Short-term high-fat diet alters substrate utilization during exercise but not glucose tolerance in highly trained athletes

We determined the effect of a high-fat diet and carbohydrate (CHO) restoration on substrate oxidation and glucose tolerance in 7 competitive ultra-endurance athletes (peak oxygen uptake [VO2peak] 68 +/- 1 ml(.)kg-1(.)min-1; mean +/- SEM). For 6 days, subjects consumed a random order of a high-fat (69% fat; FAT-adapt) or a high-CHO (70% CHO; HCHO) diet, each followed by 1 day of a high-CHO diet. Treatments were separated by an 18-day wash out. Substrate oxidation was determined during submaximal cycling (20 min at 65% VO2peak) prior to and following the 6 day dietary interventions. Fat oxidation at baseline was not different between treatments (17.4 +/- 2.1 vs. 16.1 +/- 1.3 g(.)20 min-1 for FAT-adapt and HCHO, respectively) but increased 34% after 6 days of FAT-adapt (to 23.3 +/- 0.9 g(.)20 min-1, p < .05) and decreased 30% after HCHO (to 11.3 +/- 1.4 g(.)20 min-1, p < .05). Glucose tolerance, determined by the area under the plasma [glucose] versus time curve during an oral glucose tolerance (OGTT) test, was similar at baseline (545 +/- 21 vs. 520 +/- 28 mmol(.)L-1(.)90 min-1, after 5-d of dietary intervention (563 +/- 26 vs. 520 +/- 18 mmol(.)L-1(.)90 min-1) and after 1 d of high-CHO (491 +/- 28 vs. 489 +/- 22 mmol(.)L-1(.)90 min-1 for FAT- adapt and HCHO, respectively). An index of whole-body insulin sensitivity (S(I), 10000/divided by fasting [glucose] X fasting [insulin] X mean [glucose] during OGTT X mean [insulin] during OGTT) was similar at baseline (15 +/- 2 vs. 17 +/- 5 arbitrary units), after 5-d of dietary intervention (15 +/- 2 vs. 15 +/- 2) and after 24 h of CHO loading (17 +/- 3 vs. 18 +/- 2 for FAT- adapt and HCHO, respectively). We conclude that despite marked changes in the pattern of substrate oxidation during submaximal exercise, short-term adaptation to a high-fat diet does not alter whole-body glucose tolerance or an index of insulin sensitivity in highly-trained individuals.