Effects of pre-exercise carbohydrate feedings on muscle glycogen use during exercise in well-trained runners

Summary The purpose of this study was to examine the effects of pre-exercise glucose and fructose feedings on muscle glycogen utilization during exercise in six well-trained runners ( =68.2±3.4 ml·kg^–1·min^–1). On three separate occasions, the runners performed a 30 min treadmill run at 70% . Thirty minutes prior to exercise each runner ingested 75 g of glucose (trial G), 75 g of fructose (trial F) or 150 ml of a sweetened placebo (trial C). During exercise, no differences were observed between any of the trials for oxygen uptake, heart rate or perceived exertion. Serum glucose levels were elevated as a result of the glucose feeding (P<0.05) reaching peak levels at 30 min post-feeding (7.90±0.24 mmol·l^–1). With the onset of exercise, glucose levels dropped to a low of 5.89±0.85 mmol·l^–1 at 15 min of exercise in trial G. Serum glucose levels in trials F and C averaged 6.21±0.31 mmol·l^–1 and 5.95±0.23 mmol·l^–1 respectively, and were not significantly different (P<0.05). There were also no differences in serum glucose levels between any of the trials at 15 and 30 min of exercise. Muscle glycogen utilization in the first 15 min of exercise was similar in trial C (18.8±8.3 mmol·kg^–1), trial F (16.3±3.8 mmol·kg^–1) and trial G (17.0±1.8 mmol·kg^–1), and total glycogen use was also similar in trial C (25.6±7.9 mmol·kg^–1), trial F (35.4±5.7 mmol·kg^–1) and trial G (24.6±3.2 mmol·kg^–1). In contrast to previous research, these results suggest that pre-exercise feedings of fructose or glucose do not affect the rate of muscle glycogen utilization during 30 min of treadmill running in trained runners.     

Carbohydrate supplementation during prolonged cycling exercise spares muscle glycogen but does not affect intramyocellular lipid use

Using contemporary stable-isotope methodology and fluorescence microscopy, we assessed the impact of carbohydrate supplementation on whole-body and fiber-type-specific intramyocellular triacylglycerol (IMTG) and glycogen use during prolonged endurance exercise. Ten endurance-trained male subjects were studied twice during 3 h of cycling at 63 ± 4% of maximal O₂ uptake with either glucose ingestion (CHO trial; 0.7 g CHO kg⁻¹ h⁻¹) or without (CON placebo trial; water only). Continuous infusions with [U-¹³C] palmitate and [6,6-²H₂] glucose were applied to quantify plasma free fatty acids (FFA) and glucose oxidation rates and to estimate intramyocellular lipid and glycogen use. Before and after exercise, muscle biopsy samples were taken to quantify fiber-type-specific IMTG and glycogen content. Plasma glucose rate of appearance (R _a) and carbohydrate oxidation rates were substantially greater in the CHO vs CON trial. Carbohydrate supplementation resulted in a lower muscle glycogen use during the first hour of exercise in the CHO vs CON trial, resulting in a 38 ± 19 and 57 ± 22% decreased utilization in type I and II muscle-fiber glycogen content, respectively. In the CHO trial, both plasma FFA R _a and subsequent plasma FFA concentrations were lower, resulting in a 34 ± 12% reduction in plasma FFA oxidation rates during exercise (P < 0.05). Carbohydrate intake did not augment IMTG utilization, as fluorescence microscopy revealed a 76 ± 21 and 78 ± 22% reduction in type I muscle-fiber lipid content in the CHO and CON trial, respectively. We conclude that carbohydrate supplementation during prolonged cycling exercise does not modulate IMTG use but spares muscle glycogen use during the initial stages of exercise in endurance-trained men.     

Effect of creatine supplementation on creatine and glycogen content in rat skeletal muscle

The effects of high dose creatine feeding (5 g kg(-1) BW day(-1), 5 days) on creatine content, glucose transport, and glycogen accumulation in white gastrocnemius, red gastrocnemius and soleus muscles of the rat was investigated. Isolated rat hindquarters of creatine fed and control rats were perfused with a standard medium containing either insulin alone (0, 100 or 20 000 microU mL(-1)) or in combination with creatine (2 or 10 mmol L(-1)). Furthermore, plasma insulin concentration was measured in normal rats during creatine feeding, as well as in anaesthetized rats during intravenous creatine infusion. Five days of creatine feeding increased (P < 0.05) total creatine content in soleus (+ 20%) but not in red gastrocnemius (+15%, n.s.) and white gastrocnemius (+ 10%, n.s.). In parallel, glycogen content was markedly elevated (P < 0.05) in soleus (+ 40%), less (P < 0.05) in red gastrocnemius (+ 15%), and not in white gastrocnemius (+ 10%, n.s.). Glucose transport rate, muscle GLUT-4 content, glycogen synthase activity in perfused muscles and glycogen synthesis rate were not significantly altered by creatine feeding in either muscle type. Furthermore, high dose creatine feeding raised (P < 0.05) plasma creatine concentration fivefold but did not alter circulating insulin level. It is concluded that short-term high dose creatine feeding enhances creatine disposal and glycogen storage in rat skeletal muscle. However, the creatine and glycogen response to creatine supplementation is markedly greater in oxidative than in glycolytic muscles.     

Post-exercise ketosis and the glycogen content of liver and muscle in rats on a high carbohydrate diet

Summary Post-exercise ketosis is known to be suppressed by physical training and by a high carbohydrate diet. As a result it has often been presumed, but not proven, that the development of post-exercise ketosis is closely related to the glycogen content of the liver. We therefore studied the effect of 1 h of treadmill running on the blood 3-hydroxybutyrate and liver and muscle glycogen concentrations of carbohydrate-loaded trained (n=72) and untrained rats (n=72). Resting liver and muscle glycogen levels were 25%–30% higher in the trained than in the untrained animals. The resting 3-hydroxybutyrate concentrations of both groups of rats were very low: <0.08 mmol·1⁻¹. Exercise did not significantly influence the blood 3-hydroxybutyrate concentrations of trained rats, but caused a marked post-exercise ketosis (1.40±0.40 mmol·1⁻¹ 1 h after exercise) in the untrained animals, the time-course of which was the approximate inverse of the changes in liver glycogen concentration. Interpreting the results in the light of similar data obtained after a normal and low carbohydrate diet it has been concluded that trained animals probably owe their relative resistance to post-exercise ketosis to their higher liver glycogen concentrations as well as to greater peripheral stores of mobilizable carbohydrate.     

The effects of carbohydrate intake and muscle glycogen content on self-paced intermittent-sprint exercise despite no knowledge of carbohydrate manipulation

The aim of this study was to determine the effects of carbohydrate (CHO) ingestion and muscle glycogen content, without the influence of knowledge of CHO consumption, on intermittent-sprint performance. Ten males completed two conditions on two consecutive days. Day 1 involved 2?×?40?min of leg cycling separated by 15?min of arm cycling, followed by an overnight diet consuming either a high [HCHO; 7?g/kg body weight (bw)] or low (LCHO; 2?g/kg bw) CHO diet. Participants were blinded to the knowledge CHO was being examined or manipulated. Day 2 included a 60-min intermittent-sprint exercise (ISE) protocol that included 15-m maximal sprints every minute and self-paced efforts of varying intensities. Pre and post-ISE muscle biopsies were obtained on Day 2. Pre- and post-exercise maximal voluntary torque (MVT), voluntary activation (VA) and twitch contractile properties were assessed during 15 maximal isometric contractions. Blood glucose and lactate, heart rate (HR) and rating of perceived exertion (RPE) were also recorded. Pre-ISE muscle glycogen was greater in HCHO compared with LCHO (597?±?115 vs. 318?±?72?mmol?kg?dry weight; P?=?0.001). Total distance and hard running distance were 4.9 and 8.1% greater in HCHO, respectively (P?=?0.02-0.04). Peak MVT, VA, HR and RPE were not different between conditions (P?>?0.05). Blood glucose was higher pre-ISE for LCHO but lower post-ISE compared with HCHO (P?相似文献   

Fat and carbohydrate metabolism during low intensity exercise: Effects of the availability of muscle glycogen

Summary Four subjects were studied during exercise at 50% of maximum oxygen uptake after a normal diet, after a low carbohydrate (CHO) diet following exercise-induced glycogen depletion, and after a high CHO diet. This regime has previously been shown to cause changes in the amount of glycogen stored in the exercising muscles. Metabolic and respiratory parameters were measured during the exercise. The respiratory exchange ratio, blood lactate, blood pyruvate, blood glucose and plasma triglycerides were lower than normal following the low CHO diet and higher than normal following the high CHO diet. Plasma free fatty acids and plasma glycerol were higher than normal after the low CHO diet and lower than normal after the high CHO diet. The contribution of CHO to metabolism was less than normal after the low CHO diet and greater than normal after the high CHO diet. The altered availability of FFA does not appear to be a result of the variations in the blood lactate content. R. J. M. is in receipt of a Science Research Council Postgraduate Studentship award     

Effects of timing of pre-exercise ingestion of carbohydrate on subsequent metabolism and cycling performance

The occurrence of rebound hypoglycaemia may depend on the timing of carbohydrate intake. The aim of the present study was to investigate the metabolic and performance responses to the ingestion of carbohydrate at differing times before exercise. Eight subjects [mean (SEM)] [28 (3) years, 74.5 (2.6) kg, maximal oxygen uptake 63.1 (3.1) ml·kg^–1·min^–1] performed three experiments. They ingested 75 g glucose dissolved in 500 ml water, thereafter resting for either 15, 45 or 75 min (15-Pre, 45-Pre and 75-Pre) before exercising for 20 min at 65% maximal power output followed by a time trial [total work 685 (18) kJ]. There were no differences in performance between conditions [mean powers 268 (10), 269 (7) and 276 (12) W for 15-Pre, 45-Pre and 75-Pre, respectively]. There were significant differences in plasma glucose concentration between 15-Pre [6.6 (0.6) mmol·l^–1; P<0.05] and both 45-Pre [4.5 (0.2) mmol·l^–1] and 75-Pre [3.7 (0.2) mmol·l^–1] immediately before exercise. Insulin concentrations immediately before exercise were higher (P<0.05) during 15-Pre [72.6 (10.4) μU·ml^–1] than during 45-Pre [50.8 (9.9) μU·ml^–1], which was higher (P<0.05) than during75-Pre [33.9 (5.5) μU·ml^–1]. These differences disappeared within 10 min of exercise. Two subjects became hypoglycaemic (plasma glucose concentration of less than 3.5 mmol·l^–1) in the 15-Pre while three and five subjects were transiently hypoglycaemic in the 45-Pre and 75-Pre, respectively. Performance and rating of perceived exertion did not seem to be related to hypoglycaemia. Altering the timing of the ingestion of carbohydrate before exercise resulted in differences in plasma glucose /insulin responses which disappeared within 10 min of exercise and which had no effect on performance. Hypoglycaemia was observed in some subjects during the first 10 min but this did not affect performance. Electronic Publication     

The effects of carbohydrate supplementation during the second of two prolonged cycling bouts on immunoendocrine responses

The purpose of this study was to examine the effect of carbohydrate (CHO) feeding during the second of two 90-min cycling bouts (EX1 started at 09:00 and EX2 started at 13:30) at 60% on leucocyte redistribution, neutrophil degranulation and oxidative burst and plasma IL-6 and stress hormone responses. This study consisted of two trials, which were completed in a counterbalanced order and separated by at least 4 days. Subjects (n=9) consumed a lemon flavoured 10% w/v CHO (glucose) or placebo (PLA) beverage during EX2: 500 ml just before exercise and 250 ml every 20 min during exercise. Venous blood samples were taken 5 min before exercise, immediately post-exercise, and 18-h post-EX2 for both trials. The main findings of this study were that ingestion of CHO compared with PLA during EX2 better maintained plasma glucose concentration, blunted the responses of plasma adrenaline, ACTH, cortisol, GH and IL-6, and attenuated the leukocytosis and monocytosis, but had no effect on neutrophil degranulation and oxidative burst activity. Furthermore, the immunoendocrine disturbances induced by two bouts of prolonged exercise returned to resting values within 18 h. These findings suggest that ingestion of CHO compared with PLA during the second of two bouts of 90-min cycling at 60% better maintains plasma glucose, blunts hypothalamic–pituitary–adrenal activation, and attenuates leucocyte trafficking, but does not affect neutrophil function. Furthermore, the disturbances of immunoendocrine responses induced by two bouts of prolonged exercise on the same day recover within 18 h.     

Effect of creatine supplementation on metabolism and performance in humans during intermittent sprint cycling

This double blind study investigated the effect of oral creatine supplementation (CrS) on 4 × 20 s of maximal sprinting on an air-braked cycle ergometer. Each sprint was separated by 20 s of recovery. A group of 16 triathletes [mean age 26.6 (SD 5.1) years. mean body mass 77.0 (SD 5.8) kg, mean body fat 12.9 (SD 4.6)%, maximal oxygen uptake 4.86 (SD 0.7) l · min⁻¹] performed an initial 4 × 20 s trial after a muscle biopsy sample had been taken at rest. The subjects were then matched on their total intramuscular creatine content (TCr) before being randomly assigned to groups to take by mouth either a creatine supplement (CRE) or a placebo (CON) before a second 4 × 20 s trial. A muscle biopsy sample was also taken immediately before this second trial. The CrS of 100 g comprised 4 × 5 g for 5 days. The initial mean TCr were 112.5 (SD 8.7) and 112.5 (SD 10.7) mmol · kg⁻¹ dry mass for CRE and CON, respectively. After creatine loading and placebo ingestion respectively, CRE [128.7 (SD 11.8) mmol · kg⁻¹ dry mass] had a greater (P=0.01) TCr than CON [112.0 (SD 10.0) mmol · kg⁻¹ dry mass]. While the increase in free creatine for CRE was statistically significant (P=0.034), this was not so for the changes in phosphocreatine content [trial 1: 75.7 (SD 6.9), trial 2: 84.7 (SD 11.0) mmol · kg⁻¹ dry mass, P=0.091]. There were no significant differences between CRE and CON for citrate synthase activity (P=0.163). There was a tendency towards improved performance in terms of 1 s peak power (in watts P=0.07; in watts per kilogram P=0.05), 5 s peak power (in watts P=0.08) and fatigue index (P=0.08) after CrS for sprint 1 of the second trial. However, there was no improvement for mean power (in watts P=0.15; in watts per kilogram P=0.1) in sprint 1 or for any performance values in subsequent sprints. Our results suggest that, while CrS elevates the intramuscular stores of free creatine, this does not have an ergogenic effect on 4 × 20 s all-out cycle sprints with intervening 20-s rest periods. Accepted: 2 October 2000     

Effects of pre-exercise ingestion of differing amounts of carbohydrate on subsequent metabolism and cycling performance

Studies on the effect of the pre-exercise ingestion of carbohydrate on metabolism and performance have produced conflicting results, perhaps because of differences in the designs of the studies. The purpose of the present study was to examine the effects of ingesting differing amounts of glucose pre-exercise on the glucose and insulin responses during exercise and on time-trial (TT) performance. Nine well-trained male cyclists completed four exercise trials separated by at least 3 days. At 45 min before the start of exercise subjects consumed 500 ml of a beverage containing either 0 g (PLAC), 25 g (LOW), 75 g (MED) or 200 g (HIGH) of glucose. The exercise trials consisted of 20 min of submaximal steady-state exercise (SS) at 65% of maximal power output immediately followed by a [mean (SEM)] 691 (12) kJ TT. Plasma insulin concentrations at the onset of exercise were significantly higher (P<0.05) in MED and HIGH compared with LOW and PLAC. Plasma glucose concentration fell rapidly (P<0.05) during SS exercise in all glucose trials, but remained steady in PLAC. No difference in plasma glucose concentration was observed between the glucose trials at any time. Hypoglycaemia (less than 3.5 mmol·l^–1) was observed in six subjects during SS but only after ingesting glucose pre-exercise. However, there was no difference in TT performance between the four trials. The ingestion of 0, 25, 75 or 200 g of glucose 45 min before a 20 min submaximal exercise bout did not affect subsequent TT performance. In addition, mild rebound hypoglycaemia following pre-exercise glucose ingestion did not negatively affect performance. Electronic Publication     

Mobilization and oxidative burst of neutrophils are influenced by carbohydrate supplementation during prolonged cycling in humans

Prolonged, strenuous exercise may lead to suppressive effects on the immune system, which might be responsible for a greater susceptibility to opportunistic infections. The aim of this study was to examine the influence of carbohydrate substitution (CHS) during prolonged, strenuous exercise on neutrophil granulocytes and their oxidative burst (intracellular oxidation of dihydrorhodamine₁₂₃ to rhodamine₁₂₃ after induction by formylized 1-methionyl-1-leucyl-1-phenylalanin) using flow cytometry. In three trials different concentrations of CHS (placebo compared to 6% and 12% CHS; 50 ml·kg^–1) were given randomly to 14 endurance trained cyclists [mean (SD) age 25 (5) years, maximal oxygen uptake 67 (6) ml·min^–1·kg^–1] cycling for 4 h in a steady state at 70% of their individual anaerobic threshold. Blood samples were taken before, immediately after cessation, 1 h and 19 h after exercise. A significant rise in neutrophil counts was observed immediately after cessation and 1 h after exercise with a return to normal rest values 19 h after exercise for all three conditions (P<0.001). The relative proportions of rhodamine₁₂₃+ neutrophils were significantly diminished in all three conditions 1 h after exercise (P<0.01), while the mean fluorescence intensity was lowest in the placebo trial and differed significantly to the 12% CHS trial (P=0.024) and almost significantly to the 6% CHS trial (P=0.052). In conclusion, these data suggest a beneficial effect of CHS on the neutrophil oxidative burst and a possible attenuation of the susceptibility to infections, presumably due to the reduction of metabolic stress in prolonged, strenuous exercise. Electronic Publication     

Effects of acute carbohydrate supplementation during sessions of high-intensity intermittent exercise

Abs tract The present study evaluated the acute effects of carbohydrate supplementation on heart rate (HR), rate of perceived exertion (RPE), metabolic and hormonal responses during and after sessions of high-intensity intermittent running exercise. Fifteen endurance runners (26 ± 5 years, 64.5 ± 4.9 kg) performed two sessions of intermittent exercise under carbohydrate (CHO) and placebo (PLA) ingestion. The sessions consisted of 12 × 800 m separated by intervals of 1 min 30 s at a mean velocity corresponding to the previously performed 3-km time trial. Both the CHO and PLA sessions were concluded within ∼28 min. Blood glucose was significantly elevated in both sessions (123.9 ± 13.2 mg dl⁻¹ on CHO and 147.2 ± 16.3 mg dl⁻¹ on PLA) and mean blood lactate was significantly higher in the CHO (11.4 ± 4.9 mmol l⁻¹) than in the PLA condition (8.4 ± 5.1 mmol l⁻¹) (P < 0.05). The metabolic stress induced by the exercise model used was confirmed by the elevated HR (∼182 bpm) and RPE (∼18 on the 15-point Borg scale) for both conditions. No significant differences in plasma insulin, cortisol or free fatty acids were observed during exercise between the two trials. During the recovery period, free fatty acid and insulin concentrations were significantly lower in the CHO trial. Supplementation with CHO resulted in higher lactate associated with lipolytic suppression, but did not attenuate the cortisol, RPE or HR responses.     

Effects of carbohydrate feedings on plasma free tryptophan and branched-chain amino acids during prolonged cycling

Summary Brain serotonin (5-hydroxytryptamine, 5-HT) has been suggested to be involved in central fatigue during prolonged exercise. Changes in the ratio of plasma free tryptophan (free Trp) to branched-chain amino acids (BCAA) are associated with altered brain 5-HT synthesis. The purposes of this study were to describe systematically the effects of prolonged exercise on changes in plasma free Trp and BCAA and to examine the effects of carbohydrate (CHO) feedings on these same variables. Eight well-trained men [ _max = 57.8 (SE 4.1) ml kg^–1 min^–1] cycled for up to 255 min at a power output corresponding toVO₂ at lactate threshold (approximately 68%VO_2max) on three occasions separated by at least 1 week. Subjects drank 5 ml kg^–1 body wt^–1 of either a water placebo, or a liquid beverage containing a moderate (6% CHO) or high (12% CHO) concentration of carbohydrate beginning at min 14 of exercise and every 30 min thereafter. Exercise time to fatigue was shorter in subjects receiving placebo [190 (SE 4) min] as compared to 6% CHO [235 (SE 10) min] and 12% CHO [234 (SE 9) min] (P<0.05). Glucose and insulin decreased in the placebo group, and free Trp, free-Trp/BCAA, and free fatty acids increased approximately five- to sevenfold (P < 0.05). These changes were attenuated in a dose-related manner by the carbohydrate drinks. Plasma free Trp and plasma free fatty acids were highly correlated (r=0.86,P<0.001). Plasma BCAA did not change in the placebo group, but decreased slightly in those receiving 6% CHO and 12% CHO (P<0.05). No differences in heart rate, , plasma volume and respiratory exchange ratio were found. The results indicate that free Trp and free Trp/BCAA increase progressively during prolonged cycling to fatigue. This response was attenuated by CHO feedings. Changes in plasma free fatty acids probably play a prominent role in these responses.     

The effects of 3000-m swimming on subsequent 3-h cycling performance: implications for ultraendurance triathletes

The purpose of this study was to examine the physiological effects of 3000-m swimming on subsequent 3-h cycling time trial performance in ultraendurance triathletes. Eight highly trained ultraendurance triathletes [mean (SEM) age 34 (2) years, body fat 12.5 (0.8)%, maximum oxygen consumption 63.2 (2.1) ml · kg⁻¹ · min⁻¹] completed two randomly assigned trials 1 week apart. The swim/bike trial (SB) involved 3000 m of swimming [min:s 52:28 (1:48)] immediately followed by a 3-h cycling performance at a self-selected time-trial pace. The control trial (CON) consisted of an identical 3-h cycling time trial but without prior swimming. Subjects consumed an 8% carbohydrate (CHO)/electrolyte beverage during both trials at the rate of 60 g CHO · h⁻¹ and 1 l · h⁻¹. No significant differences were evident between CON and SB on the dependent measures (CON vs SB): power output [W˙, 222 (14) W vs 212 (13) W], heart rate [f _c, 147 (5) beats · min⁻¹ vs 143 (4) beats · min⁻¹; %f _cmax 80.0 (1.6)% vs 78.4 (1.5)%], oxygen uptake [3.10 (0.12) l · min⁻¹ vs 2.97 (0.15) l · min⁻¹], minute ventilation [82.5 (4.4) l · min⁻¹ vs 77.3 (3.7) l · min⁻¹], rating of perceived exertion [14.6 (0.4) vs 14.0 (0.1)], blood lactate [6.1 (0.5) mmol · l⁻¹ vs 4.8 (0.5) mmol · l⁻¹], and blood glucose [5.0 (0.2) mmol · l⁻¹ vs 5.3 (0.1) mmol · l⁻¹; all non-significant at the P > 0.05 level]. However, the CON respiratory exchange ratio was significantly greater than for SB [0.91 (0.01) vs 0.89 (0.01); P < 0.05], suggesting that the SB trial required a greater reliance on lipid as a fuel substrate. Hence, the main finding in the present study was that 3000 m of swimming had no significant performance effect (in terms of W˙) on subsequent 3-h cycling performance in ultraendurance triathletes. Accepted: 2 March 2000     

Oral creatine supplementation decreases plasma markers of adenine nucleotide degradation during a 1-h cycle test

We investigated the effect of oral creatine supplementation (20 g d(-1) for 7 days) on metabolism during a 1-h cycling performance trial. Twenty endurance-trained cyclists participated in this double-blind placebo controlled study. Five days after familiarization with the exercise test, the subjects underwent a baseline muscle biopsy. Thereafter, a cannula was inserted into a forearm vein before performing the baseline maximal 1-h cycle (test 1 (T1)). Blood samples were drawn at regular intervals during exercise and recovery. After creatine (Cr) loading, the muscle biopsy, 1-h cycling test (test 2 (T2)) and blood sampling were repeated. Resting muscle total creatine (TCr), measured by high performance liquid chromatography, was increased (P < 0.001) in the creatine group from 123.0 +/- 3.8 - 159.8 +/- 7.9 mmol kg(-1) dry wt, but was unchanged in the placebo group (126.7 +/- 4.7 - 127.5 +/- 3.6 mmol kg(-1) dry wt). The extent of Cr loading was unrelated to baseline Cr levels (r=0.33, not significant). Supplementation did not significantly improve exercise performance (Cr group: 39.1 +/- 0.9 vs. 39.8 +/- 0.8 km and placebo group: 39.3 +/- 0.8 vs. 39.2 +/- 1.1 km) or change plasma lactate concentrations. Plasma concentrations of ammonia (NH(3)) (P < 0.05) and hypoxanthine (Hx) (P < 0.01) were lower in the Cr group from T1 to T2. Our results indicate that Cr supplementation alters the metabolic response during sustained high-intensity submaximal exercise. Plasma data suggest that nett intramuscular adenine nucleotide degradation may be decreased in the presence of enhanced intramuscular TCr concentration even during submaximal exercise.     

Effects of a rapid change in muscle glycogen availability on metabolic and hormonal responses during exercise

Summary To evaluate the metabolic and hormonal adaptations following a rapid change in muscle glycogen availability, 14 subjects had their muscle glycogen content increased in one leg (IG) and decreased in the other (DG). In group A (n=7), subjects exercised on a bicycle ergometer at 70% maximal oxygen uptake for 20 min using the DG leg. Without resting these same subjects exercised another 20 min using the IG leg. Subjects in group B (n=7) followed the same single-leg exercise protocol but in the reverse order. In order to get some information on the time sequence of these possible adaptations, blood samples were collected at rest and at the beginning and the end of each exercise period (min 5, 20, 25, and 40). Results indicated that 5 min after the switch from the DG leg to the IG leg. transient increases in plasma free fatty acids (1.20 to 1.39 meq·l^–1) and serum insulin (10.1 to 12 mU·l^–1) concentrations occured. Between minute 25 and 40 of exercise, the DG to IG switch was accompanied by a decrease in free fatty acids and glycerol concentrations as well as an increase in lactate levels. An opposite response was observed in the IG to DG condition during the same time span. Plasma norepinephrine, epinephrine, glucagon, and serum cortisol concentrations were not significantly affected by the leg change. These results suggest a rapid preferential use of muscle glycogen when available and a time lag in the response of the extramuscular substrate mobilization factors.     

Effects of supranormal liver glycogen content on hyperglucagonemia-induced liver glycogen breakdown

The purpose of the present study was to test the hypothesis that a higher hepatic glycogen level is associated with higher glucagon-induced hepatic glycogen depletion. Four groups of anesthetized rats received three injections (at times 0, 30, and 60 min) of glucagon (intravenously, 20?μg/kg). Among these groups, hepatic glycogen levels had previously been manipulated either by an overloading diet (Fast-refed), a reduction in food intake (1/2-fast), or exercise (75?min of running, 26?m/min, 0% grade). A fourth group had normal hepatic glycogen levels. A fifth group of rats was injected only with saline (0.9% NaCl). Liver glycogen concentrations were measured every 30?min during the course of the 90-min experiment, using liver samples obtained from the open liver biopsy technique. Plasma glucagon concentrations were significantly higher (P?<?0.05) in the glucagon-injected groups than in the saline-injected group. As expected, liver glycogen levels were significantly higher (P?<?0.01; 1.6-fold) in the Fast-refed group than in all other groups. Glucagon-induced decreases in liver glycogen concentrations were similar in Fast-refed than in normally fed and exercised rats when the overall 90-min period was considered. However, during the course of the last 30-min period, liver glycogen was significantly (P?<?0.01) decreased only in the Fast-refed group. The Fast-refed, normally fed, and exercised groups had a similar glucagon-induced hyperglycemia that was significantly more elevated (P?<?0.01) than glucose levels measured in the saline-injected group. Glucagon-induced reactive hyperinsulinemia was observed only in the Fast-refed and normally fed rats, and not in the exercised and 1/2-fast rats. It is concluded that supranormal levels of liver glycogen may be associated with a larger hyperglucagonemia-induced liver glycogen breakdown.