We compared the effects of high-fat and high-carbohydrate dietary conditions on metabolism and short- and ultra-endurance cycling performance. Seven cyclists (VO(2)max 72 +/- 7 mL x kg(-1) x min(-1)) underwent a 2-week adaptation to each of the following 3 diets: 14-day high carbohydrate with 70 +/- 9 percent energy (%E) carbohydrate, 16 +/- 5%E fat, and 14 +/- 2%E protein; 14-day high fat with 66 +/- 10%E fat, 20 +/- 3%E protein, and 15 +/- 4%E carbohydrate; and 11.5-day high-fat diet followed by 2.5-day carbohydrate-loading. The conditions included a pre-exercise meal of the same composition as the preceding diet. Each diet condition was preceded by a 2-week standardizing normal diet. The exercise test lasted approximately 5 hours and comprised a 15-minute trial, an incremental test to measure the peak fat-oxidation rate, and a 100-km trial. Sports bars and a 5% carbohydrate solution were ingested during the tests. The diets had no statistically significant effect on 15-minute performance, although the high-fat condition tended to reduce distance covered by -2.4% (95% confidence interval: -5.4% to +0.6%, P =.11) relative to the fat with carbo-loading condition. In the 100-km time trial, the high-fat and the fat with carbohydrate-loading conditions attenuated the decline in power output observed in the high-carbohydrate condition (P =.03 to.07), although the corresponding improvement in performance time of 3% to 4% (-2% to 10%) was not statistically significant (P =.16 to.22). Power output during the final 5 km of the time trial in the fat with carbo-loading condition was 1.3-fold (1.0 to 1.6, P =.04) greater than in the high-carbohydrate condition. Overall, for every 10%E increase in dietary fat, 100-km mean power increased by 2% (-0.0% to 4%, P =.06). Relative to the high-carbohydrate condition, the high-fat conditions resulted in the following metabolic changes consistent with greater lipolysis and fuel availability: lower plasma insulin concentration before exercise (P <.0001), and during exercise a 10% to 20% higher plasma-glucose concentration (P <.01), higher plasma glycerol (P <.05), and a 2.5-fold to 2.9-fold increase in the peak fat-oxidation rate (P <.0001). In conclusion, high-fat dietary conditioning increased fat oxidation, and although the main effects were not statistically significant, there was some evidence for enhanced ultra-endurance cycling performance relative to high-carbohydrate.
