This study was intended to compare exogenous [(13)C]glucose (Glu(exo)) oxidation in boys with insulin-dependent diabetes mellitus (IDDM) and healthy boys of similar age, weight, and maximal O(2) uptake. In a control trial with water intake (CT) and in a (13)C-enriched glucose trial (GT), subjects cycled for 60 min (58.8 +/- 0.9% maximal O(2) uptake) while the utilization of total glucose, total fat, and Glu(exo) was assessed. In CT, total glucose was 84.7 +/- 9.2 vs. 91.3 +/- 6.6 g/60 min (not significantly different) and total fat was 13.3 +/- 2.2 vs. 11.1 +/- 1.7 g/60 min (not significantly different) in IDDM vs. healthy boys, respectively. In GT, Glu(exo) was 10.4 +/- 1.7 vs. 14.8 +/- 1.1 g/60 min, corresponding to 9.0 +/- 1.0 vs. 12.4 +/- 0.5% of the total energy supply in IDDM and healthy boys, respectively (P < 0.05). Endogenous glucose was spared in both groups by 12.6 +/- 3.5% (P < 0.05). Blood glucose and plasma insulin concentrations were two- to threefold higher in IDDM vs. healthy boys in both trials. In conclusion, Glu(exo) is impaired in exercising boys with IDDM, even when plasma insulin levels are elevated.