The purpose of this study was to compare metabolic and performance effects of ingestion of glucose, acid-modified high amylose starch and placebo during endurance exercise at 60% of V02 peak. Specifically, we sought to determine effects on plasma glucose maintenance, total carbohydrate oxidation as measured by indirect calorimetry, exogenous carbohydrate oxidation using 13C02 excretion, and endurance performance. Ten male subjects cycled to fatigue at 60% of V02peak, after ingesting acid-modified high amylose starch (AMS), glucose, or a sweet non-caloric placebo. The ingestion protocol consisted of a bolus feeding of 4 ml/kg (280 ml, 22 g CHO for a 70 kg person) at the onset of exercise, and 2 ml/kg body weight every 15 minutes for the duration of the exercise. All trials were separated by 1 week and treatments were administered in a random, double blind manner. The plasma glucose derived from exogenous carbohydrates (CHO) at the time of fatigue for AMS, and glucose were 23% and 46% respectively. A significant difference was seen among treatments. Maximum exogenous 13C oxidation rates were 0.69 g/min, 0.21 g/min for the GLU and AMS trials respectively. Exogenous carbohydrate oxidized was different among treatments. Overall time to exhaustion was significantly lower in the CON treatments (138 min), compared to the AMS (158 min) and GLU (159 min) treatments. Carbohydrate in the form of an acid-modified starch was digested, absorbed, and metabolized when ingestion during endurance exercise, as supported by the direct measured of absorption and oxidation of the exogenous carbohydrate using 13C methodology.