Metabolic availability of oral glucose during exercise: a reassessment.

The purpose of this study was to reassess the metabolic availability of oral glucose during prolonged exercise in man, using 13C-labeling and a computation procedure (J Appl Physiol 69:1047-1052, 1990) that correctly takes into account changes in isotopic composition of CO2 arising from oxidation of endogenous substrates (Rendo). These changes are due to glucose ingestion associated with exercise. Each of the seven subjects completed three 2-hour periods of exercise at 67% maximum oxygen consumption (VO2max) on an ergocycle, with ingestion of water (1,000 mL) or 60 g (in 1,000 mL water) of 13C-labeled glucose at two levels of enrichment (13C/12C = 1.11482% and 1.13303%). As expected, Rendo significantly increased from rest to exercise with water ingestion (1.09888% +/- .00196% to 1.09970% +/- .00175%) and with glucose ingestion (1.10002% +/- .00159%) due to changes in the respective contributions of endogenous carbohydrates and fat to energy requirements as assessed by the respiratory exchange ratio (RER). When changes in Rendo were taken into account, the estimated amount of exogenous glucose oxidized was 38.8 +/- 10.3 g. Much higher values were found when Rendo at rest or during exercise with water ingestion were used in the computation (42.3 +/- 10.3 to 65.1 +/- 20.5 g) according to the commonly used method. Examination of data in the literature indicates that the reported oxidation rate of exogenous glucose (g/min) is significantly related to oxygen consumption (VO2) (L/min; r = .592) and that exogenous glucose contributes approximately 14% to 17% to the energy requirement.(ABSTRACT TRUNCATED AT 250 WORDS)