Effect of endogenous carbohydrate availability on oral medium-chain triglyceride oxidation during prolonged exercise

The present study examined the medium-chain triglyceride (MCT) oxidation rate of oral carbohydrate (CHO) + MCT supplements after a glycogen-depletion trial [low glycogen (LG)] and in the glycogen-loaded state [normal-to-high glycogen (HG)]. Eight elite athletes cycled four times 90 min at 50% maximal workload (57% maximal O2 uptake). In two trials, they followed a LG protocol to achieve low-glycogen stores in the leg muscles the evening before the experiment, and in two trials they followed a HG protocol. Subjects received a bolus of 4 ml/kg at the start and 2 ml/kg every 20 min during exercise of either a 15% CHO (long-chain glucose polymer) solution or an equicaloric CHO + MCT suspension. Exogenous MCT oxidation was measured by adding a [1,1,1-13C]trioctanoate tracer to the MCT oil and measuring 13CO2 production in the breath. The results show that 85% of MCT ingested was oxidized in LG and 69% in HG during the 60- to 90-min period. There was no statistically significant difference in MCT utilization between LG and HG. Peak oxidation rates were 0.15 and 0.13 g/min, respectively. MCT contributed 7.6% (LG) and 6.5% (HG) to total energy expenditure during the 60- to 90-min period. Total fatty acid oxidation was significantly elevated in the LG trial but was not influenced by MCT ingestion. Concomitantly, CHO oxidation was reduced in LG but no effect of MCT was observed. We conclude that 1) the contribution of MCT to total energy expenditure was small and 2) strenuous exercise the day before the experiment, followed by a low CHO intake and leading to a low CHO availability, substantially increased total fat oxidation but did not significantly increase MCT oxidation.