OXIDATION RATES OF ORALLY INGESTED CARBOHYDRATES DURING PROLONGED EXERCISE IN MEN

Six male volunteers exercised on a cycle ergometer at 65% of maximal work load for 120 min on six occasions while ingesting water (W) only, four doses of maltodextrin (M) [0.92, 1.85, 2.77, and 3.70 g/kg body wt (4, 8, 12, and 16% M, respectively)], and sucrose (8) [1.85 g/kg body wt (8% S)]. Drinks were given during warm-up (8 ml/kg body wt) and each 15 min during exercise (2 ml/kg body wt). M and S were of high C-13 natural abundance. Total carbohydrate (CHO) and fat oxidations were calculated from the nonprotein respiratory exchange ratio. M and S increased total CHO oxidation compared with W; no difference was observed between CHO solutions. Total CHO oxidation decreased continuously with time and more rapidly after W than after M or S. Fat oxidation increased continuously in all treatments. Oxidation rates of ingested CHO were 52 +/- 19, 76 +/- 12, 86 +/- 10, and 91 +/- 9 g/2 h for 4, 8, 12, and 16% M, respectively. The oxidation rate of 8 was 81 +/- 10 g/2 h (not different from 8% M), which indicated that the glucose polymer had no advantage over S. Oxidation rates of M and S increased to a plateau after 90-120 min of exercise. For all solutions except 4% M, the plateau oxidation rate was close to 1.0 g/min. Differences between 8, 12, and 16% M and 8% S were minimal such that ingestion of 8% M or S may well have had an optimal ergogenic effect. With 12 and 16% M, ingestion exceeded the plateau oxidation observed during the last 30-min period, which indicated the accumulation of exogenous CHO in the gastrointestinal tract and/or in unidentified endogenous pools.