Fat is the muscles’ primary fuel for low- to moderate-intensity exercise. Carbohydrate is the muscles’ primary fuel for moderately high- to high-intensity exercise. Carbohydrate fuel supplies are very limited in the body, such that carbohydrate fuel depletion is a major cause of fatigue during prolonged exercise at higher intensities (e.g. triathlons and marathons). Fat fuel supplies 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, endurance athletes could theoretically delay fatigue and perform better. Endurance training and increased fat consumption are known to increase fat burning during exercise. But is there solid proof that endurance athletes can actually perform better by training and eating to burn more fat? Let’s explore the question.
Several years ago, researchers from the University of Buffalo published an interesting study on the performance effects of various levels of fat consumption in men and women. Endurance and VO2max tests were performed 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.
One major limitation of this study 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 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 research, however, has 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 100-km cycling test. 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 100-km 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. In a recent review of the existing literature, researchers from Kansas State University wrote, “We and others have observed that although fat oxidation may be increased, 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.”
That’s why the New Zealand researchers mentioned above thought to include a hybrid diet—a high-fat base diet followed by a short carbohydrate-loading period—in their study. The rationale for this approach is that a couple of weeks 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 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 20-km time trial. The high-fat diet resulted in increased total fat oxidation and reduced total carbohydrate oxidation during exercise. Most noteworthy, the high-fat treatment was also associated with improved time trial times. On average, the cyclists completed the 20-km time trial 4.5 percent faster after the high-fat 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.
What About Training for Fat Burning?
Some endurance sports coaches believe in emphasizing training in the “fat-burning” zone (approximately 60 percent VO2max) to increase fat-burning capacity and thereby increase fat-reliance in races. Perhaps the best-known advocate of this approach was Phil Maffetone, an endurance sports coach who made his name by developing a training philosophy that was characterized by an extreme emphasis on the importance of fat metabolism. He taught his athletes to do virtually all of their training at a very low intensity to maximize fat metabolism and stimulate physiological adaptations that increased the body’s capacity for fat oxidation in subsequent workouts. Over time, Maffetone believed, the athlete would be able to swim, bike or run faster and faster at the same, low, fat-burning intensity.
Research has shown that training in the fat-burning zone does improve fat-burning capacity. However, it only improves fat-burning capacity within the fat-burning zone itself—that is, at lower exercise intensities. No matter how fit they are or in what manner they’ve trained, all runners rely on carbohydrate when racing at intensities that are near or above the lactate threshold. Indeed, despite being well adapted for fat burning, elite male marathon runners oxidize carbohydrate almost exclusively during competition. Only slower marathon runners (3:30-plus) and ultramarathon runners are likely to benefit from emphasizing training in their fat-burning zone.
In summary, there is probably nothing special you need to do with your diet or your training to become a fat-burning machine and, as such, a faster racer. Proper training will automatically increase your fat-burning capacity, but this adaptation will not directly influence your performance in races lasting less than several hours. If you care to, 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 slight chance it might help.