Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability

Purpose: Five days of a high-fat diet produce metabolic adaptations that increase the rate of fat oxidation during prolonged exercise, We investigated whether enhanced rates of fat oxidation during submaximal exercise after 5 d of a high-fat diet would persist in the face of increased carbohydrate (CHO) availability before and during exercise. Methods: Eight well-trained subjects consumed either a high-CHO (9.3 g(.)kg(-1.)d(-1) CHO, 1.1 g(.)kg(-1.)d(-1) fat; HCHO) or an isoenergetic high-fat diet (2.5 g(.)kg(-1.)d(-1) CHO, 4.3 g(.)kg(-1.)d(-1) fat; FAT-adapt) for 5 d followed by a high-CHO diet and rest on day 6. On day 7, performance testing (2 h steady-state (SS) cycling at 70% peak O-2 uptake [(V) over dot O-2peak] + time trial [TT]) of 7 kJ(.)kg(-1)) was undertaken after a CHO breakfast (CHO 2 g(.)kg(-1)) and intake of CHO during cycling (0.8 g(.)kg(-1).h(-1)). Results: FAT-adapt reduced respiratory exchange ratio (RER) values before and during cycling at 70% (V) over dot O-2peak; was restored by 1 d CHO and CHO intake during cycling (0.90 +/- 0.01, 0.80 +/- 0.01, 0.91 +/- 0.01, for days 1, 6, and 7, respectively), RER values were higher with HCHO (0.90 +/- 0.01, 0,88 +/- 0.01 (HCHO > FAT-adapt, P < 0.05), 0.95 +/- 0.01 (HCHO > FAT-adapt, P < 0,05)). On day 7, fat oxidation remained elevated (73 +/- 4 g vs 45 +/- 3 g, P < 0.05), whereas CHO oxidation was reduced (354 +/- 11 g vs 419 +/- 13 g, P < 0.05) throughout SS in FAT-adapt versus HCHO. TT performance was similar for both trials (25.53 +/- 0.67 min vs 25.45 +/- 0.96 min. NS). Conclusion: Adaptations to a short-term high-fat diet persisted in the face of high CHO availability before and during exercise, but failed to confer a performance advantage during a TT lasting similar to 25 min undertaken after 2 h of submaximal cycling.