Metabolic availability of glucose ingested 3 h before prolonged exercise in humans.

The aim of the present study was to investigate the extent to which an oral load of glucose ingested 3 h before a 4-h exercise bout of moderate intensity represents an energy source readily available during that exercise. Therefore, five healthy male volunteers drank 100 g of naturally labeled [13C]glucose dissolved in 400 ml of water, rested for 3 h, and then exercised on a treadmill for the next 4 h at about 45% of their individual maximum O2 consumption. Total glucose oxidation was derived from nonprotein respiratory quotient and exogenous glucose oxidation evaluated by the 13C methodology as previously described. Total carbohydrate oxidation averaged 285 +/- 17 g during the 7 h of the test, the global amount of carbohydrate oxidized during the exercising period was 253.1 +/- 16.9 g/4 h. Exogenous glucose oxidation averaged 11.3 +/- 0.7 g during the 3-h period of rest and increased markedly after the beginning of exercise, reaching 18.9 +/- 2.2 g/30 min during the first 30 min of exercise; the total amount of exogenous glucose oxidized during the 4 h of exercise was 67.5 +/- 9.4 g. Throughout the whole period of exercise, blood glucose concentrations remained between 3.5 and 4.0 mmol/l. Exercise induced a major fall in plasma insulin levels that reached undetectable values after 3 and 4 h, whereas plasma glucagon levels tended to rise, but their level never significantly exceeded the basal values; plasma free fatty acids and glycerol increased markedly during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)