Lack of effect of NaCl and/or metoclopramide on exogenous (13C)-glucose oxidation during exercise

The purpose of this study was to compare the oxidation rate of ingested glucose during prolonged exercise, without and with the addition of sodium to the solution. The effect of metoclopramide, a drug which favors gastric emptying, was also investigated since gastric emptying could be a factor limiting the bioavailability of ingested glucose. Six subjects performed four bouts of exercise of 2 hours each at 64 +/- 4% VO2max on a cycle ergometer during which they ingested 100 g of glucose enriched with 13C, without (trials 1 and 3) and with (trials 2 and 4) addition of 25 mmol.l(-1) of NaCl. The glucose solution was ingested in four equal volumes (175 ml containing 25 g of glucose) at 0, 30, 60 and 90 min of the exercise period. For the trials 3 and 4, the subjects were given 10 mg of metoclopramide orally 60 min before the beginning of exercise. The VO2, VCO2 and heart rate were similar in response to exercise between the four trials. No significant difference was observed between trials for the oxidation rates of ingested glucose during the first as well as the second hour of exercise. Over the 120 min of exercise, the amounts of exogenous glucose oxidized were 52.0 +/- 9.6, 54.3 +/- 10.9, 52.7 +/- 12.3 and 53.3 +/- 10.4 grams for trials 1 to 4, respectively. The contribution of exogenous glucose oxidation to the energy yield represented 13.0 +/- 1.8% without and 13.2 +/- 1.9% with addition of NaCl. The amounts of endogenous carbohydrate and fat oxidized were also similar in the four trials. These results suggest that neither the addition of NaCl to glucose solutions nor the ingestion of metoclopramide increases the contribution of the oxidation of ingested glucose to the total energy yield during prolonged exercise.