PLASMA-GLUCOSE METABOLISM DURING EXERCISE - EFFECT OF ENDURANCE TRAINING IN HUMANS

It has long been recognized that endurance training reduces the relian ce on carbohydrate as a source of energy during submaximal exercise. H istorically, this has been ascribed to a decrease in muscle glycogen u tilization. However, recent studies have demonstrated that, at least i n humans, training also reduces the production and utilization of plas ma-borne glucose during exercise. The latter is true not only during m oderate exercise performed at the same absolute intensity before and a fter training, but also during intense exercise performed at the same relative intensity in the trained and untrained stales. Moreover, this adaptation is often quantitatively just as important as the decline i n muscle glycogen utilization in accounting for the overall carbohydra te-sparing effect of training. This reduced reliance on plasma glucose , which appears to result from a decrease in muscle glucose transport, seems to be related to the training-induced increase in muscle mitoch ondrial respiratory capacity. On the other hand, the training-induced decrease in glucose production (which is the result of reductions in b oth hepatic glycogenolysis and gluconeogenesis) is probably largely du e to alterations in the glucoregulatory hormone response to exercise, although other factors (such as changes in hepatic hormone sensitivity and/or responsiveness) map also play a role. By minimizing the possib ility of hypoglycemia, these adaptations in glucose production and uti lization likely contribute to the increased endurance that results fro m exercise training.