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.
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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. Can you achieve this effect by maintaining a high-fat diet? If so, is there a downside?
Fat as Fuel?
Several years ago, researchers from the University of Buffalo published a study on the performance effects of various levels of fat consumption in men and women. Endurance and VO2max tests were completed at the end of four-week periods in which runners consumed diets of 16 percent, 31 percent, and 44 percent fat. Time to exhaustion in the endurance test was 14 percent greater at the end of the medium-fat diet than it was at the end of the low-fat diet. However, there was no change in VO2max.
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These results would seem to suggest that increasing fat intake increases endurance, perhaps by boosting fat burning during exercise. One major limitation of this study, however, was that the order of the diets was not random, therefore we cannot rule out the possibility that the runners performed better in the second endurance test simply because they were more familiar with it, or in better shape, not because of their diet.
Also, there was no difference in the rate of fat burning in the second endurance test versus the first. If higher fat intake was the cause of superior endurance, we would expect increased fat burning during exercise to be the mechanism.
Other studies have found that increased fat intake does result in greater fat oxidation during exercise. Researchers from New Zealand compared the effects of a 14-day high-carbohydrate diet, a 14-day high-fat diet, and an 11.5-day high-fat diet followed by a 2.5-day carbo-loading diet on fat oxidation and performance in a 15-minute cycling test and a 100km cycling test.
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Performance in the 15-minute test was slightly better after the high-carb diet, but not to a statistically significant degree, while performance in the 100km test was slightly better, but again not to a statistically significant degree, following the high-fat diet. Fat oxidation was significantly greater during the 100-km test following the high-fat diet.
Like this study, other studies have also suggested that, while increased fat intake may increase endurance, it may also reduce performance in shorter, higher-intensity races. This was shown in a 2003 study from the University of Connecticut.
Twenty volunteers were divided into two groups and placed on either an endurance training program and a high-fat diet (61 percent fat) or an endurance training program and a moderate-fat diet (25 percent fat) for six weeks.
They performed a VO2max test and a 45-minute time trial before and after the study period. Members of the high-fat diet group exhibited a marked increase in fat burning during the 45-minute time trial, but their work output dropped by 18 percent relative to the moderate-fat group.
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The Best of Both Worlds
In a recent review of the existing literature on this topic, researchers from Kansas State University concluded, "We and others have observed that although fat oxidation may be increased [with a high-fat diet], the ability to maintain high-intensity exercise (above the lactate threshold) seems to be compromised or at least indifferent when compared with consumption of more carbohydrate."