Carbohydrate feedings and exercise performance: effect of initial muscle glycogen concentration

Rapid carbohydrate loading after a short bout of near maximal-intensity exercise

PURPOSE

One limitation shared by all published carbohydrate-loading regimens is that 2-6 d are required for the attainment of supranormal muscle glycogen levels. Because high rates of glycogen resynthesis are reported during recovery from exercise of near-maximal intensity and that these rates could in theory allow muscle to attain supranormal glycogen levels in less than 24 h, the purpose of this study was to examine whether a combination of a short bout of high-intensity exercise with 1 d of a high-carbohydrate intake offers the basis for an improved carbohydrate-loading regimen.

METHODS

Seven endurance-trained athletes cycled for 150 s at 130% VO2peak followed by 30 s of all-out cycling. During the following 24 h, each subject was asked to ingest 12 g.kg-1 of lean body mass (the equivalent of 10.3 g.kg-1 body mass) of high-carbohydrate foods with a high glycemic index.

RESULTS

Muscle glycogen increased from preloading levels (+/- SE) of 109.1 +/- 8.2 to 198.2 +/- 13.1 mmol.kg-1 wet weight within only 24 h, these levels being comparable to or higher than those reported by others over a 2- to 6-d regimen. Densitometric analysis of muscle sections stained with periodic acid-Schiff not only corroborated these findings but also indicated that after 24 h of high-carbohydrate intake, glycogen stores reached similar levels in Type I, IIa, and IIb muscle fibers.

CONCLUSION

This study shows that a combination of a short-term bout of high-intensity exercise followed by a high-carbohydrate intake enables athletes to attain supranormal muscle glycogen levels within only 24 h.

Reduced neuromuscular activity and force generation during prolonged cycling

We examined neuromuscular activity during stochastic (variable intensity) 100-km cycling time trials (TT) and the effect of dietary carbohydrate manipulation. Seven endurance-trained cyclists performed two 100-km TT that included five 1-km and four 4-km high-intensity epochs (HIE) during which power output, electromyogram (EMG), and muscle glycogen data were analyzed. The mean power output of the 4-km HIE decreased significantly throughout the trial from 319 +/- 48 W for the first 4-km HIE to 278 +/- 39 W for the last 4-km HIE (P < 0.01). The mean integrated EMG (IEMG) activity during the first 4-km HIE was 16.4 +/- 9.8% of the value attained during the pretrial maximal voluntary contraction (MVC). IEMG decreased significantly throughout the trial, reaching 11.1 +/- 5.6% during the last 4-km HIE (P < 0.01). The study establishes that neuromuscular activity in peripheral skeletal muscle falls parallel with reduction in power output during bouts of high-intensity exercise. These changes occurred when <20% of available muscle was recruited and suggest the presence of a central neural governor that reduces the active muscle recruited during prolonged exercise.

Reliability of power in physical performance tests

The reliability of power in tests of physical performance affects the precision of assessment of athletes, patients, clients and study participants. In this meta-analytic review we identify the most reliable measures of power and the factors affecting reliability. Our measures of reliability were the typical (standard) error of measurement expressed as a coefficient of variation (CV) and the percent change in the mean between trials. We meta-analysed these measures for power or work from 101 studies of healthy adults. Measures and tests with the smallest CV in exercise of a given duration include field tests of sprint running (approximately 0.9%), peak power in an incremental test on a treadmill or cycle ergometer (approximately 0.9%), equivalent mean power in a constant-power test lasting 1 minute to 3 hours on a treadmill or cycle ergometer (0.9 to 2.0%), lactate-threshold power (approximately 1.5%), and jump height or distance (approximately 2.0%). The CV for mean power on isokinetic ergometers was relatively large (> 4%). CV were larger for nonathletes versus athletes (1.3 x), female versus male nonathletes (1.4 x), shorter (approximately 1-second) and longer (approximately 1-hour) versus 1-minute tests (< or = 1.6 x), and respiratory- versus ergometer-based measures of power (1.4 to 1.6 x). There was no clear-cut effect of time between trials. The importance of a practice trial was evident in studies with > 2 trials: the CV between the first 2 trials was 1.3 times the CV between subsequent trials; performance also improved by 1.2% between the first 2 trials but by only 0.2% between subsequent trials. These findings should help exercise practitioners and researchers select or design good measures and protocols for tests of physical performance.

Effect of oral glucose on leucine turnover in human subjects at rest and during exercise at two levels of dietary protein

The aim of this study was to determine the effect of glucose supplementation on leucine turnover during and after exercise and whether variation in the previous dietary protein content modulated this effect. Postabsorptive subjects received a primed constant [1-13C, 15N]leucine infusion for 6 h, after previous consumption of a high (1.8 g kg-1 day-1, HP, n = 16) or low (0.7 g kg-1 day-1, LP, n = 16) protein diet for 7 days. The subjects were studied at rest; during 2 h of exercise, during which half of the subjects from each dietary protocol received 0.75 g kg-1 h-1 glucose (HP + G, LP + G) and the other half received water (HP + W, LP + W); then again for 2 h of rest. Glucose supplementation suppressed leucine oxidation (P < 0.01) by 20% in subjects consuming the high protein diet (58.2 +/- 2.8 micromol kg-1 h-1, HP + G; 72.4 +/- 3.9 micromol kg-1 h-1, HP + W) but not the low protein diet (51.1 +/- 5.9 micromol kg-1 h-1, LP + G; 51.7 +/- 5.5 micromol kg-1 h-1, LP + W), with no difference in skeletal muscle branched-chain 2-oxo acid dehydrogenase (BCOADH) activity between groups. Glucose supplementation did not alter the rate of whole-body protein synthesis or breakdown. The sparing effect of glucose on leucine oxidation appears only to occur if previous protein intake was high. It was not mediated by a suppression of BCOADH fractional activity but may be due to reduced substrate availability.

Carbohydrate loading failed to improve 100-km cycling performance in a placebo-controlled trial

We evaluated the effect of carbohydrate (CHO) loading on cycling performance that was designed to be similar to the demands of competitive road racing. Seven well-trained cyclists performed two 100-km time trials (TTs) on separate occasions, 3 days after either a CHO-loading (9 g CHO. kg body mass(-1). day(-1)) or placebo-controlled moderate-CHO diet (6 g CHO. kg body mass(-1). day(-1)). A CHO breakfast (2 g CHO/kg body mass) was consumed 2 h before each TT, and a CHO drink (1 g CHO. kg(.)body mass(-1). h(-1)) was consumed during the TTs to optimize CHO availability. The 100-km TT was interspersed with four 4-km and five 1-km sprints. CHO loading significantly increased muscle glycogen concentrations (572 +/- 107 vs. 485 +/- 128 mmol/kg dry wt for CHO loading and placebo, respectively; P < 0.05). Total muscle glycogen utilization did not differ between trials, nor did time to complete the TTs (147.5 +/- 10.0 and 149.1 +/- 11.0 min; P = 0.4) or the mean power output during the TTs (259 +/- 40 and 253 +/- 40 W, P = 0.4). This placebo-controlled study shows that CHO loading did not improve performance of a 100-km cycling TT during which CHO was consumed. By preventing any fall in blood glucose concentration, CHO ingestion during exercise may offset any detrimental effects on performance of lower preexercise muscle and liver glycogen concentrations. Alternatively, part of the reported benefit of CHO loading on subsequent athletic performance could have resulted from a placebo effect.

The effectiveness of commercially available sports drinks

The purpose of this review is to evaluate the effectiveness of commercially available sports drinks by answering the questions: (i) will consuming a sports drink be beneficial to performance? and (ii) do different sports drinks vary in their effectiveness? To answer these questions we have considered the composition of commercially available sports drinks, examined the rationale for using them, and critically reviewed the vast number of studies that have investigated the effectiveness of sports drinks on performance. The focus is on the drinks that contain low carbohydrate concentrations (<10%) and are marketed for general consumption before and during exercise rather than those with carbohydrate concentrations >10%, which are intended for carbohydrate loading. Our conclusions are 3-fold. First, because of variations in drink composition and research design, much of the sports drinks research from the past cannot be applied directly to the effectiveness of currently available sports drinks. Secondly, in studies where a practical protocol has been used along with a currently available sports beverage, there is evidence to suggest that consuming a sports drinks will improve performance compared with consuming a placebo beverage. Finally, there is little evidence that any one sports drink is superior to any of the other beverages on the market.

Noninvasive measurement of muscle high-energy phosphates and glycogen concentrations in elite soccer players by 31P- and 13C-MRS

PURPOSE

The purpose of this study was to measure noninvasively the absolute concentrations of muscle adenosine triphosphate [ATP], phosphocreatine [PCr], inorganic phosphate (Pi), and glycogen [Gly] of elite soccer players.

METHODS

Magnetic resonance spectroscopy (31P- and 13C-MRS) was used to measure the concentrations of metabolites in the calf muscles of 18 young male players [age = 17.5 +/- 1.0 (SD) yr].

RESULTS

Average muscle [PCr] and [ATP] were 17.8 +/- 3.3 and 6.0 +/- 1.2 mmol x (kg wet weight)(-1), respectively. The ratios of Pi/PCr and PCr/ATP were 0.15 +/- 0.05 and 3.00 +/- 0.26, respectively. The muscle [Gly] was 144 +/- 54 mmol x (kg wet weight)(-1). There was a high correlation (r = 0.93, P < 0.0001) between muscle ATP and PCr concentrations, but there was no correlation between [Gly] and [PCr] or [ATP]. The concentrations of the different metabolites determined in the present study with noninvasive MRS methods were within the ranges of values reported in human muscle from biochemical analysis of muscle biopsies.

CONCLUSION

MRS methods can be utilized to assess noninvasively the muscle energetic status of elite soccer players during a soccer season. The high correlation between ATP and PCr might be indicative of fiber type differences in the content of these two metabolites.

Role of carbohydrate in physical activity

Carbohydrate is an essential fuel for prolonged, strenuous exercise, although the carbohydrate stores of the body are limited. Research studies have provided evidence that carbohydrate depletion is associated with fatigue, decrease in exercise intensity, and even exercise cessation. With the appropriate diet and exercise protocol, however, the carbohydrate stores of the body can be substantially increased and exercise performance improved by carbohydrate supplementation before and during exercise. In this article, the role of carbohydrate supplementation for increasing carbohydrate stores before exercise, maintaining blood glucose during exercise, and the rapid replenishment of the carbohydrate stores after exercise are discussed. Considered in the discussion are the types, amounts and forms of carbohydrate supplements that are most effective, and the most appropriate times for their ingestion.

Oral [13C]glucose oxidation during prolonged exercise after high- and low-carbohydrate diets

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.

Fuel substrate kinetics of carbohydrate loading differs from that of carbohydrate ingestion during prolonged exercise

This study compared fuel substrate kinetics in trained cyclists who ingested a 10% carbohydrate (CHO) drink without prior CHO-loading ([NLC] n=9) with those in cyclists who ingested a water placebo after CHO-loading ([CLP] n=7) during 180 minutes of cycling at 70% maximum oxygen consumption (Vo2 max). Muscle glycogen at the start of exercise was 194 +/- 4 and 124 +/- 8 mmol/kg wet weight (mean +/- SEM) in CLP and NLC subjects, respectively . Total CHO oxidation was similar. Total rate of appearance of glucose from endogenous (Raend) and exogenous (Raexog) origin and plasma glucose oxidation increased significantly (P<.05), with NLC subjects ending significantly higher than CLP subjects (104 +/- 17 v 79 +/- 9 and 115 +/- 16 v 74 +/- 11 micromol/min/kg fat-free mass [FFM], respectively). However, Raend was lower (P<.05) in NLC than in CLP subjects (40 +/- 10 v 79 +/- 9 micromol/min/kg FFM), as was endogenous plasma glucose oxidation (42 +/- 13 v 75 +/- 11 micromol/min kg FFM). Muscle glycogen disappearance was identical in the first hour, but declined thereafter in NLC subjects. Two NLC subjects with the lowest muscle glycogen content were unable to complete the trial despite CHO ingestion. We conclude that with respect to the groups studied (1) CHO loading before exercise reduces the relative contribution of plasma glucose oxidation to total CHO oxidation, but may prolong time to exhaustion as a function of higher muscle glycogen concentration; (2) CHO ingestion has a liver glycogen-sparing effect, causes a reduction in gluconeogenesis, or both, that should delay the onset of hypoglycemia; (3) the progressive increase in plasma glucose oxidation that occurs during prolonged exercise is related to muscle glycogen status and occurs irrespective of whether CHO is ingested: (4) the effects of CHO ingestion and CHO-loading on fuel substrate kinetics are different.

Effect of carbohydrate ingestion subsequent to carbohydrate supercompensation on endurance performance

This investigation determined whether carbohydrate ingestion during prolonged moderate-intensity exercise enhanced endurance performance when the exercise was preceded by carbohydrate supercompensation. Seven male trained cyclists performed two trials at an initial power output corresponding to 71 +/- 1% of their peak oxygen consumption. During the trials, subjects ingested either a 6% glucose/sucrose (C) solution or an equal volume of artificially flavored and sweetened placebo (P) every 20 min throughout exercise. Both C and P were preceded by a 6-day carbohydrate supercompensation procedure in which subjects undertook a depletion-taper exercise sequence in conjunction with a moderate- and high-carbohydrate diet regimen. Statistical analysis of time to exhaustion, plasma glucose concentration, carbohydrate oxidation rate, fat oxidation rate, and plasma glycerol concentration indicated that in spite of a carbohydrate supercompensation procedure administered prior to exercise, carbohydrate ingestion during exercise can exert an additional ergogenic effect by preventing a decline in blood glucose levels and maintaining carbohydrate oxidation during the later stages of moderate-intensity exercise.

The effects of carbohydrate loading on muscle glycogen content and cycling performance

This study compared the effects of supplementing the normal diets of 8 endurance-trained cyclists with additional carbohydrate (CHO), in the form of potato starch, for 3 days on muscle glycogen utilization and performance during a 3-hr cycle ride. On two occasions prior to the trial, the subjects ingested in random order either their normal CHO intake of 6.15 +/- 0.23 g/kg body mass/day or a high-CHO diet of 10.52 +/- 0.57 g/kg body mass/day. The trial consisted of 2 hr of cycling at approximately 75% of VO2peak with five 60-s sprints at 100% VO2peak at 20-min intervals, followed by a 60-min performance ride. Increasing CHO intake by 72 +/- 9% for 3 days prior to the trial elevated preexercise muscle glycogen contents, improved power output, and extended the distance covered in 1 hr. Muscle glycogen contents were similar at the end of the 3-hr trial, indicating a greater utilization of glycogen when subjects were CHO loaded, which may have been responsible for their improved cycling performance.

Effect of branched-chain amino acid and carbohydrate supplementation on the exercise-induced change in plasma and muscle concentration of amino acids in human subjects

Five male endurance-trained subjects performed exhaustive exercise on a cycle ergometer at a work rate corresponding to 75% of their VO2max after reduction of their muscle glycogen stores. During exercise the subjects were given in random order a 6% carbohydrate solution continuing 7 g L-1 of branched-chain amino acids (BCAA), a 6% CHO solution and flavoured water. The physical performance was lowered in four of the five subjects when they were given flavoured water during exercise as compared with the two conditions when CHO was supplied. No difference in performance was found when the subjects were given CHO + BCAA or only CHO during exercise. When CHO + BCAA was supplied the plasma and muscle (vastus lateralis) concentrations of BCAA increased during exercise by 120 and 35%, respectively. In the other conditions there was no change or a slight decrease in the plasma concentrations of BCAA, but the muscle concentrations of BCAA were decreased after exercise. The plasma concentration of glutamine over the whole exercise period and 5 min after exercise was higher when CHO + BCAA were supplied during exercise compared with a supply of CHO alone or water. However, exercise caused no change in the muscle concentration of glutamine, whereas that of glutamate decreased in all three conditions. A supply of CHO + BCAA or CHO alone did not affect the exercise-induced increase in the plasma and muscle concentration of aromatic amino acids, indicating that neither BCAA nor CHO influenced the net protein degradation during exercise.