Fat adaptation and prolonged exercise performance
Journal of applied physiology
A. Hawley Effect of fat adaptation and carbohydrate restoration on prolonged endurance exercise. J Appl Physiol 91:  2001.-We determined the effect of fat adaptation on metabolism and performance during 5 h of cycling in seven competitive athletes who consumed a standard carbohydrate (CHO) diet for 1 day and then either a high-CHO diet (11 g ⅐ kg Ϫ1 ⅐ day Ϫ1 CHO, 1 g ⅐ kg Ϫ1 ⅐ day Ϫ1 fat; HCHO) or an isoenergetic high-fat diet (2.6 g ⅐ kg Ϫ1 ⅐ day Ϫ1 CHO,
... kg Ϫ1 ⅐ day Ϫ1 CHO, 4.6 g ⅐ kg Ϫ1 ⅐ day Ϫ1 fat; fat-adapt) for 6 days. On day 8, subjects consumed a high-CHO diet and rested. On day 9, subjects consumed a preexercise meal and then cycled for 4 h at 65% peak O2 uptake, followed by a 1-h time trial (TT). Compared with baseline, 6 days of fat-adapt reduced respiratory exchange ratio (RER) with cycling at 65% peak O2 uptake [0.78 Ϯ 0.01 (SE) vs. 0.85 Ϯ 0.02; P Ͻ 0.05]. However, RER was restored by 1 day of high-CHO diet, preexercise meal, and CHO ingestion (0.88 Ϯ 0.01; P Ͻ 0.05). RER was higher after HCHO than fat-adapt (0.85 Ϯ 0.01, 0.89 Ϯ 0.01, and 0.93 Ϯ 0.01 for days 2, 8, and 9, respectively; P Ͻ 0.05). Fat oxidation during the 4-h ride was greater (171 Ϯ 32 vs. 119 Ϯ 38 g; P Ͻ 0.05) and CHO oxidation lower (597 Ϯ 41 vs. 719 Ϯ 46 g; P Ͻ 0.05) after fat-adapt. Power output was 11% higher during the TT after fat-adapt than after HCHO (312 Ϯ 15 vs. 279 Ϯ 20 W; P ϭ 0.11). In conclusion, compared with a high-CHO diet, fat oxidation during exercise increased after fat-adapt and remained elevated above baseline even after 1 day of a high-CHO diet and increased CHO availability. However, this study failed to detect a significant benefit of fat adaptation to performance of a 1-h TT undertaken after 4 h of cycling.