrolyte solutions (a 5.5% (E) and a 6.9% (L) were ingested for the first hour of exercise; water was ingested until exhaustion. On the third occasion water (W) was ingested throughout the run. The order testing was randomly assigned. Exhaustion times for the W, E, and L trials were 109.6 +/- 9.6 min, 124.5 +/- 8.4 min, and 121.4 +/- 9.4 min, respectively. There was no difference between the two CHO trials, but time to exhaustion was longer only for the E trial (P < 0.05), compared with the W trial. Nevertheless the average performance times for the combined results of the two CHO trials were longer than the water trial. Carbohydrate ingestion resulted in higher blood glucose concentration (P < 0.01) at 20 min in the E trail only and lower (P < 0.05) serum growth hormone and plasma FFA and glycerol concentrations at 60 min but not at exhaustion in both E and L trials compared with the W trial. Blood lactate, plasma ammonia, electrolytes, catecholamines, and serum insulin and cortisol concentrations were not different in the three trials. In conclusion, CHO ingestion during the first hour of exercise improves endurance capacity go a greater extent compared with water alone.     

The effects of pre-exercise starch ingestion on endurance performance

This study compared the physiological responses and performance following the ingestion of a waxy starch (WS), resistant starch (RS), glucose (GL) and an artificially-sweetened placebo (PL) ingested prior to exercise. Ten college-age, male competitive cyclists completed four experimental protocols consisting of a 30 min isokinetic, self-paced performance ride preceded by 90 min of constant load cycling at 66% VO2max. Thirty min prior to exercise, they ingested 1 g.kg-1 body weight of GL, WS, RS, or PL At rest, GL elicited greater (p < 0.05) serum glucose and insulin responses than all other trials. During exercise, however, serum glucose, insulin, blood C-peptide and glucagon responses were similar among trials. The mean total carbohydrate oxidation rates (CHOox) were higher (p < 0.05) during the GL, WS, and RS trials (2.59 +/- 0.13, 2.49 +/- 0.10, and 2.71 +/- 0.15 g.min-1, respectively) compared to PL (2.35 +/- 0.12 g.min-1). Subjects were able to complete more work (p < 0.05) during the performance ride when they ingested GL (434 +/- 25.2 kj) or WS (428 +/- 22.5 kj) compared to PL (403 +/- 35.1 kj). They also tended to produce more work with RS ingestion (418 +/- 31.4 kj), although this did not reach statistical significance (p < 0.09). These results indicate that preexercise CHO ingestion in the form of starch or glucose maintained higher rates of total carbohydrate oxidation during exercise and provided an ergogenic benefit during self-paced cycling.     

Uptake and release of hormones and metabolites by tissues of exercising leg in man

To examine release of insulin from tissues of the exercising llin, growth hormone, cortisol, and circulating metabolites were studied in five men before, during, and after exercise on a bicycle ergometer at 60% of their maximum work capacity. At rest, insulin, growth hormone, and cortisol were taken up by leg tissues. During exercise arterial plasma insulin concentration fell, but cortisol and growth hormone levels rose; there was net release of insulin into venous blood but little change in uptake of cortisol and growth hormone. Insulin release persisted after exercise for 15-30 min. During exercise arterial concentrations and uptake of glucose and free fatty acids (FFA) increased. Examination of the changes in hormones and metabolites failed to identify any single hormonal or metabolic factor causing the observed reversal of insulin uptake with exercise.     

Effect of carbohydrate ingestion on adipose tissue lipolysis during long-lasting exercise in trained men

To study whether sucrose administration acts on lipid mobilization during prolonged exercise, we used subcutaneous abdominal adipose tissue microdialysis in eight well-trained subjects submitted at random to two 100-min exercises (50% maximal aerobic power) on separate days. After 50 min of exercise, the subjects ingested either a sucrose solution (0.75 g/kg body wt) or water. By using a microdialysis probe, dialysate was obtained every 10 min from the subjects at rest, during exercise, and during a 30-min recovery period. During exercise without sucrose, plasma and dialysate glycerol increased significantly. With sucrose, the response was significantly lower for dialysate glycerol (P < 0.05). Plasma free fatty acid level was lower after sucrose than after water ingestion (P < 0.05). With water ingestion, plasma catecholamines increased significantly, whereas insulin fell (P < 0.05). With sucrose ingestion, the epinephrine response was blunted, whereas the insulin level was significantly increased. In conclusion, the use of adipose tissue microdialysis directly supports a lower lipid mobilization during exercise when sucrose is supplied, which confirms that the availability of carbohydrate influences lipid mobilization.     

Human muscle glycogen metabolism during exercise. Effect of carbohydrate supplementation

Carbohydrate (CHO) ingestion during exercise, in the form of CHO-electrolyte beverages, leads to performance benefits during prolonged submaximal and variable intensity exercise. However, the mechanism underlying this ergogenic effect is less clear. Euglycaemia and oxidation of blood glucose at high rates late in exercise and a decreased rate of muscle glycogen utilisation (i.e. glycogen 'sparing') have been proposed as possible mechanisms underlying the ergogenic effect of CHO ingestion. The prevalence of one or the other mechanism depends on factors such as the type and intensity of exercise, amount, type and timing of CHO ingestion, and pre-exercise nutritional and training status of study participants. The type and intensity of exercise and the effect of these on blood glucose, plasma insulin and catecholamine levels, may play a major role in determining the rate of muscle glycogen utilisation when CHO is ingested during exercise. The ingestion of CHO (except fructose) at a rate of > 45 g/h, accompanied by a significant increase in plasma insulin levels, could lead to decreased muscle glycogen utilisation (particularly in type I fibres) during exercise. Endurance training and alterations in pre-exercise muscle glycogen levels do not seem to affect exogenous glucose oxidation during submaximal exercise. Thus, at least during low intensity or intermittent exercise, CHO ingestion could result in reduced muscle glycogen utilisation in well trained individuals with high resting muscle glycogen levels. Further research needs to concentrate on factors that regulate glucose uptake and energy metabolism in different types of muscle fibres during exercise with and without CHO ingestion.     

State-space models of insulin and glucose responses to diets of varying nutrient content in men and women

Discrete-time state-space models were developed to describe contemporaneous responses of plasma insulin and glucose of normal human subjects. Male and female subjects ingested three consecutive identical meals from isocaloric diets classified as high-carbohydrate, high-fat, high-protein, or standard. Distinctly different glucose and insulin responses were measured in men and women. A seven-state system of linear equations, three in insulin and four in glucose, was identified and estimated to describe responses in men. A six-state system, three in insulin and three in glucose, describes responses in women. Model simulations at 15-min intervals closely match measured concentrations over a 12-h period. Effects of diet content and meal timing on insulin and glucose concentrations were quantified. Dynamic insulin and glucose responses to isocaloric meals of pure carbohydrate, fat, and protein diets were projected on the basis of models developed from mixed diets. The symmetry of the projections indicates that positive excursions in glucose concentrations associated with carbohydrate intake may be matched with negative excursions associated with fat and protein intake to help manage postmeal glucose excursions.     

Effect of added fat on plasma glucose and insulin response to ingested potato in individuals with NIDDM

OBJECTIVE: In normal subjects, ingestion of butter with potato resulted in considerably lower blood glucose levels but similar or higher insulin concentrations compared with those observed in the same subjects after potato ingestion alone. We determined whether butter ingested with potato would result in a greater stimulation in insulin secretion than ingestion of potato alone in subjects with NIDDM. RESEARCH DESIGN AND METHODS: Seven male subjects with untreated NIDDM ingested 50 g CHO alone or 50 g CHO with 5, 15, 30, or 50 g fat as a breakfast meal. Fat was ingested in the form of butter, and CHO was given in the form of potato. Subjects received 50 g glucose on two separate occasions for comparative purposes. The subjects also were given only water and were studied over the same time period (water control). Plasma glucose, glucagon, alpha-amino nitrogen, nonesterified fatty acids, serum insulin, C-peptide, and triglyceride concentrations were determined over 5 h. The integrated area responses were quantified over the 5-h period using the water control as a baseline. RESULTS: The mean plasma glucose area response after ingestion of potato with or without the various amounts of butter were all similar and were 82% of that observed after ingestion of 50 g glucose. The mean insulin area response to potato alone was 532 pmol.h.L-1. The mean insulin area responses to potato plus 5,15,30, and 50 g of fat meals were 660,774,750, and 756 pmol.h.L-1, respectively. Thus, the mean insulin areas were all greater than for ingestion of potato alone, and a maximal response was observed with addition of 15 g fat (1.4-fold). The C-peptide data did not confirm an increase in insulin secretion. Overall the area responses after ingestion of meals containing fat were not different from the response to potato ingestion alone, although the responses were erratic. The glucagon area response was positive after ingestion of all fat containing meals except for that containing only 5 g fat, and there was a dose-response relationship. The plasma alpha-amino nitrogen and nonesterified fatty acid area responses were negative after potato ingestion and were not significantly different when fat was added. The serum triglyceride concentration increase was greater after the ingestion of butter with the potato as expected. CONCLUSIONS: In contrast to the results in normal subjects after ingestion of butter with potato, the glucose response was not smaller in subjects with NIDDM. The insulin response was greater. The insulin area response data indicated the presence of a dose-response relationship. Whether similar responses will be observed with other dietary fat and CHO sources remains to be determined.     

Muscle sympathetic nerve activity is reduced in IDDM before overt autonomic neuropathy

Studies of heart-rate variability have demonstrated that abnormal cardiac parasympathetic activity in individuals with IDDM precedes the development of other signs or symptoms of diabetic autonomic neuropathy. To determine whether IDDM patients have impaired sympathetic activity compared with normal control subjects before the onset of overt neuropathy, we directly recorded MSNA. We also examined the effects of changes in plasma glucose and insulin on sympathetic function in each group. MSNA was recorded by using microneurographic techniques in 10 IDDM patients without clinically evident diabetic complications and 10 control subjects. MSNA was compared during a 15-min fasting baseline period and during insulin infusion (120 mU.m-2.min-1) with 30 min of euglycemia. A cold pressor test was performed at the end of euglycemia. Power spectral analysis of 24-h RR variability was used to assess cardiac autonomic function. IDDM patients had lower MSNA than control subjects at baseline (8 +/- 1 vs. 18 +/- 3 burst/min, P < 0.02). MSNA increased in both groups with insulin infusion (P < 0.01) but remained lower in IDDM patients (20 +/- 3 vs. 28 +/- 3 burst/min, P < 0.01). In the IDDM group, we found no relationships between MSNA and plasma glucose, insulin, or HbA1c concentrations. BP levels did not differ at rest or during insulin. Heart-rate variability and the MSNA response to cold pressor testing in IDDM patients did not differ from those in healthy control subjects. IDDM patients had reduced MSNA at rest and in response to insulin. The lower MSNA is not attributable to differences in plasma glucose or insulin, but, rather, is most likely an early manifestation of diabetic autonomic neuropathy that precedes impaired cardiac parasympathetic control.     

Effects of tennis training on lipid metabolism and lipoproteins in recreational players

OBJECTIVE: Metabolic and ergogenic effects of carbohydrate and caffeine concentrations, common in commercial available beverages, were investigated in 16 tournament players (8 males and 8 females) during a 4 hrs interrupted tennis match (30 min rest after 150 min). METHODS: On three double-blind occasions players ingested a placebo (PLA), carbohydrate (CHO) or caffeine drink (CAF) at court changeover and during the resting period. In men (women) total intake consisted of 2.8 l (2.0 l) fluid, supplemented with 243 g (182 g) carbohydrates (CHO) or with 364 mg (260 mg) caffeine (CAF), respectively. Postexercise all players performed a ball-machine test (BMT) and a tennis-sprint test (TST). RESULTS: During match play blood glucose (GLU) was higher in CHO and did not differ between CAF and PLA. Immediately after the resting period GLU temporary declined in CHO and PLA, while no significant changes occurred in CAF. Increases of serum FFA and glycerol as well as the decrease of insulin were similar during the PLA and CAF trials and less pronounced in CHO. Postexercise urine concentrations of epinephrine and caffeine were significantly higher in CAF. Perception ratings and hitting accuracy (BMT) were not affected by treatment. CHO resulted in higher blood lactate levels during match play and a better post-exercise sprint performance (TST). Under CAF women won significantly more games than during both other treatments. CONCLUSIONS: CHO enhances tennis-specific running-speed but has no ergogenic effect on tennis performance under the conditions of our study. CAF improves glucose homeostasis at the beginning of work load after rest and may increase tennis success in women.     

Ultraviolet-B induced hyperplasia and squamous cell carcinomas in the cornea of XPA-deficient mice

The purpose of this study was to determine whether presweetened breakfast cereals with various fiber contents and a moderate glycemic index optimize glucose availability and improve endurance exercise performance. Six recreationally active women ate 75 g of available carbohydrate in the form of breakfast cereals: sweetened whole-grain rolled oats (SRO, 7 g of dietary fiber) or sweetened whole-oat flour (SOF, 3 g of dietary fiber) and 300 ml of water or water alone (Con). The meals were provided 45 min before semirecumbent cycle ergometer exercise to exhaustion at 60% of peak O2 consumption (VO2peak). Diet and physical activity were controlled by having the subjects reside in the General Clinical Research Center for 2 days before each trial. Blood samples were drawn from an antecubital vein for glucose, free fatty acid (FFA), glycerol, insulin, epinephrine, and norepinephrine determination. Breath samples were obtained at 15-min intervals after meal ingestion and at 30-min intervals during exercise. Muscle glycogen concentration was determined from biopsies taken from the vastus lateralis muscle before the meal and immediately after exercise. Plasma FFA concentrations were lower (P < 0.05) during the SRO and SOF trials for the first 60 and 90 min of exercise, respectively, than during the Con trial. Respiratory exchange ratios were higher (P < 0.05) at 90 and 120 min of exercise for the SRO and SOF trials, respectively, than for the Con trial. At exhaustion, glucose, insulin, FFA, glycerol, epinephrine, and norepinephrine concentrations, respiratory exchange ratio, and muscle glycogen use in the vastus lateralis muscle were similar for all trials. Exercise time to exhaustion was 16% longer (P < 0.05) during the SRO than during the Con trial: 266.5 +/- 13 and 225.1 +/- 8 min, respectively. There was no difference in exercise time for the SOF (250.8 +/- 12) and Con trials. We conclude that eating a meal with a high dietary fiber content and moderate glycemic index 45 min before prolonged moderately intense exercise significantly enhances exercise capacity.     

Extended visual fixation in young infants: look distributions, heart rate changes, and attention

Feeding a high-carbohydrate (CHO) diet and administration of alkalinizing agents have both been shown to improve performance in high-intensity exercise. The effect of these treatments in combination was investigated in the present study. Six healthy male subjects exercised to exhaustion on an electrically braked cycle ergometer at a power output equivalent to 100% of their maximum oxygen uptake (VO2,max) on four separate occasions. Each subject consumed either a diet with the same composition as his normal diet (termed the experimental normal (N) diet; 54 +/- 7% CHO, 13 +/- 2% protein, 33 +/- 7% fat) or a high-CHO diet (81 +/- 2% CHO, 13 +/- 2% protein, 6 +/- 1% fat) that had the same energy and protein content for the 3 days prior to the exercise tests. Subjects then ingested either a placebo (CaCO3) or trisodium citrate (0.3 g (kg body mass)-1) 3 h before exercise. Time to fatigue was not different between experimental conditions. Consumption of the high-CHO diet had no effect on blood acid-base status, but the ingestion of sodium citrate induced a mild metabolic alkalosis after both the N diet and the high-CHO diet. This alkalinizing effect was also evident after exercise, since blood pH, plasma bicarbonate and blood base excess were higher (P < 0.05) after the ingestion of sodium citrate than under the placebo conditions. The changes in blood lactate, pyruvate and glucose and plasma glycerol after exercise were similar for all experimental conditions. Blood lactate, glucose and pyruvate and plasma glycerol concentrations increased from resting values (P < 0.01) following exercise but this increase was similar under all experimental conditions. These data demonstrate that when the energy and protein content of the diets is the same, exercise capacity and the metabolic response to intense exercise are similar following consumption either of a high-CHO diet or a more normal diet. Acute ingestion of sodium citrate prior to exercise resulted in a reduction in post-exercise acidosis despite a blood lactate concentration that was similar to that observed after the ingestion of a placebo, but did not affect exercise performance under the conditions of this study.     

Insulin-independent acute restoration of euglycemia normalizes the impaired glucose clearance during exercise in diabetic dogs

At rest and during exercise, chronic hyperglycemia, high free fatty acid (FFA) oxidation, and insulin deficiency in diabetes are well known to impair glucose clearance (metabolic clearance rate [MCR]). The effect of acute restoration of glycemia per se on MCR has been less well characterized. We therefore studied normal and alloxan-diabetic dogs both at rest and during exercise, as diabetic hyperglycemic or after acutely induced euglycemia (<160 min) generated by infusion of either insulin or phlorizin. Glucose uptake was similar under hyperglycemic and normoglycemic conditions both at rest and during exercise, indicating a precise balance between the mass effect of glucose and decreased MCR. Rest and exercise MCR was fourfold lower under conditions of hyperglycemia, but insulin-independent restoration of euglycemia improved basal MCR threefold and normalized MCR during exercise. High FFA turnover did not affect glucose uptake but was correlated with plasma lactate concentrations (r = 0.72, P < 0.001), suggesting that muscle fuel requirements are controlled by glucose oxidation and not uptake. We conclude that in alloxan-diabetic dogs, the impaired MCR may be an adaptive phenomenon because correction of hyperglycemia corrects MCR despite partial insulin deficiency and high FFA turnover. We speculate that constant glucose uptake despite hyperglycemia in diabetes may protect the muscle from excessive exposure to glucose.     

Kappa-opioid potentiation of tumor necrosis factor-alpha-induced anti-HIV-1 activity in acutely infected human brain cell cultures

The aim of this study was to investigate the effect of medium-chain triacylglycerol (MCT) ingestion during exercise on subsequent time-trial cycling performance. Seven well-trained cyclists performed four exercise trials consisting of 2 h at 60% of maximal oxygen uptake followed by a simulated time trial (ie, completion of a preset amount of work as fast as possible) of approximately 15 min duration. During the trials, subjects ingested 1) a 10% carbohydrate solution (CHO; 170 +/- 6 g glucose), 2) a 10% carbohydrate electrolyte with 5% MCT solution (CHO + MCT; 85 +/- 3 g MCT), 3) a 5% MCT solution, or 4) artificially colored and flavored water (placebo). Neither CHO nor CHO + MCT ingestion had any effect on performance compared with placebo ingestion, whereas ingestion of MCT had a negative effect on performance. Average work rates during the time trial were 314 +/- 19, 314 +/- 13, and 312 +/- 18 with CHO, CHO + MCT, and placebo, respectively, and was 17-18% lower in the MCT trial (263 +/- 22 W). In addition, compared with placebo ingestion, MCT ingestion had no effect on total rates of fat or carbohydrate oxidation, nor did it affect exogenous or endogenous carbohydrate utilization. The negative effect of MCT ingestion was associated with increased gastrointestinal complaints (ie, intestinal cramping). These data suggest that large amounts of MCTs (85 g) ingested during prolonged submaximal exercise may provoke gastrointestinal problems leading to decreased exercise performance.     

Postprandial lipemia: effects of intermittent versus continuous exercise

PURPOSE: The purpose of this study was to assess whether exercise performed in continuous and discontinuous formats reduced postprandial lipemia to a similar degree. METHODS: Fifteen normolipidemic and three borderline hyperlipidemic healthy males (ages 30.6 +/- 9.0 (mean +/- SD) yr, BMI 23.1 +/- 1.4 kg.m-2) participated in three trials, each conducted over 2 d. Subjects refrained from exercise for the 2 d preceding each trial. On day one, subjects rested (control trial), or ran at 60% of maximal oxygen uptake in either one 90-min session (continuous exercise trial), or three 30-min sessions (intermittent exercise trial). On day two, subjects ingested a high-fat test breakfast (1.2 g fat, 1.2 g carbohydrate, 70 kJ energy per kilogram body mass). Blood samples were obtained in the fasted state and at intervals for 6 h postprandially. RESULTS: Fasting plasma triacylglycerol (TAG) concentrations did not differ between trials. Areas under the TAG versus time curves were 18.1 +/- 6.7% (mean +/- SEM) and 17.7 +/- 7.6% (both P < 0.05) lower than control in the continuous exercise and intermittent exercise trials, respectively. Plasma glucose responses to the test meal did not differ between trials, but the serum insulin response was lower in the intermittent exercise trial compared with that in the control. CONCLUSION: The results suggest that both intermittent and continuous exercise can reduce postprandial lipemia.     

Effect of carbohydrate ingestion on sprint performance following continuous and intermittent exercise

PURPOSE: This investigation was conducted to study the effects on sprint performance of glucose and fructose ingestion during a 15-min rest period half way through 90 min of continuous and intermittent exercise. On three occasions, eight subjects cycled at 76 +/- 2% VO2max for 90 min (continuous trials: CON trials) with a 15-min half-time break. METHODS: On another three occasions, they cycled for 90 min between moderate (65% VO2max) and high (100% VO2max) intensity (intermittent trials: INT trials) with the same half-time. In both trials, 90-min exercise was followed by a 40-s Wingate test to evaluate remaining sprint capacity. During half-time, they consumed either 20% glucose polymer (G), 20% fructose (F) or sweet placebo (P). Ingestion of G maintained plasma glucose levels, carbohydrate oxidation rate and lower value of ratings of perceived exertion (RPE) in both trials and indicated higher sprint performance compared with P (mean power of CON trials: 614.3 +/- 23.3 W vs 574.0 +/- 22.7 W, P < 0.001, INT trials: 629.5 +/- 27.6 W vs 596.3 +/- 25.5 W, P < 0.01). RESULTS: Ingestion of F showed similar effect in CON trials (603.8 +/- 26.1 W vs 574.0 +/- 22.7 W, P < 0.01) but had no positive effect in INT trials. Additionally, mean power of G was higher than F (629.5 +/- 27.6 W vs 598.4 +/- 34.2 W, P < 0.01) in INT trials. CONCLUSIONS: These results indicated that ingestion of G during half-time of 90-min exercise could maintain carbohydrate utilization and improve sprint performance in both CON and INT trials.     

Glucose infusion partially attenuates glucose production and increases uptake during intense exercise

Glucose infusion can prevent the increase in glucose production (Ra) and increase glucose uptake (Rd) during exercise of moderate intensity. We postulated that 1) because in postabsorptive intense exercise (>80% maximal O2 uptake) the eightfold increase in Ra may be mediated by catecholamines rather than by glucagon and insulin, exogenous glucose infusion would not prevent the Ra increment, and 2) such infusion would cause greater Rd. Fit young men were exercised at >85% maximal O2 uptake for 14 min in the postabsorptive state [controls (Con), n = 12] or at minute 210 of a 285-min glucose infusion. In seven subjects, the infusion was constant (CI; 4 mg . kg-1 . min-1), and in seven subjects it was varied (VI) to mimic the exercise Ra response in Con. Although glucose suppressed Ra to zero (with glycemia approximately 6 mM and insulin approximately 150 pM), an endogenous Ra response to exercise occurred, to peak increments two-thirds those in Con, in both CI and VI. Glucagon was unchanged, and very small increases in the glucagon-to-insulin ratio occurred in all three groups. Catecholamine responses were similar in all three groups, and correlation coefficients of Ra with plasma norepinephrine and epinephrine were significant in all. In all CI and VI, Rd at rest was 2x Con, increased earlier in exercise, and was higher for the 1 h of recovery with glucose infusion. Thus the Ra response was only partly attenuated, and the catecholamines are likely to be the regulators. This suggests that an acute endogenous Ra rise is possible even in the postprandial state. Furthermore, the fact that more circulating glucose is used by muscle during exercise and early recovery suggests that muscle glycogen is spared.     

Coca chewing for exercise: hormonal and metabolic responses of nonhabitual chewers

To determine the effects of acute coca use on the hormonal and metabolic responses to exercise, 12 healthy nonhabitual coca users were submitted twice to steady-state exercise (approximately 75% maximal O2 uptake). On one occasion, they were asked to chew 15 g of coca leaves 1 h before exercise, whereas on the other occasion, exercise was performed after 1 h of chewing a sugar-free chewing gum. Plasma epinephrine, norepinephrine, insulin, glucagon, and metabolites (glucose, lactate, glycerol, and free fatty acids) were determined at rest before and after coca chewing and during the 5th, 15th, 30th, and 60th min of exercise. Simultaneously to these determinations, cardiorespiratory variables (heart rate, mean arterial blood pressure, oxygen uptake, and respiratory gas exchange ratio) were also measured. At rest, coca chewing had no effect on plasma hormonal and metabolic levels except for a significantly reduced insulin concentration. During exercise, the oxygen uptake, heart rate, and respiratory gas exchange ratio were significantly increased in the coca-chewing trial compared with the control (gum-chewing) test. The exercise-induced drop in plasma glucose and insulin was prevented by prior coca chewing. These results contrast with previous data obtained in chronic coca users who display during prolonged submaximal exercise an exaggerated plasma sympathetic response, an enhanced availability and utilization of fat (R. Favier, E. Caceres, H. Koubi, B. Sempore, M. Sauvain, and H. Spielvogel. J. Appl. Physiol. 80: 650-655, 1996). We conclude that, whereas coca chewing might affect glucose homeostasis during exercise, none of the physiological data provided by this study would suggest that acute coca chewing in nonhabitual users could enhance tolerance to exercise.