Macro partitioning
Elevating plasma free fatty acid availability during intense exercise (approx. 80% VO2max) increases fat oxidation and spares carbohydrate, but does not enhance performance in exercise lasting approximately 1 hour.
For intense workouts lasting around an hour (like interval training or hard races), eating a high-fat meal beforehand will not help you perform better, even if it changes what fuel your body burns. Your body will still rely heavily on carbohydrates because the intensity is too high to use fat efficiently. Stick to your standard pre-workout nutrition strategy rather than experimenting with high-fat loads for these specific efforts.
We conclude: (1) increased fat availability during intense cycling increases the rate of fat oxidation; but (2) the reduction in the rate of carbohydrate oxidation in the presence of high circulating plasma free FA is unlikely to enhance intense exercise performance lasting about 1 h; (3) substrate selection during intense (about 80 % VO2max) exercise is dominated by carbohydrate oxidation.
Why this rating
Randomized crossover design with highly trained subjects, though small sample size (n=7).
Source
Effect of altering substrate availability on metabolism and performance during intense exercise
John A. Hawley et al. · British Journal Of Nutrition · 2000
DOI 10.1017/s0007114500002440
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