Muscle glycogen storage after different amounts of carbohydrate ingestion

The purpose of this study was to determine whether the rate of muscle glycogen storage could be enhanced during the initial 4-h period postexercise by substantially increasing the amount of the carbohydrate consumed. Eight subjects cycled for 2 h on three separate occasions to deplete their muscle glycogen stores. Immediately and 2 h after exercise they consumed either 0 (P), 1.5 (L), or 3.0 g glucose/kg body wt (H) from a 50% glucose polymer solution. Blood samples were drawn from an antecubital vein before exercise, during exercise, and throughout recovery. Muscle biopsies were taken from the vastus lateralis immediately, 2 h, and 4 h after exercise. Blood glucose and insulin declined significantly during exercise in each of the three treatments. They remained below the preexercise concentrations during recovery in the P treatment but increased significantly above the preexercise concentrations during the L and H treatments. By the end of the 4 h-recovery period, blood glucose and insulin were still significantly above the preexercise concentrations in both treatments. Muscle glycogen storage was significantly increased above the basal rate (P, 0.5 mumol.g wet wt-1.h-1) after ingestion of either glucose polymer supplement. The rates of muscle glycogen storage, however, were not different between the L and H treatments during the first 2 h (L, 5.2 +/- 0.9 vs. H, 5.8 +/- 0.7 mumol.g wet wt-1.h-1) or the second 2 h of recovery (L, 4.0 +/- 0.9 vs. H, 4.5 +/- 0.6 mumol.g wet wt-1. h-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of carbohydrate ingestion on exercise metabolism

Five male cyclists were studied during 2 h of cycle ergometer exercise (70% VO2 max) on two occasions to examine the effect of carbohydrate ingestion on muscle glycogen utilization. In the experimental trial (CHO) subjects ingested 250 ml of a glucose polymer solution containing 30 g of carbohydrate at 0, 30, 60, and 90 min of exercise; in the control trial (CON) they received an equal volume of a sweet placebo. No differences between trials were seen in O2 uptake or heart rate during exercise. Venous blood glucose was similar before exercise in both trials, but, on average, was higher during exercise in CHO [5.2 +/- 0.2 (SE) mmol/l] compared with CON (4.8 +/- 0.1, P less than 0.05). Plasma insulin levels were similar in both trials. Muscle glycogen levels were also similar in CHO and CON both before and after exercise; accordingly, there was no difference between trials in the amount of glycogen used during the 2 h of exercise (CHO = 62.8 +/- 10.1 mmol/kg wet wt, CON = 56.9 +/- 10.1). The results of this study indicate that carbohydrate ingestion does not influence the utilization of muscle glycogen during prolonged strenuous exercise.     

Carbohydrate ingestion and muscle glycogen depletion during marathon and ultramarathon racing

Two studies were undertaken to characterize the effects of carbohydrate ingestion on fuel/hormone response to exercise and muscle glycogen utilization during prolonged competitive exercise. In study 1, eighteen subjects were divided into three groups, matched for maximum oxygen consumption (VO2max) and blood lactate turnpoint. All subjects underwent a 3-day carbohydrate (CHO) depletion phase, followed by 3 days of CHO loading (500-600 g.day-1). During the race, the groups drank either 2% glucose (G), 8% glucose polymer (GP), or 8% fructose (F). Muscle biopsies were performed before and after the race and venous blood was sampled before and at regular intervals during the race. In study 2, eighteen subjects divided into 2 matched groups ingested either a 4% G or 10% GP solution during a 56 km race. Despite significantly greater CHO ingestion by GP and F in study 1 and by GP in study 2, blood glucose, free fatty acids and insulin concentrations, muscle glycogen utilization and running performance were not different between groups. These studies show (i) that hypoglycaemia is uncommon in athletes competing in races of up to 56 km provided they CHO-load before and ingest a minimum of 10 g CHO.h-1 during competition; (ii) that neither the amount (10 g vs 40 g.h-1) nor the type of carbohydrate (G vs GP vs F) has any effect on the extent of muscle glycogen depletion or running performance in matched subjects racing over distances up to 56 km.     

Effects of recovery beverages on glycogen restoration and endurance exercise performance

The restorative capacities of a high carbohydrate-protein (CHO-PRO) beverage containing electrolytes and a traditional 6% carbohydrate-electrolyte sports beverage (SB) were assessed after glycogen-depleting exercise. Postexercise ingestion of the CHO-PRO beverage, in comparison with the SB, resulted in a 55% greater time to exhaustion during a subsequent exercise bout at 85% maximum oxygen consumption (VO(2)max). The greater recovery after the intake of the CHO-PRO beverage could be because of a greater rate of muscle glycogen storage. Therefore, a second study was designed to investigate the effects of after exercise CHO-PRO and SB supplements on muscle glycogen restoration. Eight endurance-trained cyclists (VO(2)max = 62.1 +/- 2.2 ml.kg(-1) body wt.min(-1)) performed 2 trials consisting of a 2-hour glycogen-depletion ride at 65-75% VO(2)max. Carbohydrate-protein (355 ml; approximately 0.8 g carbohydrate (CHO).kg(-1) body wt and approximately 0.2 g protein.kg(-1) body wt) or SB (355 ml; approximately 0.3 g CHO.kg(-1) body wt) was provided immediately and 2 hours after exercise. Trials were randomized and separated by 7-15 days. Ingestion of the CHO-PRO beverage resulted in a 17% greater plasma glucose response, a 92% greater insulin response, and a 128% greater storage of muscle glycogen (159 +/- 18 and 69 +/- 32 micromol.g(-1) dry weight for CHO-PRO and SB, respectively) compared with the SB (p < 0.05). These findings indicate that the rate of recovery is coupled with the rate of muscle glycogen replenishment and suggest that recovery supplements should be consumed to optimize muscle glycogen synthesis as well as fluid replacement.     

Time course of glycogen accumulation after eccentric exercise.

This study examined the time course of glycogen accumulation in skeletal muscle depleted by concentric work and subsequently subjected to eccentric exercise. Eight men exercised to exhaustion on a cycle ergometer [70% of maximal O2 consumption (VO2max)] and were placed on a carbohydrate-restricted diet. Approximately 12 h later they exercised one leg to subjective failure by repeated eccentric action of the knee extensors against a resistance equal to 120% of their one-repetition maximum concentric knee extension force (ECC leg). The contralateral leg was not exercised and served as a control (CON leg). During the 72-h recovery period, subjects consumed 7 g carbohydrate.kg body wt-1.day-1. Moderate soreness was experienced in the ECC leg 24-72 h after eccentric exercise. Muscle biopsies from the vastus lateralis of the ECC and CON legs revealed similar glycogen levels immediately after eccentric exercise (40.2 +/- 5.2 and 47.6 +/- 6.4 mmol/kg wet wt, respectively; P greater than 0.05). There was no difference in the glycogen content of ECC and CON legs after 6 h of recovery (77.7 +/- 7.9 and 85.1 +/- 4.9 mmol/kg wet wt, respectively; P greater than 0.05), but 18 h later, the ECC leg contained 15% less glycogen than the CON leg (90.2 +/- 8.2 vs. 105.8 +/- 8.9 mmol/kg wet wt; P less than 0.05). After 72 h of recovery, this difference had increased to 24% (115.8 +/- 8.0 vs. 153.0 +/- 12.2 mmol/kg wet wt; P less than 0.05). These data confirm that glycogen accumulation is impaired in eccentrically exercised muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate

The purpose of this study was to determine whether the postponement of fatigue in subjects fed carbohydrate during prolonged strenuous exercise is associated with a slowing of muscle glycogen depletion. Seven endurance-trained cyclists exercised at 71 +/- 1% of maximal O2 consumption (VO2max), to fatigue, while ingesting a flavored water solution (i.e., placebo) during one trial and while ingesting a glucose polymer solution (i.e., 2.0 g/kg at 20 min and 0.4 g/kg every 20 min thereafter) during another trial. Fatigue during the placebo trial occurred after 3.02 +/- 0.19 h of exercise and was preceded by a decline (P less than 0.01) in plasma glucose to 2.5 +/- 0.5 mM and by a decline in the respiratory exchange ratio (i.e., R; from 0.85 to 0.80; P less than 0.05). Glycogen within the vastus lateralis muscle declined at an average rate of 51.5 +/- 5.4 mmol glucosyl units (GU) X kg-1 X h-1 during the first 2 h of exercise and at a slower rate (P less than 0.01) of 23.0 +/- 14.3 mmol GU X kg-1 X h-1 during the third and final hour. When fed carbohydrate, which maintained plasma glucose concentration (4.2-5.2 mM), the subjects exercised for an additional hour before fatiguing (4.02 +/- 0.33 h; P less than 0.01) and maintained their initial R (i.e., 0.86) and rate of carbohydrate oxidation throughout exercise. The pattern of muscle glycogen utilization, however, was not different during the first 3 h of exercise with the placebo or the carbohydrate feedings. The additional hour of exercise performed when fed carbohydrate was accomplished with little reliance on muscle glycogen (i.e., 5 mmol GU X kg-1 X h-1; NS) and without compromising carbohydrate oxidation. We conclude that when they are fed carbohydrate, highly trained endurance athletes are capable of oxidizing carbohydrate at relatively high rates from sources other than muscle glycogen during the latter stages of prolonged strenuous exercise and that this postpones fatigue.     

Rat lung glucose metabolism after 24 h of exposure to 100% oxygen

Previous studies with lung homogenates and isolated cells have suggested oxygen cell injury results from the inhibition of key enzymes involved in both cytosolic and mitochondrial energy generation. In this study, the extent and pattern of metabolism of D-[U-14C, 5-3H]glucose was examined in perfused lungs isolated from rats before and after 24 h of in vivo exposure to 100% O2. Lung ATP levels after O2 exposure were maintained by a 53% increase in glucose utilization from an unexposed control value of 18.0 +/- 3.2 to 27.5 +/- 3.0 mumol 3H2O.h-1.g dry wt-1, accounted for by an enhanced rate of lactate plus pyruvate production from 15.7 +/- 2.0 to 32.7 +/- 4.1 mumol.h-1.g dry wt-1 with no alteration in lactate-to-pyruvate ratio. CO2 production was unaltered from a control rate of 27.5 +/- 4.0 14CO2 mumol.h-1.g dry wt-1. Maximal rates of glucose metabolism were determined by perfusion with 0.8 mM dinitrophenol, giving for air-exposed lungs a rate of 53.5 +/- 5.0 mumol 3H2O.h-1.g dry wt-1 and increased lactate plus pyruvate and 14CO2 production rates of 46.5 +/- 6.5 and 128.3 +/- 19.6 mumol.h-1.g dry wt-1, respectively. Although this maximal rate of glucose utilization was unaltered in oxygen-exposed lungs, lactate plus pyruvate production was further increased to 80.0 +/- 9.1 mumol.h-1.g dry wt-1 with a concomitant decrease in the dinitrophenol-induced rate of 14CO2 production to 81.5 +/- 9.2 mumol.h-1.g dry wt-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Early postexercise muscle glycogen recovery is enhanced with a carbohydrate-protein supplement.

In the present study, we tested the hypothesis that a carbohydrate-protein (CHO-Pro) supplement would be more effective in the replenishment of muscle glycogen after exercise compared with a carbohydrate supplement of equal carbohydrate content (LCHO) or caloric equivalency (HCHO). After 2.5 +/- 0.1 h of intense cycling to deplete the muscle glycogen stores, subjects (n = 7) received, using a rank-ordered design, a CHO-Pro (80 g CHO, 28 g Pro, 6 g fat), LCHO (80 g CHO, 6 g fat), or HCHO (108 g CHO, 6 g fat) supplement immediately after exercise (10 min) and 2 h postexercise. Before exercise and during 4 h of recovery, muscle glycogen of the vastus lateralis was determined periodically by nuclear magnetic resonance spectroscopy. Exercise significantly reduced the muscle glycogen stores (final concentrations: 40.9 +/- 5.9 mmol/l CHO-Pro, 41.9 +/- 5.7 mmol/l HCHO, 40.7 +/- 5.0 mmol/l LCHO). After 240 min of recovery, muscle glycogen was significantly greater for the CHO-Pro treatment (88.8 +/- 4.4 mmol/l) when compared with the LCHO (70.0 +/- 4.0 mmol/l; P = 0.004) and HCHO (75.5 +/- 2.8 mmol/l; P = 0.013) treatments. Glycogen storage did not differ significantly between the LCHO and HCHO treatments. There were no significant differences in the plasma insulin responses among treatments, although plasma glucose was significantly lower during the CHO-Pro treatment. These results suggest that a CHO-Pro supplement is more effective for the rapid replenishment of muscle glycogen after exercise than a CHO supplement of equal CHO or caloric content.     

Influence of carbohydrate dosage on exercise performance and glycogen metabolism

This study was undertaken to examine the effects of ingestion of carbohydrate (CHO) solutions of 0 (WP), 6 (CHO-6), 12 (CHO-12), and 18 g CHO/100 ml (CHO-18) on performance and muscle glycogen use. Ten trained cyclists performed five 120-min cycling trials. The first 105 min of each trial was at 70% of maximal O2 consumption (VO2max), and the final 15 min was an all-out performance ride on an isokinetic cycle ergometer equipped to measure total work output. In one of the trials (CHO-12I) the submaximal portion of the ride consisted of seven 15-min rides at 70% of VO2max with a 3-min rest between each ride. Every 15 min the men consumed 8.5 ml.kg-1.h-1 (approximately 150 ml) of one of the four test solutions. Venous blood samples were obtained every 15 min for glucose and insulin. Muscle biopsies were obtained from the vastus lateralis at 0 and 105 min in the WP and the CHO-12 continuous and intermittent trials. Biopsy samples were assayed for glycogen and sectioned and stained for myosin adenosinetriphosphatase and glycogen for single fiber depletion measurements. There were no differences in glycogen use (86.7 +/- 6.0, 75.5 +/- 7.9, and 83.5 +/- 5.5 mmol/kg for the WP, CHO-12C, and CHO-12I, respectively) or depletion patterns between the WP and the two CHO-12 trials. Blood glucose was significantly elevated in both the CHO-12 trials and in the CHO-18 trial compared with the WP trial.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of insulin on exercise-induced GLUT-4 protein expression and glycogen supercompensation in rat skeletal muscle.

The purpose of this study was to investigate the role of insulin on skeletal muscle GLUT-4 protein expression and glycogen storage after postexercise carbohydrate supplementation. Male Sprague-Dawley rats were randomly assigned to one of six treatment groups: sedentary control (Con), Con with streptozocin (Stz/C), immediately postexercise (Ex0), Ex0 with Stz (Stz/Ex0), 5-h postexercise (Ex5), and Ex5 with Stz (Stz/Ex5). Rats were exercised by swimming (2 bouts of 3 h) and carbohydrate supplemented immediately after each exercise session by glucose intubation (1 ml of a 50% wt/vol). Stz was administered 72-h before exercise, which resulted in hyperglycemia and elimination of the insulin response to the carbohydrate supplement. GLUT-4 protein of Ex0 rats was 30% above Con in fast-twitch (FT) red and 21% above Con in FT white muscle. In Ex5, GLUT-4 protein was 52% above Con in FT red and 47% above Con in FT white muscle. Muscle glycogen in FT red and white muscle was also increased above Con in Ex5 rats. Neither GLUT-4 protein nor muscle glycogen was increased above Con in Stz/Ex0 or Stz/Ex5 rats. GLUT-4 mRNA in FT red muscle of Ex0 rats was 61% above Con but only 33% above Con in Ex5 rats. GLUT-4 mRNA in FT red muscle of Stz/C and Stz/Ex0 rats was similar but significantly elevated in Ex5/Stz rats. These results suggest that insulin is essential for the increase in GLUT-4 protein expression following postexercise carbohydrate supplementation.     

Muscle glycogen storage postexercise: effect of mode of carbohydrate administration

The primary purpose of this study was to determine whether gastric emptying limits the rate of muscle glycogen storage during the initial 4 h after exercise when a carbohydrate supplement is provided. A secondary purpose was to determine whether liquid (L) and solid (S) carbohydrate (CHO) feedings result in different rates of muscle glycogen storage after exercise. Eight subjects cycled for 2 h on three separate occasions to deplete their muscle glycogen stores. After each exercise bout they received 3 g CHO/kg body wt in L (50% glucose polymer) or S (rice/banana cake) form or by intravenous infusion (I; 20% sterile glucose). The L and S supplements were divided into two equal doses and administered immediately after and 120 min after exercise, whereas the I supplement was administered continuously during the first 235 min of the 240-min recovery period. Blood samples were drawn from an antecubital vein before exercise, during exercise, and throughout recovery. Muscle biopsies were taken from the vastus lateralis immediately after and 120 and 240 min after exercise. Blood glucose and insulin declined during exercise and increased significantly above preexercise levels during recovery in all treatments. The increase in blood glucose during the I treatment, however, was three times greater than during the L or S treatments. The average insulin response of the L treatment (61.7 +/- 4.9 microU/ml) was significantly greater than that of the S treatment (47.5 +/- 4.2 microU/ml) but not that of the I (55.3 +/- 4.5 microU/ml) treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Energy metabolism of medium-chain triglycerides versus carbohydrates during exercise

Medium-chain triglycerides (MCT) are known to be rapidly digested and oxidized. Their potential value as a source of dietary energy during exercise was compared with that of maltodextrins (MD). Twelve subjects exercised for 1 h on a bicycle ergometer (60% VO2 max), 1 h after the test meal (1MJ). The metabolism of MCT was followed using 1-13C-octanoate (Oc) as tracer and U-13C-glucose (G) was added to the 13C-naturally enriched MD. After MCT ingestion no insulin peak was observed with some accumulation of ketone bodies (KB), blood levels not exceeding 1 mM. Total losses of KB during exercise in urine, sweat and as breath acetone were small (less than 0.2 mmol X h-1). Hence, the influence of KB loss and storage on gas exchange data was negligible. The partition of fat and carbohydrate utilization during exercise as obtained by indirect calorimetry was practically the same after the MCT and the CHO meals. Oxidation over the 2-h period was 30% of dose for Oc and 45% for G. Glycogen decrements in the Vastus lateralis muscle were equal. It appears that with normal carbohydrate stores, a single meal of MCT or CHO did not alter the contribution of carbohydrates during 1 h of high submaximal exercise. The moderate ketonemia after MCT, despite substantial oxidation of this fat, led to no difference in muscle glycogen sparing between the diets.     

Hypohydration does not impair skeletal muscle glycogen resynthesis after exercise

The purpose of this investigation was to examine the effects of moderate hypohydration (HY) on skeletal muscle glycogen resynthesis after exhaustive exercise. On two occasions, eight males completed 2 h of intermittent cycle ergometer exercise (4 bouts of 17 min at 60% and 3 min at 80% of maximal O2 consumption/10 min rest) to reduce muscle glycogen concentrations (control values 711 +/- 41 mumol/g dry wt). During one trial, cycle exercise was followed by several hours of light upper body exercise in the heat without fluid replacement to induce HY (-5% body wt); in the second trial, sufficient water was ingested during the upper body exercise and heat exposure to maintain euhydration (EU). In both trials, 400 g of carbohydrate were ingested at the completion of exercise and followed by 15 h of rest while the desired hydration level was maintained. Muscle biopsy samples were obtained from the vastus lateralis immediately after intermittent cycle exercise (T1) and after 15 h of rest (T2). During the HY trial, the muscle water content was lower (P less than 0.05) at T1 and T2 (288 +/- 9 and 265 +/- 5 ml/100 g dry wt, respectively; NS) than during EU (313 +/- 8 and 301 +/- 4 ml/100 g dry wt, respectively; NS). Muscle glycogen concentration was not significantly different during EU and HY at T1 (200 +/- 35 vs. 251 +/- 50 mumol/g dry wt) or T2 (452 +/- 34 vs. 491 +/- 35 mumol/g dry wt). These data indicate that, despite reduced water content during the first 15 h after heavy exercise, skeletal muscle glycogen resynthesis is not impaired.     

Effects of postexercise carbohydrate-protein feedings on muscle glycogen restoration.

The purpose of this investigation was to determine the effects of postexercise eucaloric carbohydrate-protein feedings on muscle glycogen restoration after an exhaustive cycle ergometer exercise bout. Seven male collegiate cyclists [age = 25.6 +/- 1.3 yr, height = 180.9 +/- 3.2 cm, wt = 75.4 +/- 4.0 kg, peak oxygen uptake (VO(2 peak)) = 4.20 +/- 0.2 l/min] performed three trials, each separated by 1 wk: 1) 100% alpha-D-glucose [carbohydrate (CHO)], 2) 70% carbohydrate-20% protein (PRO)-10% fat, and 3) 86% carbohydrate-14% amino acid (AA). All feedings were eucaloric, based on 1.0 g. kg body wt(-1). h(-1) of CHO, and administered every 30 min during a 4-h muscle glycogen restoration period in an 18% wt/vol solution. Muscle biopsies were obtained immediately and 4 h after exercise. Blood samples were drawn immediately after the exercise bout and every 0.5 h for 4 h during the restoration period. Increases in muscle glycogen concentrations for the three feedings (CHO, CHO-PRO, CHO-AA) were 118 mmol/kg dry wt; however, no differences among the feedings were apparent. The serum glucose and insulin responses did not differ throughout the restoration period among the three feedings. These results suggest that muscle glycogen restoration does not appear to be enhanced with the addition of proteins or amino acids to an eucaloric CHO feeding after exhaustive cycle exercise.     

Carbohydrate supercompensation and muscle glycogen utilization during exhaustive running in highly trained athletes

Three female and three male highly trained endurance runners with mean maximal oxygen uptake (VO2max) values of 60.5 and 71.5 ml.kg-1.min-1, respectively, ran to exhaustion at 75%-80% of VO2max on two occasions after an overnight fast. One experiment was performed after a normal diet and training regimen (Norm), the other after a diet and training programme intended to increase muscle glycogen levels (Carb). Muscle glycogen concentration in the gastrocnemius muscle increased by 25% (P less than 0.05) from 581 mmol.kg-1 dry weight, SEM 50 to 722 mmol.kg-1 dry weight, SEM 34 after Carb. Running time to exhaustion, however, was not significantly different in Carb and Norm, 77 min, SEM 13 vs 70 min, SEM 8, respectively. The average glycogen concentration following exhaustive running was 553 mmol.kg-1 dry weight, SEM 70 in Carb and 434 mmol.kg-1 dry weight, SEM 57 in Norm, indicating that in both tests muscle glycogen stores were decreased by about 25%. Periodic acid-Schiff staining for semi-quantitative glycogen determination in individual fibres confirmed that none of the fibres appeared to be glycogen-empty after exhaustive running. The steady-state respiratory exchange ratio was higher in Carb than in Norm (0.92, SEM 0.01 vs 0.89, SEM 0.01; P less than 0.05). Since muscle glycogen utilization was identical in the two tests, the indication of higher utilization of total carbohydrate appears to be related to a higher utilization of liver glycogen. We have concluded that glycogen depletion of the gastrocnemius muscle is unlikely to be the cause of fatigue during exhaustive running at 75%-80% of VO2max in highly trained endurance runners. Furthermore, diet- and training-induced carbohydrate super-compensation does not appear to improve endurance capacity in such individuals.     

Improvements in exercise performance: effects of carbohydrate feedings and diet

In an effort to determine the effects of carbohydrate (CHO) feedings immediately before exercise in both the fasted and fed state, 10 well-trained male cyclists [maximum O2 consumption (VO2 max), 4.35 +/- 0.11 l/min)] performed 45 min of cycling at 77% VO2 max followed by a 15-min performance ride on an isokinetic cycle ergometer. After a 12-h fast, subjects ingested 45 g of liquid carbohydrate (LCHO), solid carbohydrate confectionery bar (SCHO), or placebo (P) 5 min before exercise. An additional trial was performed in which a high-CHO meal (200 g) taken 4 h before exercise was combined with a confectionery bar feeding (M + SCHO) immediately before the activity. At 10 min of exercise, serum glucose values were elevated by 18 and 24% during SCHO and LCHO, respectively, compared with P. At 0 and 45 min no significant differences were observed in muscle glycogen concentration or total use between the four trials. Total work produced during the final 15 min of exercise was significantly greater (P less than 0.05) during M + SCHO (194,735 +/- 9,448 N X m), compared with all other trials and significantly greater (P less than 0.05) during LCHO and SCHO (175,204 +/- 11,780 and 176,013 +/- 10,465 N X m, respectively) than trial P (159,143 +/- 11,407 N X m). These results suggest that, under conditions when CHO stores are less than optimal, exercise performance is enhanced with the ingestion of 45 g of CHO 5 min before 1 h of intense cycling.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of elevated FFA on carbohydrate and lipid oxidation during prolonged exercise in humans

Increased availability of circulating free fatty acids (FFA) inhibits the rate of glycolysis in heart and resting skeletal muscle (Randle effect). Whether elevated FFA may play a role in decreasing carbohydrate oxidation during prolonged exercise in humans is more controversial. Using respiratory exchange measurements, we measured substrate utilization during 2.5 h of exercise at approximately 44 +/- 1% maximal O2 uptake (VO2 max) in the presence or absence of elevated FFA levels. After 30 min of base-line determinations, 1,000 U heparin was given intravenously and a 3-h constant infusion of Intralipid 10% (150 g/h) and heparin (500 U/h) was started. After an additional 30 min of rest, subjects exercised for 2.5 h (study 1, n = 6). In another five subjects (study 2) 100 g glucose was ingested after 30 min of exercise. The same protocols (studies 1 and 2) were also performed during a 0.9%-saline infusion. During exercise, without glucose ingestion, higher FFA concentrations prevailed during the Intralipid infusion (1,122 +/- 40 vs. 782 +/- 65 mumol/l), but the relative contributions of carbohydrate (49 +/- 4 vs. 50 +/- 4%) or lipid (49 +/- 4 vs. 47 +/- 6%) oxidation to the total energy expenditure were different only during the first 30 min of exercise. Similarly, higher FFA levels (1,032 +/- 62 vs. 568 +/- 46 mumol/l) did not alter the relative contributions of carbohydrate (62 +/- 4 vs. 69 +/- 2%) or lipid (36 +/- 4 vs. 29 +/- 2%) oxidation to the total energy expenditure after glucose feeding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Carbohydrate ingestion influences skeletal muscle cytokine mRNA and plasma cytokine levels after a 3-h run.

Sixteen experienced marathoners ran on treadmills for 3 h at approximately 70% maximal oxygen consumption (Vo(2 max)) on two occasions while receiving 1 l/h carbohydrate (CHO) or placebo (Pla) beverages. Blood and vastus lateralis muscle biopsy samples were collected before and after exercise. Plasma was analyzed for IL-6, IL-10, IL-1 receptor agonist (IL-1ra), IL-8, cortisol, glucose, and insulin. Muscle was analyzed for glycogen content and relative gene expression of 13 cytokines by using real-time quantitative RT-PCR. Plasma glucose and insulin were higher, and cortisol, IL-6, IL-10, and IL-1ra, but not IL-8, were significantly lower postexercise in CHO vs. Pla. Change in muscle glycogen content did not differ between CHO and Pla (P = 0.246). Muscle cytokine mRNA content was detected preexercise for seven cytokines in this order (highest to lowest): IL-15, TNF-alpha, IL-8, IL-1beta, IL-12p35, IL-6, and IFN-gamma. After subjects ran for 3 h, gene expression above prerun levels was measured for five of these cytokines: IL-1beta, IL-6, and IL-8 (large increases), and IL-10 and TNF-alpha (small increases). The increase in mRNA (fold difference from preexercise) was attenuated in CHO (15.9-fold) compared with Pla (35.2-fold) for IL-6 (P = 0.071) and IL-8 (CHO, 7.8-fold; Pla, 23.3-fold; P = 0.063). CHO compared with Pla beverage ingestion attenuates the increase in plasma IL-6, IL-10, and IL-1ra and gene expression for IL-6 and IL-8 in athletes running 3 h at 70% Vo(2 max) despite no differences in muscle glycogen content.     

Postexercise muscle glycogen resynthesis in obese insulin-resistant Zucker rats.

We determined the effect of an acute bout of swimming (8 x 30 min) followed by either carbohydrate administration (0.5 mg/g glucose ip and ad libitum access to chow; CHO) or fasting (Fast) on postexercise glycogen resynthesis in soleus muscle and liver from female lean (ZL) and obese insulin-resistant (ZO) Zucker rats. Resting soleus muscle glycogen concentration ([glycogen]) was similar between genotypes and was reduced by 73 (ZL) and 63% (ZO) after exercise (P < 0.05). Liver [glycogen] at rest was greater in ZO than ZL (334 +/- 31 vs. 247 +/- 16 micromol/g wet wt; P < 0.01) and fell by 44 and 94% after exercise (P < 0.05). The fractional activity of glycogen synthase (active/total) increased immediately after exercise (from 0.22 +/- 0.05 and 0.32 +/- 0.04 to 0.63 +/- 0.08 vs. 0.57 +/- 0.05; P < 0.01 for ZL and ZO rats, respectively) and remained elevated above resting values after 30 min of recovery. During this time, muscle [glycogen] in ZO increased 68% with CHO (P < 0.05) but did not change in Fast. Muscle [glycogen] was unchanged in ZL from postexercise values after both treatments. After 6 h recovery, GLUT-4 protein concentration was increased above resting levels by a similar extent for both genotypes in both fasted (approximately 45%) and CHO-supplemented (approximately 115%) rats. Accordingly, during this time CHO refeeding resulted in supercompensation in both genotypes (68% vs. 44% for ZL and ZO). With CHO, liver [glycogen] was restored to resting levels in ZL but remained at postexercise values for ZO after both treatments. We conclude that the increased glucose availability with carbohydrate refeeding after glycogen-depleting exercise resulted in glycogen supercompensation, even in the face of muscle insulin-resistance.     

Transgenic mice overexpressing GLUT-1 protein in muscle exhibit increased muscle glycogenesis after exercise

The purpose of the present study was to determine the rates of muscle glycogenolysis and glycogenesis during and after exercise in GLUT-1 transgenic mice and their age-matched littermates. Male transgenic mice (TG) expressing a high level of human GLUT-1 and their nontransgenic (NT) littermates underwent 3 h of swimming. Glycogen concentration was determined in gastrocnemius and extensor digitorum longus (EDL) muscles before exercise and at 0, 5, and 24 h postexercise, during which food (chow) and 10% glucose solution (as drinking water) were provided. Exercise resulted in approximately 90% reduction in muscle glycogen in both NT (from 11.2 +/- 1.4 to 2. 1 +/- 1.3 micromol/g) and TG (from 99.3 +/- 4.7 to 11.8 +/- 4.3 micromol/g) in gastrocnemius muscle. During recovery from exercise, the glycogen concentration increased to 38.2 +/- 7.3 (5 h postexercise) and 40.5 +/- 2.8 micromol/g (24 h postexercise) in NT mice. In TG mice, however, the increase in muscle glycogen concentration during recovery was greater (to 57.5 +/- 7.4 and 152.1 +/- 15.7 micromol/g at 5 and 24 h postexercise, respectively). Similar results were obtained from EDL muscle. The rate of 2-deoxyglucose uptake measured in isolated EDL muscles was 7- to 10-fold higher in TG mice at rest and at 0 and 5 h postexercise. There was no difference in muscle glycogen synthase activation measured in gastrocnemius muscles between NT and TG mice immediately after exercise. These results demonstrate that the rate of muscle glycogen accumulation postexercise exhibits two phases in TG: 1) an early phase (0-5 h), with rapid glycogen accumulation similar to that of NT mice, and 2) a progressive increase in muscle glycogen concentration, which differs from that of NT mice, during the second phase (5-24 h). Our data suggest that the high level of steady-state muscle glycogen in TG mice is due to the increase in muscle glucose transport activity.