METABOLIC AVAILABILITY OF MEDIUM-CHAIN TRIGLYCERIDES COINGESTED WITH CARBOHYDRATES DURING PROLONGED EXERCISE

The present study examined the metabolic response to medium-chain triglycerides (MCTs) ingestion with or without carbohydrates (CHOs). Eight well-trained athletes cycled 4 x 180 min at 50% maximal work rate (57% maximal O-2 consumption). Subjects drank a bolus of 4 ml/kg at the start and 2 ml/kg every 20 min during exercise of either a 15% (214 g) CHO solution (CHO trial), an equicaloric 149 g CHO-29 g MCT suspension (CHO+MCT trial), 214 g CHO [high CHO (HCHO)]-29 g MCT suspension (HCHO+MCT trial) or 29 g MCT solution (MCT trial). Exogenous MCT oxidation was measured by adding a [1,1,1-C-13]trioctanoate tracer to the MCT oil. (CO2)-C-13 enrichment of breath samples was measured every 15 min. During the second hour (60- to 120-min period), the amount of MCT oxidized was 72% of the amount ingested during the CHO+MCT trial, whereas during the MCT trial only 33% was oxidized. The rate of MCT oxidation increased more rapidly during the HCHO+MCT and CHO+MCT trials compared with the MCT trial, yet in all three cases the oxidation rate stabilized at 0.12 g/min during 120-180 min of exercise. It is concluded that more MCTs are oxidized when ingested in combination with CHOs. Data do confirm the hypothesis that oral MCTs might serve as an energy source in addition to glucose during exercise because the metabolic availability of MCTs was high during the last hour of exercise, with oxidation rates being similar to 70% of the ingestion rate. However, the maximal amount of oral MCTs that can be tolerated in the gastrointestinal tract is small (similar to 30 g) and limits the contribution of oral MCTs to total energy expenditure to values between 3 and 7%.