Fat is the muscles' primary fuel for low- to moderate-intensity exercise. Carbohydrate, which is the muscles' primary fuel for moderately high- to high-intensity exercise, is stored only in small amounts in the body. Consequently, carbohydrate fuel depletion is a major cause of fatigue during prolonged exercise at higher intensities, such as triathlons.
Fat fuel supplies, by contrast, are virtually unlimited in the body. Thus, by increasing their reliance on fat fuel and decreasing their reliance on carbohydrate fuel during race-intensity exercise, runners could theoretically delay fatigue and perform better.
In summary, switching to a general high-fat diet will increase your fat burning capacity but will not affect your performance, so don't bother doing so. However, you may experiment with 10 or 11 days on a high-fat diet followed by two days of carbo-loading before a longer race, as it won't harm you and there's a chance it will have a positive impact on your performance.
Active Expert Matt Fitzgerald is the author of several books on triathlon and running, including Runner's World Performance Nutrition for Runners (Rodale, 2005) and his newest, Brain Training for Runners.
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