Lipid metabolism during exercise. II. Respiratory exchange ratio and muscle glycogen content during 4 h bicycle ergometry in two groups of healthy men.

Seven physically fit (well-trained, maximal oxygen uptake 69.6 +/- 4.4 ml x kg-1 min-1) and eight less fit (moderately trained, maximal oxygen uptake 56.1 +/- 5.7 ml x kg-1 x min-1) healthy male subjects were exercised for 4 h by bicycle ergometry against a pedalling resistance calculated to cause oxygen consumption corresponding to approximately 30% of each individual's maximal oxygen uptake value. Respiratory exchange ratio was estimated at 1 h and blood glucose and lactate concentrations and muscle glycogen content at 2 h intervals. Muscle glycogen content decreased markedly during the first 2 h of exercise in the well-trained group but was similar after 4 h exercise in both groups. No major differences were observed between the two subject groups in blood variable concentrations. Calculations based on respiratory exchange ratio showed that the proportion of carbohydrates utilized in the total energy consumption was 14% in the physically fit group and 25% in the less fit group, thus supporting previous observations that more energy is derived by fat oxidation in well-trained than in less-trained individuals during submaximal work at relatively similar oxygen consumption levels.