Effect of glucose ingestion on energy substrate utilization during prolonged muscular exercise

The distribution of substrates utilized during prolonged exercise was investigated in normal human volunteers with an without ingestion of 100 g exogenous glucose. The energy provided by protein oxidation was derived from urinary nitrogen excretion and the total energy provided by carbohydrates and lipids was calculated from respiratory quotient (RQ) determinations. The contribution of exogenous glucose to the energy supply was determined by an original procedure using "naturally labeled 13C-glucose" as metabolic tracer. Protein oxidation provided between 1 and 2% of the total energy requirement; this amount was not affected by glucose ingestion. In the absence of exogenous glucose ingestion, carbohydrate were progressively replaced by lipids as source of energy. Exogenous glucose contributed markedly to total carbohydrate oxidation and decreased the percentage of energy derived from lipids. In addition, ingestion of exogenous glucose resulted in a significant economy of endogenous carbohydrates and permitted to prolong the duration of exercise.