Medium-chain triacylglycerols (MCT) are known to hydrolyze readily and completely to fatty acids and to be metabolized more easily by beta-oxidation than long-chain triacylglycerols (LCT). Therefore, we investigated the effect of 2 wk of ingestion of food containing a small amount (6 g) of MCT on energy metabolism during moderate-intensity exercise and high-intensity exercise in recreational athletes. For comparison, the subjects were administered food containing MCT or LCT for 14 d, and were instructed to perform cycle ergometer exercise at a workload corresponding to 60% peak O(2) uptake (VO(2)) for 40 min followed by a workload corresponding to 80% peak VO(2) until exhaustion. Blood lactate concentration, VO(2), VCO(2), and rating of perceived exertion (RPE) were measured at rest and during exercise. The exercise time to exhaustion at a workload corresponding to 80% peak VO(2) was significantly (p<0.05) longer in the MCT trial (10.2+/-7.6 min; mean+/-SD) than in the LCT trial (5.8+/-3.3 min). Blood lactate concentration and RPE during exercise were significantly (p<0.05) lower after ingestion of MCT-containing food. Fat oxidation rate was higher and carbohydrate oxidation rate was lower during exercise in the MCT trial than in the LCT trial, but the differences were not significant. These results indicate that the ingestion of MCT-containing food may suppress utilization of carbohydrate for energy production because of increased utilization of fatty acids for generating energy. In conclusion, our data suggest that short-term ingestion of food containing a small amount of MCT suppresses the increase in blood lactate concentration and RPE during moderate-intensity exercise and extends the duration of subsequent high-intensity exercise, at levels higher than those achieved by ingestion of LCT-containing food.