Substrate utilization during prolonged exercise with ingestion of 13C-glucose in acute hypobaric hypoxia (4,300 m)

Energy substrate oxidation was measured using indirect respiratory calorimetry combined with tracer technique in five healthy young male subjects, during a 80-min exercise period on ergocycle with ingestion of 140 g of 13C-labelled glucose, in normoxia and acute hypobaric hypoxia (445 mmHg or 4,300 m), at the same relative \( {\left( {77\% \dot{V}_{{{\text{O}}_{2} \max }} } \right)} \) and absolute workload (161±8 W, corresponding to 77 and 54% \( \dot{V}_{{{\text{O}}_{{\text{2}}} {\text{max}}}} \) in hypoxia and normoxia). The oxidation rate of exogenous glucose was not significantly different in the three experimental situations: 21.4±2.9, 20.2±1.2 and 17.2±0.6 g over the last 40 min of exercise at ~77 and ~54% \( \dot{V}_{{{\text{O}}_{{\text{2}}} {\text{max}}}} \) in normoxia and in hypoxia, respectively, providing 12.5±1.5, 16.8±1.1 and 14.9±1.1% of the energy yield, although ingestion of glucose during exercise resulted in a higher plasma glucose concentration in hypoxia than normoxia. The contribution of carbohydrate (CHO) oxidation to the energy yield was significantly higher in hypoxia (92.0±2.1%) than in normoxia for both a given absolute (75.3±5.2%) and relative workload (78.1±1.8%). This greater reliance on CHO oxidation in hypoxia was entirely due to the significantly larger contribution of endogenous glucose oxidation to the energy yield: 75.9±1.7% versus 66.6±3.3 and 55.2±3.7% in normoxia at the same relative and absolute workload.