EFFECT OF ENDOGENOUS CARBOHYDRATE AVAILABILITY ON ORAL MEDIUM-CHAIN TRIGLYCERIDE OXIDATION DURING PROLONGED EXERCISE

The present study examined the medium-chain triglyceride (MCT) oxidati on rate of oral carbohydrate (CHO)+MCT supplements after a glycogen-de pletion trial [low glycogen (LG)] and in the glycogen-loaded state [no rmal-to-high glycogen (HG)]. Eight elite athletes cycled four times 90 min at 50% maximal workload (57% maximal O-2 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 foll owed 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-cha in glucose polymer) solution or an equicaloric CHO + MCT suspension. E xogenous MCT oxidation was measured by adding a [1,1,1-C-13]trioctanoa te tracer to the MCT oil and measuring (CO2)-C-13 production in the br eath. 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 oxi dation 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 9 0-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 w as small and 2) strenuous exercise the day before the experiment, foll owed by a low CHO intake and leading to a low CHO availability, substa ntially increased total fat oxidation but did not significantly increa se MCT oxidation.