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.<br /> <br /><br />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 [[latin capital V with dot above]O2peak] + 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).<br /> <br /><br />Results: FAT-adapt reduced respiratory exchange ratio (RER) values before and during cycling at 70% [latin capital V with dot above]O2peak; RER 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).<br /> <br /><br />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.<br />