Macro partitioning
Five days of high-fat, low-carbohydrate dieting followed by one day of carbohydrate restoration enhances fat oxidation and spares muscle glycogen during prolonged submaximal cycling, but does not improve time trial performance compared to a standard high-carbohydrate diet.
If you are an endurance athlete, switching to a high-fat, low-carb diet for 5 days and then eating high carbs for 1 day before a race will change your metabolism to burn more fat. However, this strategy does not make you faster than eating a standard high-carb diet. The metabolic shift is real, but it does not provide a performance advantage in time trials.
These data show significant metabolic adaptations with a brief period of high-fat intake, which persist even after restoration of CHO availability. However, there was no evidence of a clear benefit of fat adaptation to cycling performance.
Why this rating
Randomized crossover design with well-trained subjects, though small sample size (n=8).
Source
Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling
Louise M. Burke et al. · Journal of Applied Physiology · 2000
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