Substrate utilization during prolonged exercise after ingestion of 13C-glucose in obese and control subjects

This study was performed to investigate whether the difference in substrate utilization observed between obese and control individuals at rest still exists during prolonged exercise. Using a combination of respiratory exchange and tracer techniques, six obese and six control subjects were investigated while exercising for 2 h on a bicycle ergometer, 1 h after ingesting 100 g naturally-enriched 13C-glucose. Oxidation rates of protein, lipid and carbohydrate (CHO) were measured by indirect calorimetry and that of exogenous glucose by mass spectrometry (13CO2). Before exercise the obese subjects presented a lower rate of CHO utilization with a mean respiratory quotient of .803 compared to .858 for the controls. This impairement of CHO utilization disappeared during exercise where total CHO oxidation was found to be comparable for the obese (94.0 +/- 8.4 g) and the control group (94.3 +/- 6.1g). Exogenous-glucose oxidation was even slightly more elevated in the obese subjects (33.6 +/- 2.5 g compared with 28.1 +/- 2.3 g). Two hypotheses have been proposed to explain the improvement in CHO utilization during exercise after a glucose load: (1) The fall in FFA is sufficient to suppress the inhibition of CHO uptake and oxidation; (2) The insulin resistance decreases during exercise. In conclusion, this study supports the concept that exercise performed by obese individuals stimulates CHO utilization with a concomitant improvement in glucose tolerance.