Ten subjects were studied to determine the effects of medium chain triglyceride plus carbohydrate (MCT), long chain triglyceride plus carbohydrate (LCT), and carbohydrate (CHO) feedings on substrate utilization during 60 min of endurance exercise (70 % V̇O2max). Based on the respiratory exchange ratio, the percentage of energy obtained from lipid metabolism during the MCT, LCT, and CHO trials was similar (37.2 % to 39.1 %). This, however, differed significantly (P < 0.05) from the percentage of lipid metabolized (49.1 %) during the control (CONT) trial (fasted state). Serum glycerol paralleled the estimated lipid oxidation and was 74 % higher in the CONT trial when compared to the MCT, LCT, and CHO trials by the end of exercise. Prior to exercise, glucose levels (∼ 4.6 mM) were the same for all treatments, but insulin levels for the three experimental trials were threefold higher than those of the CONT. After 15 min of exercise glucose levels had declined significantly (P < 0.05) in the experimental trials, while rising slightly during the CONT trial. Despite this hypoglycemic state, no difference in the perceived exertion was found between the experimental and CONT trials. These data indicate that hyperinsulinemia causes a preferential oxidation of carbohydrates during exercise even when other substrates (FFA and ketones) are present in elevated amounts. Also, MCT in combination with CHO is not a viable energy source during an acute endurance exercise.