Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate

Abstract

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.

New strategies in sport nutrition to increase exercise performance

“…Whereas it was generally accepted that exogenous CHO oxidation rates were thought to be limited at approximately 1 g/min due to saturation of intestinal glucose transporters, it is now known that exogenous CHO oxidation rates can increase to 1.8 g/min with the addition of sucrose or fructose to the CHO blend [38]. When taken together, it is currently thought that CHO feeding during exercise may therefore augment exercise performance via multiple mechanisms consisting of muscle glycogen sparing [39], liver glycogen sparing [40] and maintenance of plasma glucose and CHO oxidation rates [41]. It is noteworthy, however, that exogenous CHO feeding during exercise also improves performance when exercise duration is <60 minutes [42], an effect that is not apparent when glucose is directly infused to the bloodstream during exercise [43].…”

Effect of carbohydrate availability on time to exhaustion in exercise performed at two different intensities

“…However, fat oxidation may not be sufficient to maintain the power output required during the cycling time-to-exhaustion test. In fact, a number of studies have reported that exogenous carbohydrate supplementation, especially when muscle glyco-Carbohydrate availability and time to exhaustion www.bjournal.com.br gen concentrations are low, can maintain blood glucose and total carbohydrate oxidation and delay the onset of fatigue (4)(5)(6)(7)(8)(9). Therefore, while carbohydrate-loading diets are associated with improved endurance capacity, ascribed mainly to maintenance of a high rate of carbohydrate oxidation throughout exercise, low carbohydrate diets have been shown to impair endurance exercise capacity, which has been ascribed to anticipated reduction of endogenous carbohydrate availability and to a decreased rate of carbohydrate oxidation (4)(5)(6)(7)(8)(9).…”

High-fat diet overrules the effects of training on fiber-specific intramyocellular lipid utilization during exercise

“…The elevated plasma adrenaline-to-insulin ratio, due to fasting, increases circulating FFA concentration via stimulation of peripheral lipolysis (3,19,23). At the same time, reduced liver glycogen content impairs glucoregulation (15,16), which suppresses the input of blood-borne glucose in energy turnover. Thus during exercise in the fasted state (F), the rate of fat oxidation for a given submaximal exercise intensity is significantly increased (6,19).…”