Dose-response effects of ingested carbohydrate on exercise metabolism in women

PURPOSE: The effect of different quantities of carbohydrate (CHO) intake on CHO metabolism during prolonged exercise was examined in endurance-trained females. METHOD: On four occasions, eight females performed 2 h of cycling at approximately 60% .VO2max with ingestion of beverages containing low (LOW, 0.5 g.min(-1)), moderate (MOD, 1.0 g.min(-1)), or high (HIGH, 1.5 g.min(-1)) amounts of CHO, or water only (WAT). Test solutions contained trace amounts of [U-13C] glucose. Indirect calorimetry combined with measurement of expired 13CO2 and plasma 13C enrichment enabled calculation of exogenous CHO, liver-derived glucose, and muscle glycogen oxidation during the last 30 min of exercise. RESULTS: The highest rates of exogenous CHO oxidation were observed in MOD, with no further increases in HIGH (peak rates of 0.33 +/- 0.02, 0.50 +/- 0.03, and 0.48 +/- 0.05 g.min(-1) for LOW, MOD, and HIGH, respectively; P < 0.05 for LOW vs MOD and HIGH). Endogenous CHO oxidation was lowest in MOD (0.99 +/- 0.06, 0.82 +/- 0.08, 0.70 +/- 0.07, and 0.89 +/- 0.09 g.min(-1); P < 0.05 for MOD vs all other trials). Compared with WAT, CHO ingestion reduced liver glucose oxidation during exercise by approximately 30% (P < 0.05 for WAT vs all CHO). Differential rates of muscle glycogen oxidation were observed with different CHO doses (0.57 +/- 0.07, 0.53 +/- 0.08, 0.41 +/- 0.07, and 0.60 +/- 0.09 g.min(-1) for WAT, LOW, MOD, and HIGH respectively; P < 0.05 for MOD vs HIGH). CONCLUSION: In endurance-trained women, the highest rates of exogenous CHO oxidation and greatest endogenous CHO sparing was observed when CHO was ingested at moderate rates (1.0 g.min(-1), 60 g.h(-1)) during exercise.     

Superior endurance performance with ingestion of multiple transportable carbohydrates

INTRODUCTION: The aim of the present study was to investigate the effect of ingesting a glucose plus fructose drink compared with a glucose-only drink (both delivering carbohydrate at a rate of 1.8 g.min(-1)) and a water placebo on endurance performance. METHODS: Eight male trained cyclists were recruited (age 32 +/- 7 yr, weight 84.4 +/- 6.9 kg, .VO(2max) 64.7 +/- 3.9 mL.kg(-1).min(-1), Wmax 364 +/- 31 W). Subjects ingested either a water placebo (P), a glucose (G)-only beverage (1.8 g.min(-1)), or a glucose and fructose (GF) beverage in a 2:1 ratio (1.8 g.min(-1)) during 120 min of cycling exercise at 55% Wmax followed by a time trial in which subjects had to complete a set amount of work as quickly as possible (approximately 1 h). Every 15 min, expired gases were analyzed and blood samples were collected. RESULTS: Ingestion of GF resulted in an 8% quicker time to completion during the time trial (4022 s) compared with G (3641 s) and a 19% improvement compared with W (3367 s). Total carbohydrate (CHO) oxidation was not different between GF (2.54 +/- 0.25 g.min(-1)) and G (2.50 g.min(-1)), suggesting that GF led to a sparing of endogenous CHO stores, because GF has been shown to have a greater exogenous CHO oxidation than G. CONCLUSION: Ingestion of GF led to an 8% improvement in cycling time-trial performance compared with ingestion of G.     

Glucose polymer molecular weight does not affect exogenous carbohydrate oxidation

PURPOSE: To compare the effects of high (HMW) versus low molecular weight (LMW) glucose polymer solutions on the pattern of substrate oxidation during exercise. METHODS: Eight cyclists (VO(2max): 63 +/- 8 mL.kg(-1).min(-1)) performed three 150-min cycling trials at 64 +/- 5% VO(2max) while ingesting 11.25% HMW (500-750 kg.mol(-1), 21 mOsm.kg(-1)) or LMW (8 kg.mol(-1), 110 mOsm.kg(-1)) solutions providing 1.8 g of carbohydrate per minute, or plain water. Substrate oxidation was determined using stable-isotope methods and indirect calorimetry. RESULTS: Exogenous carbohydrate oxidation rate was not affected by carbohydrate molecular weight (P = 0.89, peak rate: 0.93 x// 1.37 g.min(-1)). There was no effect of carbohydrate molecular weight on endogenous carbohydrate or fat oxidation rates (P = 0.30), plasma free fatty acid (P = 0.14), lactate (P = 0.38), or glucose concentrations (P = 0.98), nor were there any serious gastrointestinal complaints reported for either of the two solutions during exercise. CONCLUSIONS: Despite previous reports of faster gastric emptying and glycogen resynthesis suggesting enhanced glucose delivery, a markedly hypotonic HMW glucose polymer solution had no effect on exogenous and endogenous substrate oxidation rates during exercise, relative to a LMW glucose polymer solution. These data are consistent with there being no effect of carbohydrate structure or solution osmolality or viscosity on exogenous glucose oxidation and that ingested glucose polymers can only be oxidized on average up to 1.0 g.min during exercise.     

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.     

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 fluid intake volume on 2-h running performances in a 25 degrees C environment

PURPOSE: In this study, we examined the effects of greater than ad libitum rates of fluid intake on 2-h running performances. METHODS: Eight male distance runners performed three runs on a treadmill at 65% of peak oxygen uptake (VO2peak) for 90 min and then ran "as far as possible" in 30 min in an air temperature of 25 degrees C, a relative humidity of 55% and a wind speed of 13-15 km x h(-1). During the runs, the subjects drank a 6.9% carbohydrate (CHO)-electrolyte solution either ad libitum or in set volumes of 150 or 350 mL x 70 kg(-1) body mass (approximately 130 or 300 mL) every 15-20 min. RESULTS: Higher (approximately 0.9 vs 0.4 L x h(-1)) rates of fluid intake in the 350 mL x 70 kg(-1) trial than in the other trials had minimal effects on the subjects' urine production (approximately 0.1 L x h(-1)), sweat rates (approximately 1.2 L x h(-1)), declines in plasma volume (approximately 8%), and rises in serum osmolality (approximately 5 mosmol x L(-1)) and Na+ concentrations (approximately 7 mEq x L(-1)). A greater (approximately 1.0 vs 0.5 g x min(-1)) rate of CHO ingestion in the 350 mL x 70 kg(-1) trial than in the other trials also did not affect plasma concentrations of glucose (> or = 5 mmol x L(-1)) and lactate (approximately 3 mmol x L(-1)) during the performance runs. In all three performance runs, increases in running speeds from approximately 14 to 15-16 km x h(-1) and rises in exercise intensities from approximately 65% to 75% of VO2peak elevated plasma lactate concentrations from approximately 1.5 to 3 mmol x L(-1) and accelerated CHO oxidation from approximately 13 to 15 mmol x min(-1). The only effect of the additional intake of approximately 1.0 L of fluid in the 350 mL x 70 kg(-1) trial was to produce such severe gastrointestinal discomfort that two of the eight subjects failed to complete their performance runs. CONCLUSION: Greater rates of fluid ingestion had no measurable effects on plasma volume and osmolality and did not improve 2-h running performances in a 25 degrees C environment.     

Independent and combined effects of amino acids and glucose after resistance exercise

PURPOSE: This study was designed to assess the independent and combined effects of a dose of amino acids (approximately 6 g) and/or carbohydrate (approximately 35 g) consumed at 1 and 2 h after resistance exercise on muscle protein metabolism. METHODS: Following initiation of a primed constant infusion of H -phenylalanine and N-urea, volunteers performed leg resistance exercise and then ingested one of three drinks (amino acids (AA), carbohydrate (CHO), or AA and CHO (MIX)) at 1- and 2-h postexercise.(5) RESULTS: Total net uptake of phenylalanine across the leg over 3 h was greatest in response to MIX and least in CHO. The individual values for CHO, MIX, and AA were 53 +/- 6, 114 +/- 38, and 71 +/- 13 mg x leg x 3h. Stimulation of net uptake in MIX was due to increased muscle protein synthesis. CONCLUSIONS: These findings indicate that the combined effect on net muscle protein synthesis of carbohydrate and amino acids given together after resistance exercise is roughly equivalent to the sum of the independent effects of either given alone. The individual effects of carbohydrate and amino acids are likely dependent on the amount of each that is ingested. Further, prior intake of amino acids and carbohydrate does not diminish the metabolic response to a second comparable dose ingested 1h later.     

The effect of glucose infusion on glucose kinetics during a 1-h time trial

PURPOSE AND METHODS: To investigate the effect of glucose infusion on glucose kinetics and performance, six endurance cyclists (VO2max = 61.7 +/- 2.0 (mean +/- SE) mL x kg(-1) x min(-1)) completed two performance trials in which they had to accomplish a set amount of work as quickly as possible (991 +/- 41 kJ). Subjects were infused with either glucose (20% in saline; carbohydrate (CHO)) at a rate of 1 g x min(-1) or saline (0.9% saline; placebo (PLA)). It was hypothesized that time trial performance would be unaffected by the infusion of glucose, as endogenous stores of CHO would not be limiting in the PLA trial. RESULTS: Plasma glucose concentration increased from 4.8 +/- 0.1 mmol x L(-1) to 5.9 +/- 0.3 mmol x L(-1) during the PLA trial and from 4.9 +/- 0.1 mmol x L(-1) at rest to 12.4 +/- 1.1 mmol x L(-1) during the CHO trial. These values were significantly higher at all time points during the CHO trial compared with PLA (P < 0.001). In the final stages of the time trial, Rd in the PLA trial was 49 +/- 5 micromol x kg(-1) x min(-1) compared with 88 +/- 7 micromol x kg(-1) x min(-1) in the CHO trial (P < 0.05). Despite these differences, there was no difference in performance time between PLA and CHO (60.04 +/- 1.47 min, PLA, vs 59.90 +/- 1.49 min, CHO, respectively). Infused carbohydrate oxidation in the last 25% of the CHO trial was at least 675 +/- 120 micromol x kg(-1) and contributed 17 +/- 4% to total carbohydrate oxidation. CONCLUSION: The results demonstrate that glucose infusion had no effect on 1-h cycle time-trial performance, despite an increased availability of plasma glucose for oxidation and evidence of increased glucose uptake into the tissues.     

RPE during prolonged cycling with and without carbohydrate ingestion in boys and men

PURPOSE: To examine the effect of prolonged cycling on ratings of perceived exertion (RPE) in boys and men and whether carbohydrate (CHO) ingestion would lower RPE during exercise. METHODS: Ten boys (9-10 yr) and 10 men (20-25 yr) cycled for 60 min at approximately 70% VO2peak on two occasions. In a double-blind, counterbalanced design, a total volume of 24 mL.kg(-1) body mass of either a 6% CHO-electrolyte (CT) or flavored water (WT) beverage was consumed intermittently before and during exercise in each trial. Oxygen consumption (VO2), ventilation (VE), respiratory rate (RR), RPE (Borg's 6-20 scale), and heart rate (HR) were recorded periodically throughout exercise. Plasma glucose (GLU) was determined before and after exercise. RESULTS: Postexercise GLU was not different between age groups but higher (P<0.001) during CT (5.6 +/- 0.2 mmol.L(-1)) compared with WT (4.7 +/- 0.1 mmol.L(-1)). CHO ingestion had no effect (P>0.05) on VO2, VE, RR, or RPE in either group. RR during exercise was higher (P<0.01) in boys (39.0 +/- 2.2 breaths.min(-1)) than in men (30.9 +/- 1.3 breaths.min(-1)). HR was slightly higher (P=0.047) during CT (160 +/- 3 beats.min(-1)) compared with WT (156 +/- 4 beats.min(-1)) and increased less over time (P<0.01) in boys compared with men. RPE at 5 min of exercise was similar (P>0.05) between boys (11.8 +/- 0.7) and men (12.0 +/- 0.7) but increased faster (P<0.01) over time in boys. The average exercise RPE was higher (P<0.01) in boys (15.8 +/- 0.5) than in men (14.0 +/- 0.4). CONCLUSIONS: The higher and faster increase in RPE during exercise in boys, compared with men, may reflect a sensitivity to RR that outweighed any effect of CHO ingestion on RPE.     

Effect of an isocaloric carbohydrate-protein-antioxidant drink on cycling performance

PURPOSE: Fourteen male cyclists were studied to compare the effect of carbohydrate-protein-antioxidant beverage (CHOPA) to an isocaloric carbohydrate-only (CHO) beverage on time to fatigue and muscle damage. METHODS: Subjects performed two sets of rides to exhaustion on a cycle ergometer. In each set, the first ride was performed at 70% VO2peak, and the second was performed 24 h later at 80%. CHO or CHOPA was consumed every 15 min during exercise and immediately afterward. Plasma CK and LDH and muscle soreness were measured pre- and postexercise. RESULTS: Time to fatigue was not different between CHO and CHOPA at 70% VO2peak (95.8 +/- 29.7 vs 98.1 +/- 28.7 min), 80% VO2peak (42.3 +/- 18.6 vs 42.9 +/- 21.8 min), or total performance time (138.1 +/- 39.3 vs 140.9 +/- 43.7 min). Postexercise CK was increased (P < 0.05) from baseline in CHO (203 +/- 120 vs 582 +/- 475 U.L(-1)) but not with CHOPA (188 +/- 119 vs 273 +/- 169 U.L(-1)). Similarly, LDH values increased over baseline in CHO (437 +/- 46 vs 495 +/- 64 U.L(-1)) but not with CHOPA (432 +/- 40 vs 451 +/- 43 U.L(-1)). Postexercise CPK and LDH were higher after the CHO trial than after the CHOPA trial. Median postexercise muscle soreness was higher in CHO (3.0 +/- 5.0) than with CHOPA (1.0 +/- 3.0). CONCLUSION: No differences in time to fatigue were observed between the beverages, despite lower total carbohydrate content in the CHOPA beverage. The CHOPA beverage attenuated postexercise muscle damage, as evidenced by CK and LDH values, compared with an isocaloric CHO beverage.     

Comparaison de l'oxydation de glucose, d'un mélange de glucose et de fructose, et de saccharose ingérés en bolus ou en doses fractionnées au cours de l'exercice

The purpose of this study was to compare the oxidation rates ofisocaloric amounts of glucose (G), of a mixture of glucose and fructose (G+F), and of sucrose (S) ingested during prolonged exercise (120 min, 58 % V0₂max). The mode of ingestion, that is in a bolus at the onset of exercise or in fractionated doses during the exercise, was also investigated. Six young male subjects ingested a placebo (P), 100 g of G, 50 g of G + 50 g of F, or 95 g of S (13 % concentration) in fractionated doses during exercise, and 100 g of G and 50 g of G + 50 g of F in a bolus at the beginning of exercise. Blood samples were taken before and during the last minute of exercise period. Ingestion of carbohydrates (CHO) does not influence the blood glucose and insulin levels, but decreases by 50% the response of plasma free fatty acid concentration. During the 120-min exercise period, the amounts of ingested carbohydrates which were oxidized were similar for G (53 ± 5 g), G+F (57 ± 6 g) and S (50 ± 7 g). Exogenous CHO oxidation contributed to increasing the total CHO utilization (from 181 to 200 g/120 min) and to reducing fat oxidation (from 86 to 72 g/120 min). Ingestion of CHO as a bolus at the beginning of exercise does not influence G oxidation (58 ± 6 g) but significantly increased the oxidation of the mixture G+F (67 ± 6), probably by favoring fructose utilization which requires a delay. From a practical viewpoint, these results show that the oxidation of S, which is the most widely available oligosaccharide, is similar to that of G or a mixture of G+F. Moreover, in order to maximize exogenous CHO oxidation during exercise, CHO should be ingested early in the exercise period, and not in fractionated doses throughout the exercise period.     

Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research

Although it is known that carbohydrate (CHO) feedings during exercise improve endurance performance, the effects of different feeding strategies are less clear. Studies using (stable) isotope methodology have shown that not all carbohydrates are oxidised at similar rates and hence they may not be equally effective. Glucose, sucrose, maltose, maltodextrins and amylopectin are oxidised at high rates. Fructose, galactose and amylose have been shown to be oxidised at 25 to 50% lower rates. Combinations of multiple transportable CHO may increase the total CHO absorption and total exogenous CHO oxidation. Increasing the CHO intake up to 1.0 to 1.5 g/min will increase the oxidation up to about 1.0 to 1.1 g/min. However, a further increase of the intake will not further increase the oxidation rates. Training status does not affect exogenous CHO oxidation. The effects of fasting and muscle glycogen depletion are less clear. The most remarkable conclusion is probably that exogenous CHO oxidation rates do not exceed 1.0 to 1.1 g/min. There is convincing evidence that this limitation is not at the muscular level but most likely located in the intestine or the liver. Intestinal perfusion studies seem to suggest that the capacity to absorb glucose is only slightly in excess of the observed entrance of glucose into the blood and the rate of absorption may thus be a factor contributing to the limitation. However, the liver may play an additional important role, in that it provides glucose to the bloodstream at a rate of about 1 g/min by balancing the glucose from the gut and from glycogenolysis/gluconeogenesis. It is possible that when large amounts of glucose are ingested absorption is a limiting factor, and the liver will retain some glucose and thus act as a second limiting factor to exogenous CHO oxidation.     

Metabolic responses to exercise after carbohydrate loads in healthy men and women

PURPOSE: This study aimed to investigate gender differences in i) pancreatic insulin secretory (beta-cell sensitivity) and whole body insulin sensitivity responses to an intravenous carbohydrate (CHO) load, and (ii) metabolic responses to exercise after both intravenous and oral CHO loads. METHODS: Seven untrained healthy men and seven age-, body mass-, and VO2max-matched women performed two trials. In one trial they cycled for 60 min at 50% VO2max, starting 60 min after ingestion of a carbohydrate-rich meal (ME trial). In the other trial, subjects were infused with 20% dextrose solution to maintain blood glucose concentration at approximately 8 mmol x L(-1) for 60 min (INF trial), then the infusion rate was maintained constant during the following 60 min while exercising at 50% VO2max. RESULTS: There was no gender effect on beta-cell sensitivity (serum insulin: 161 +/- 37 and 159 +/- 28 pmol x L(-1) for men and women, respectively) and whole body insulin sensitivity (155 +/- 24 and 135 +/- 29 mg x KgFFM(-1) x min(-1) per pmol x L(-1) x 100 for men and women, respectively). This may explain the similarity in glycemic, substrate oxidation and other metabolic responses to exercise after both intravenous and oral CHO loads in men and women. CONCLUSION: These results suggest that moderate exercise performed in the postprandial state presents a similar challenge to the ability of healthy, untrained men and women to perform exercise without a substantial decline in plasma glucose concentration below fasting values.     

Effects of a carbohydrate-protein beverage on cycling endurance and muscle damage

INTRODUCTION: The purpose of this study was to determine whether endurance cycling performance and postexercise muscle damage were altered when consuming a carbohydrate and protein beverage (CHO+P; 7.3% and 1.8% concentrations) versus a carbohydrate-only (CHO; 7.3%) beverage. METHODS: Fifteen male cyclists (mean (.-)VO(2peak) = 52.6 +/- 10.3 mL x kg x min) rode a cycle ergometer at 75% (.-)VO(2peak) to volitional exhaustion, followed 12 - 15 h later by a second ride to exhaustion at 85% (.-)VO(2peak). Subjects consumed 1.8 mL x kg BW of randomly assigned CHO or CHO+P beverage every 15 min of exercise, and 10 mL x kg BW immediately after exercise. Beverages were matched for carbohydrate content, resulting in 20% lower total caloric content per administration of CHO beverage. Subjects were blinded to treatment beverage and repeated the same protocol seven to 14 d later with the other beverage. RESULTS: In the first ride (75% (.-)VO(2peak)), subjects rode 29% longer (P < 0.05) when consuming the CHO+P beverage (106.3 +/- 45.2 min) than the CHO beverage (82.3 +/- 32.6 min). In the second ride (85% (.-)VO(2peak)), subjects performed 40% longer when consuming the CHO+P beverage (43.6 +/- 12.5 min) than when consuming the CHO beverage (31.2 +/- 8.7 min). Peak postexercise plasma CPK levels, indicative of muscle damage, were 83% lower after the CHO+P trial (216.3 +/- 122.0 U x L) than the CHO trial (1318.1 +/- 1935.6 U x L). There were no significant differences in exercising levels of (.-)VO(2), ventilation, heart rate, RPE, blood glucose, or blood lactate between treatments in either trial. CONCLUSION: A carbohydrate beverage with additional protein calories produced significant improvements in time to fatigue and reductions in muscle damage in endurance athletes. Further research is necessary to determine whether these effects were the result of higher total caloric content of the CHO+P beverage or due to specific protein-mediated mechanisms.     

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.     

Influence of different amounts of carbohydrate on endurance running capacity following short term recovery

This study examined the effect of ingesting different amounts of carbohydrate (CHO) during 4 h recovery (REC) from prolonged running, on subsequent endurance running capacity when subjects were fully rehydrated. Nine men ran at 70% VO2max on a treadmill for 90 min (T1), followed by the REC and a run to exhaustion at the same speed (T2) on two occasions. Thirty minutes into REC, subjects ingested 50 g of CHO from a 6.5% CHO-electrolyte solution (CE) on both occasions. Thereafter, subjects ingested either the same CE or a placebo (PL) every 30 min for the first 3 h of REC. The total volume ingested was equal to 150% of the body mass lost during T1 which achieved rehydration during REC in both trials. Higher blood glucose and serum insulin concentrations (P<0.05) were observed during REC in the CE trial. Nevertheless, similar run times were achieved during T2 in both trials (CE: 56.9+/-8.1 min and PL: 65.4+/-7.8 min) (+/- S.E.M) (NS). Therefore, these results suggest that ingestion of 50 g of CHO immediately after prolonged exercise, and rehydration with a placebo solution, results in a similar endurance capacity, after a 4 h recovery, as ingesting 3 times more CHO (approximately 167 g CHO) over the same period.     

Effect of different post-exercise sugar diets on the rate of muscle glycogen synthesis

The effect of repeated ingestions of fructose, sucrose, and various amounts of glucose on muscle glycogen synthesis during the first 6 h after exhaustive bicycle exercise was studied. Muscle biopsies for glycogen determination were taken before and after exercise, and every second hour during recovery. Blood samples for plasma glucose and insulin determination were taken before and after exercise, and every hour during recovery. When 0.35 (low glucose: N = 5), 0.70 (medium glucose: N = 5), or 1.40 (high glucose: N = 5) g.kg-1 body weight of glucose were given orally at 0, 2, and 4 h after exercise, the rates of glycogen synthesis were (mean +/- SE) 2.1 +/- 0.5, 5.8 +/- 1.0, and 5.7 +/- 0.9 mmol.kg-1.h-1, respectively. When 0.70 g.kg-1 body weight of sucrose (medium sucrose: N = 5), or fructose (medium fructose: N = 7) was ingested accordingly, the rates were 6.2 +/- 0.5 and 3.2 +/- 0.7 mmol.kg-1.h-1. Average plasma glucose level during recovery were similar in low glucose, medium glucose, and high glucose groups (5.76 +/- 0.24, 6.31 +/- 0.64, and 6.52 +/- 0.24 mM), while average plasma insulin levels were higher with higher glucose intake (16 +/- 1, 21 +/- 3, and 38 +/- 4 microU.ml-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of carbohydrate feedings on muscle glycogen utilization and exercise performance

Ten men were studied during 4 h of cycling to determine the effect of solid carbohydrate (CHO) feedings on muscle glycogen utilization and exercise performance. In the experimental trial (E) the subjects ingested 43 g of sucrose in solid form along with 400 ml of water at 0, 1, 2 and 3 h of exercise. During the control trial (C) they received 400 ml of an artificially sweetened drink without solid CHO. No differences in VO2, heart rate, or total energy expenditure were observed between trials; however, respiratory exchange ratios were significantly (P less than 0.05) higher during E. Blood glucose was significantly (P less than 0.05) elevated 20 min post-feeding in E; however, by 50 min no differences were observed between trials until 230 min (E = 4.5 +/- 0.2 mmol X l-1 vs C = 3.9 +/- 0.2, means +/- SE; P less than 0.05). Muscle glycogen utilization was significantly (P less than 0.05) lower during E (100.7 +/- 10.2 mmol X kg-1 w.w.) than C (126.2 +/- 5.5). During a sprint (100% VO2max) ride to exhaustion at the end of each trial, subjects performed 45% longer when fed CHO (E = 126.8 +/- 24.7 s vs C = 87.2 +/- 17.5; P less than 0.05). It was concluded that repeated solid CHO feedings maintain blood glucose levels, reduce muscle glycogen depletion during prolonged exercise, and enhance sprint performance at the end of such activity.     

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.