Human muscle glycogen metabolism during exercise. Effect of carbohydrate supplementation

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

Effects of 3 days of carbohydrate supplementation on muscle glycogen content and utilisation during a 1-h cycling performance

This study compared the effects of supplementing the normal diets of six trained cyclists [maximal oxygen uptake (VO2max) 4.5 (0.36) l.min-1; values are mean (SD)] with additional carbohydrate (CHO) on muscle glycogen utilisation during a 1-h cycle time-trial (TT). Using a randomised crossover design, subjects consumed either their normal diet (NORM) for 3 days, which consisted of 426 (137) g.day-1 CHO [5.9 (1.4) g. kg-1 body mass (BM)], or additional CHO (SUPP) to increase their intake to 661 (76) g.day-1 [9.3 (0.7) g. kg-1 BM]. The SUPP diet elevated muscle glycogen content from 459 (83) to 565 (62) mmol.kg-1 dry weight (d.w.) (P < 0.05). However, despite the increased pre-exercise muscle glycogen stores, there was no difference in the distance cycled during the TT [40.41 (1.44) vs 40.18 (1.76) km for NORM and SUPP, respectively]. With NORM, muscle glycogen declined from 459 (83) to 175 (64) mmol.kg-1 d.w., whereas with SUPP the corresponding values were 565 (62) and 292 (113) mmol.kg-1 d.w. Accordingly, both muscle glycogen utilisation [277 (64) vs 273 (114) mmol.kg-1 d.w.] and total CHO oxidation [169 (20) vs 165 (30) g.h-1 for NORM and SUPP, respectively] were similar. Neither were there any differences in plasma glucose or lactate concentrations during the two experimental trials. Plasma glucose concentration averaged 5.5 (0.5) and 5.6 (0.6) mmol.l-1, while plasma lactate concentration averaged 4.4 (1.9) and 4.4 (2.3) mmol.l-1 for NORM and SUPP, respectively. The results of this study show that when well-trained subjects increase the CHO content of their diet for 3 days from 6 to 9 g.kg-1 BM there is only a modest increase in muscle glycogen content. Since supplementary CHO did not improve TT performance, we conclude that additional CHO provides no benefit to performance for athletes who compete in intense, continuous events lasting 1 h. Furthermore, the substantial muscle CHO reserves observed at the termination of exercise indicate that whole-muscle glycogen depletion does not determine fatigue at this exercise intensity and duration.     

Glycemic index--a new tool in sport nutrition?

The glycemic index (GI) provides a way to rank foods rich in carbohydrate (CHO) according to the glucose response following their intake. Consumption of low-GI CHO foods may attenuate the insulin-mediated metabolic disturbances associated with CHO intake in the hours prior to exercise, better maintaining CHO availability. However, there is insufficient evidence that athletes who consume a low-GI CHO-rich meal prior to a prolonged event will gain clear performance benefits. The ingestion of CHO during prolonged exercise promotes CHO availability and enhances endurance and performance, and athletes usually chose CHO-rich foods and drinks of moderate to high GI to achieve this goal. Moderate- and high-GI CHO choices appear to enhance glycogen storage after exercise compared with low GI CHO-rich foods. However, the reason for this is not clear. A number of attributes of CHO-rich foods may be of value to the athlete including the nutritional value of the food or practical issues such as palatability, portability, cost gastric comfort, or ease of preparation.     

Neutron and X-ray reflectivity studies of human serum albumin adsorption onto functionalized surfaces of self-assembled monolayers

Endurance exercise training increases fat oxidation during large muscle mass exercise. Although the source of this fat has been thought to be plasma free fatty acids (FFA) released from adipose tissue, the training-induced decrease in lipolytic hormonal responses to exercise is not consistent with this concept. The purpose of this communication is to review findings, from our laboratory indicating that, in young healthy subjects, endurance exercise training reduces plasma FFA turnover and oxidation during moderate intensity prolonged 2-leg cycling while simultaneously enhancing depletion of triglycerides from the active musculature. Evidence is presented that metabolism of intramuscular triglycerides can explain the increase in total fat oxidation observed in the trained state during large muscle mass exercise. However, these results may not be applicable to exercise involving small muscle groups, a distinction that is likely to be important in explaining the apparent conflict between our findings and those from other laboratories where experimental conditions were different. In summary, for large muscle mass exercise up to 2 h in duration, plasma FFA are a less important fuel source in the trained state, and intramuscular triglycerides supply the major portion of the increase in oxidized fatty acids.     

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

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

Metabolic catecholamine, and endurance responses to caffeine during intense exercise

This study examined the possible effects of caffeine ingestion on muscle metabolism and endurance during brief intense exercise. We tested 14 subjects after they ingested placebo or caffeine (6 mg/kg) with an exercise protocol in which they cycled for 2 min, rested 6 min, cycled 2 min, rested 6 min, and then cycled to voluntary exhaustion. In each exercise the intensity required the subject's maximal O2 consumption. Eight subjects had muscle and venous blood samples taken before and after each exercise period. The caffeine ingestion resulted in a significant increase in endurance (4.12 +/- 0.36 and 4.93 +/- 0.60 min for placebo and caffeine, respectively) and resulted in a significant increase in plasma epinephrine concentration throughout the protocol but not in norepinephrine concentration. During the first two exercise bouts, the power and work output were not different; blood lactate concentrations were not affected significantly by caffeine ingestion, but during the exercise bouts muscle lactate concentration was significantly increased by caffeine. The net decrease in muscle glycogen was not different between treatments at any point in the protocol, and even at the time of fatigue there was at least 50% of the original glycogen concentration remaining. the data demonstrated that caffeine ingestion can be an effective ergogenic aid for exercise that is as brief as 4-6 min. However, the mechanism is not associated with muscle glycogen sparing. It is possible that caffeine is exerting actions directly on the active muscle and/or the neural processes that are involved in the activity.     

An essential role for gamma interferon in innate resistance to Shigella flexneri infection

The purpose of this experiment was to study endurance performance and substrate storage and utilization in fat- or carbohydrate-fed rats. Ninety-nine rats were randomly divided into three groups and over 4 wk were fed either a carbohydrate-rich [CHO; 10% total energy content in the diet (E%) fat, 20 E% protein, 70 E% carbohydrate] diet or one of two fat-rich diets (65 E% fat, 20 E% protein, 15 E% carbohydrate) containing either saturated (Sat) or monounsaturated fatty acids (Mono). Each dietary group was randomly assigned to a trained (6 days/wk, progressive to 60 min, 28 m/min at a 10% incline) or a sedentary group. Rats were killed either before or after a treadmill endurance run to exhaustion. Training increased endurance (206%), but diet composition did not affect endurance in either trained or sedentary rats. beta-Hydroxyacyl-CoA dehydrogenase activity was increased in fat-fed but not carbohydrate-fed rats (P < 0.05). Respiratory exchange ratio during the initial phase of exercise was lower after the Mono compared with the Sat diet (P < 0. 05) and higher after the CHO than the Sat diet (P < 0.05). Thus adaptation to a high-fat diet containing a moderate amount of carbohydrates did not induce enhanced endurance in either trained or untrained rats; however, substrate utilization was modulated by both amount and type of dietary fat during the initial stage of exercise in trained and sedentary rats.     

Influence of carbohydrate supplementation early in exercise on endurance running capacity

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

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

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

Importance of the 'crossover' concept in exercise metabolism

1. The 'crossover' concept is a model of substrate supply during exercise which makes the following predictions. 2. Lipid is the major fuel (approximately 60%) for non-contracting skeletal muscle and the body at rest. 3. Energy flux, as determined by exercise intensity, is the major factor in determining the balance of substrate utilization during exercise. Thus, moderate and greater exercise intensities increase contraction-induced muscle glycogenolysis and glycolysis, increase recruitment of fast-twitch muscle fibres, increase sympathetic nervous system activity and down-regulate mitochondrial fatty acid uptake. 4. Glycogen and glucose utilization scales exponentially to relative exercise power output with a greater gain in glycogen than in glucose use at high power. The relationship between free fatty acid flux and power output is an inverted hyperbola. Consequently, at high power outputs, the role of lipid oxidation is diminished. 5. Factors such as endurance training, energy supply, as influenced by dietary manipulation, and prior exercise play secondary roles in determining the balance of substrate utilization during exercise. 6. Comparisons of the metabolic responses in subjects engaged in activities requiring vastly different metabolic rates or comparisons of subjects of different gender, age or training status require normalization of data to total energy flux.     

The acute reversal of a diet-induced metabolic acidosis does not restore endurance capacity during high-intensity exercise in man

The present experiment was designed to investigate whether a diet-induced metabolic acidosis was a major factor in the earlier onset of fatigue during high-intensity exercise. Six healthy males cycled to exhaustion at a workload equivalent to 95 percent of maximum oxygen uptake on four separate occasions. Exercise tests were performed after an overnight fast and each test was preceded by one of four experimental conditions. Two experimental diets were designed, either to replicate each subject's own normal diet [N diet, mean (SD) daily energy intake (E) = 13 (0.7) MJ, 14.5 (0.8) percent protein (Pro), 37.5 (2.2) percent fat (Fat) and 47.5 (2.1) percent carbohydrate (CHO)], or a low-carbohydrate diet [E = 12.6 (0.8) MJ, 33.6 (1.3) percent Pro, 64.4 (1.5) percent Fat and 2.2 (0.4) percent CHO]. These diets were prepared and consumed within the department over a 3-day period. Over a 3-period prior to the exercise trial subjects ingested either NaHCO(3) or CaCO(3) (3.6 and 3.0 mmol*kg body mass), thus giving four experimental conditions: N diet and treatment, N diet and placebo, low-CHO diet and treatment and low-CHO diet and placebo. Treatments were assigned using a randomised protocol. Arterialised venous blood samples were taken for the determination of acid-base status and metabolite concentrations at rest prior to exercise and at intervals for 30 min following exhaustion. Consumption of the low-CHO diet induced a mild metabolic acidosis which was reversed by the ingestion of NaHCO(3). Blood pH, bicarbonate (HCO-(3)) and base excess (BE) were higher following NaHCO(3) ingestion after the normal diet than all of the other experimental conditions (P <0.01). Exercise time following the low-CHO diet was less than on the normal diet conditions (P <0.05): bicarbonate ingestion had no effect on exercise time on either of the diet conditions. Post-exercise blood pH, HCO-(3); and BE were higher following the ingestion of NaHCO(3) irrespective of the pre-exercise diet (P <0.05). Blood lactate concentration was higher 2 min after exercise following the N diet with NaHCO(3) when compared to the low-CHO diets with either NaHCO(3) or placebo (P <0.05). Plasma ammonia accumulation was not significantly different between experimental conditions. These data confirm previous data showing that the ingestion of a low-CHO diet reduces the capacity to perform high-intensity exercise, but it appears that the metabolic acidosis induced by the low-CHO diet is not the cause of the reduced exercise capacity observed during high-intensity exercise under these conditions.     

Radiotherapy results in early stage low grade nodal non-Hodgkin''s lymphoma

Severe lactic acidosis usually accompanies intense endurance exercise. It has been postulated that glycogen depletion working in concert with elevated muscle and plasma lactate levels lead to a concomitant reduction in pH. Their cumulative effect during prolonged physical exertion now leads to muscular fatigue and eventually limit endurance capacity. Therefore in the present study, dichloroacetate (DCA), a compound which enhances the rate of pyruvate oxidation thus reducing lactate formation, has been evaluated in a validated rat model of sub-maximal exercise performance. Male rats (350 g) were divided into two groups (control-saline, i.v. and DCA 5 mg/kg, i.v.) and were exercised to exhaustion in a chamber (26 degrees C) on a treadmill (11 m/min, 6 degrees incline). When compared to controls, the DCA-treated rats had longer run times (169 vs 101 min) and a decreased heating rate (0.020 vs 0.029 degrees C/min). In addition, DCA attenuated the increase in plasma lactate (28 vs 40 mg/dl) and significantly reduced both the rate and absolute amount of depletion of muscle glycogen stores. These results suggest that the activation of pyruvate dehydrogenase activity by DCA resulted in a reduction in the rate of glycogenolysis in addition to decreasing lactate accumulation by presumably limiting the availability of pyruvate for conversion to lactate, therefore increasing muscle carbohydrate oxidation via the TCA cycle. Thus DCA effected a significant delay in muscle fatigue.     

Effects of computer feedback on adherence to exercise

The effect of a diet either high or low in carbohydrates (CHO) on exogenous 13C-labeled glucose oxidation (200 g) during exercise (ergocycle: 120 min at 64.0 +/- 0.5% maximal oxygen uptake) was studied in six subjects. Between 40 and 80 min, exogenous glucose oxidation was significantly higher after the diet low in CHO (0.63 +/- 0.05 vs. 0.52 +/- 0.04 g/min), but this difference disappeared between 80 and 120 min (0.71 +/- 0.03 vs. 0.69 +/- 0.04 g/min). The oxidation rate of plasma glucose, computed from the volume of 13CO2 produced the 13C-to-12C ratio in plasma glucose at 80 min, and of glucose released from the liver, computed from the difference between plasma glucose and exogenous glucose oxidation, was higher after the diet low in CHO (1.68 +/- 0.26 vs. 1.41 +/- 0.17 and 1.02 +/- 0.20 vs. 0.81 +/- 0.14 g/min, respectively). In contrast the oxidation rate of glucose plus lactate from muscle glycogen (computed from the difference between total CHO oxidation and plasma glucose oxidation) was lower (0.31 +/- 0.35 vs. 1.59 +/- 0.20 g/min). After a diet low in CHO, the oxidation of exogenous glucose and of glucose released from the liver is increased and partly compensates for the reduction in muscle glycogen availability and oxidation.     

Effect of fluid ingestion on muscle metabolism during prolonged exercise

Five trained men were studied during 2 h of cycling exercise at 67% peak oxygen uptake at 20-22 degrees C to examine the effect of fluid ingestion on muscle metabolism. On one occasion, the subjects completed this exercise without fluid ingestion (NF) while on the other they ingested a volume of distilled deionized water that prevented loss of body mass (FR). No differences in oxygen uptake during exercise were observed between the two trials. Heart rate was lower (P < 0.01) throughout exercise when fluid was ingested, and rectal temperature after 2 h of exercise was lower (38.0 +/- 0.2 and 38.6 +/- 0.2 degrees C for FR and NF, respectively; P < 0.01), as was muscle (vastus lateralis) temperature (38.5 +/- 0.4 and 39.1 +/- 0.5 degrees C for FR and NF, respectively; P < 0.05). Resting muscle ATP, creatine phosphate, creatine, glycogen, and lactate levels were similar in the two trials, as were the postexercise ATP, creatine phosphate, and creatine levels. In contrast, muscle glycogen was higher (P < 0.05) and muscle lactate was lower (P < 0.05) after 2 h of exercise in FR compared with NF. Net muscle glycogen utilization during exercise was reduced by 16% when fluid was ingested (318 +/- 46 and 380 +/- 53 mmol/kg dry weight for FR and NF, respectively; P < 0.05). These results indicate that fluid ingestion reduces muscle glycogen use during prolonged exercise, which may account, in part, for the improved performance previously observed with fluid ingestion.     

Mammalian fuel utilization during sustained exercise

The 'crossover' and 'lactate shuttle' concepts of substrate utilization in humans during exercise are extended to describe metabolic responses on other mammalian species. The 'crossover concept' is that lipid plays a predominant role in sustaining efforts requiring half or less aerobic capacity (VO2max); however, greater relative efforts depend increasingly on blood glucose and muscle glycogen as substrates. Thus, as exercise intensity increases from mild to moderate and hard, fuel selection switches (crosses over) from lipid to carbohydrate dependence. Glycogen and glucose catabolic rates are best described as exponential functions of exercise intensity, but with a greater gain in slope of the glycogen than glucose response. In contrast, plasma free fatty acid flux is described as an inverted hyperbola with vertex at approximately 50% VO2max. Both endocrine and intra-cellular factors play critical roles in determining substrate balance during sustained exercise. Moreover, genotypic adaptation for aerobic capacity as well as phenotypic adaptations to short- and long-term chronic activity affect the balance of substrate utilization during exercise. The concept of a 'lactate shuttle' is that during hard exercise, as well as other conditions of accelerated glycolysis, glycolytic flux in muscle involves lactate formation regardless of the state of oxygenation. Further, according to the lactate shuttle concept, lactate represents a major means of distributing carbohydrate potential energy for oxidation and gluconeogenesis. In humans and other mammals, the formation, distribution and disposal of lactate (not pyruvate) represent key steps in the regulation of intermediary metabolism during sustained exercise.     

Postexercise recovery of skeletal muscle malonyl-CoA, acetyl-CoA carboxylase, and AMP-activated protein kinase

Previous studies have demonstrated that oxygen consumption and fat oxidation remain elevated in the postexercise period. The purpose of this study was to determine whether malonyl-CoA, an inhibitor of fatty acid oxidation, remains depressed in muscle after exercise. Rats were sprinted for 5 min (40 m/min, 5% grade) or run for 30 min (21 m/min, 15% grade). Red quadriceps malonyl-CoA returned to resting values by 90 min postexercise in the sprinting rats and remained significantly lower at least 90 min postexercise in the 30-min exercise group. AMP-activated protein kinase activity remained significantly elevated (P < 0.05) for 10 min after exercise in both groups. The most rapid rate of glycogen repletion was in the first 30 min postexercise. The respiratory exchange ratio decreased from a nonexercise value of 0.87 +/- 0.01 to an average 0.82 +/- 0.01 during the 90-min period after 30 min of exercise. Thus muscle malonyl-CoA remains depressed and fat oxidation is elevated for relatively prolonged periods after a single bout of exercise. This may allow fat oxidation to contribute more to muscle energy requirements, thus leaving more glucose for replenishment of muscle glycogen.     

Mechanisms of whole-body glycogen deposition after oral glucose in normal subjects. Influence of the nutritional status

It is known that prior fasting enhances whole-body glycogen retention after glucose ingestion. To identify the involved mechanisms, 33 normal volunteers underwent a total fast, varying between 14 h and 4 days, and ingested thereafter 75 g glucose labeled with [14C]glucose. Measurements of oral glucose oxidation (expired 14CO2, corrected for incomplete recovery) and total carbohydrate (CHO) oxidation (indirect calorimetry) were performed over the following 5 h. These data allowed us to calculate oral glucose storage (uptake oxidation), glycogen oxidation (CHO oxidation - oral glucose oxidation), and net CHO balance (oral glucose uptake - CHO oxidation). As compared with an overnight fast, prolonged fasting (4 days) inhibited the uptake (64.8 vs. 70.3 g/5 h; P < 0.01) and the oxidation (10.9 vs. 20.0 g/5 h; P < 0.001) of oral glucose and stimulated slightly its conversion to glycogen (53.9 vs. 50.3 g/5 h; P < 0.05). The latter effect played only a minor role in the marked increase in net CHO balance (52.3 vs. 25.2 g/5 h; P < 0.001), which was almost entirely related to a decrease in glycogen oxidation (1.6 vs. 25.1 g/5 h; P < 0.001). Considering the whole series of data, including intermediate durations of fast, it was observed that the modifications in postprandial CHO metabolism, induced by fasting, correlated strongly with basal CHO oxidation, suggesting that the degree of initial glycogen depletion is a major determinant of glycogen oxidation and net CHO storage. Thus, prior fasting stimulates postprandial glycogen retention, mainly through an inhibition of the glycogen turnover that exists in overnight-fasted subjects, during the absorptive period.     

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

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

Thyroid status and exercise tolerance. Cardiovascular and metabolic considerations

Both hypo- and hyperthyroidism are characterised by exercise intolerance. In hypothyroidism, inadequate cardiovascular support appears to be the principal factor involved. Insufficient skeletal muscle blood flow compromises exercise capacity via reduced oxygen delivery, and endurance through decreased delivery of blood-borne substrates. The latter effect results in increased dependence on intramuscular glycogen. Additionally, decreased mobilisation of free fatty acids from adipose tissue and, consequently, lower plasma free fatty acid levels compound the problem of reduced lipid delivery to active skeletal muscle in the hypothyroid state. In contrast, cardiovascular support is enhanced in hyperthyroidism, implicating other factors in exercise tolerance. Greater reliance on muscle glycogen appears to be the primary reason for decreased endurance. Biochemical changes with hyperthyroidism that would favour enhanced flux through glycolysis may account for this dependence on glycogen. Deviations from normal thyroid function, and the ensuing exercise tolerance, require appropriate medical therapy to attain euthyroid status.     

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

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