Perhaps, however, athletes could have it both ways by eating a high-fat base diet followed by short carbohydrate-loading periods before competition, as the New Zealand researchers mentioned above had subjects do in their study.
More: Why Are Carbs Important?
The rationale for this approach is that an extended period of time (two weeks or more) on a high-fat diet will stimulate increases in fat oxidation capacity during exercise, and that following this adaptation period with a couple of days of carbo-loading immediately preceding a race or other maximal endurance effort will maximize muscle glycogen stores, so the athlete has the best of both worlds.
A recent study from the University of Cape Town, South Africa, suggests that this strategy just might work. Researchers examined the effects of a high-fat diet versus a habitual diet prior to carbohydrate loading on fuel metabolism and cycling time-trial performance.
Five trained cyclists participated in two 14-day randomized cross-over trials during which they consumed either a 65 percent fat diet or their habitual 30 percent fat diet for 10 days, before switching to a 70 percent carbohydrate diet for three days.
All subjects then performed a cycling test consisting of 2.5 hours at 70 percent of peak oxygen uptake followed immediately by a 20km time trial. The high-fat/carbo-loading diet resulted in increased total fat oxidation and reduced total carbohydrate oxidation during exercise.
Most noteworthy, the high-fat/carbo-loading treatment was also associated with improved time trial times. On average, the cyclists completed the 20km time trial 4.5 percent faster after the high-fat/carbo-loading diet.
More: The Evolving Art of Carbo-Loading
Applying This Data to Your Diet
The problem with this study is that the design of the exercise test was biased to take advantage of improved fat burning. The initial 2.5-hour ride at a moderately high intensity ensured that the cyclists' muscles were significantly glycogen depleted before they even started the time trial, forcing a greater reliance on fat, of which the cyclists were more capable after the high-fat diet.
But if this study had instead involved a time trial after a standard warm-up, it is unlikely that the high-fat diet would have been seen to result in better performance. Indeed, other studies have found that a high-fat diet followed by a carbo-loading phase impairs performance in high-intensity time trials that are not immediately preceded by long endurance efforts.
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.
More: 3 Reasons to Eat More Fat
Sign up for your next race.