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 O₂ uptake [V̇O_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% V̇O_2peak; RER was restored by I d CHO and CHO intake during cycling (0.90 ± 0.01, 0.8 ± 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 ∼ 25 min undertaken after 2 h of submaximal cycling.