Carbohydrate metabolism and muscular exercise

Courtice and Douglas (1936) showed that, although a prolonged period of moderate muscular work may be associated with some rise of respiratory quotient and an enhanced carbohydrate oxidation, a persistent low respiratory quotient and ketonuria may result during subsequent rest. They concluded that this low quotient and ketonuria were in the main attributable to the reduced ratio of carbohydrate to fat oxidized owing to the depletion of readily available carbohydrate in the body as a result of the preceding muscular exercise. A second period of exercise was, however, still capable of causing a distinct rise of the respiratory quotient which was maintained throughout the exercise, to fall again abruptly to a persistent low level when the exercise stopped. Finding in one subject (Courtice) that after the ingestion of sucrose, glucose or fructose during post-exercise rest there was distinct evidence of a reduction of sugar tolerance in comparison with observations made during an initial resting period before exercise, they suggested the possibility that the activity of the endocrine organs associated with carbohydrate metabolism may be correlated with the varying activity of the muscles and so afford a partial explanation of the changes of carbohydrate metabolism which result from muscular exercise. Mills (1938) confirmed on Douglas the reduction of glucose tolerance which ensues after a prolonged period of muscular exercise, but in his own case obtained inconstant results, some experiments showing a lowered glucose tolerance, others no significant change. He also examined the influence of the carbohydrate content of the diet taken previous to the experiments, and tested the influence of intravenous injection of insulin on the subject without any previous exercise and after exercise.