Is fat-loading the next endurance-training trend?

Does a temporary high-fat diet increase the amount of fat you burn during exercise?

Would your performance in a long race be improved by doing a fat-load diet before the traditional three days of carbohydrate loading?

These questions have stimulated some very interesting and clever sports science research during the past decade. The rationale behind this thinking is that if you can increase the amount of fat you burn, you will spare those precious carbohydrate stores.

Carbohydrate is the limiting factor in endurance. We don't store much and, the faster we go, the more we burn. When carbohydrate runs low, the pace becomes agonizingly slow.

Taking in carbohydrate in the form of sports drink or foods can delay the bonk, but not prevent it. You just can't absorb enough carbohydrate to keep pace with the amount you are burning.

At the tempos of a marathon, cross-country ski race, a long bike race or triathlon, the carbohydrate deficit accumulates.

In the ceaseless quest to improve sports performance, researchers began to ask whether a higher-fat diet translates into higher fat metabolism, lower carbohydrate dependence, better endurance and faster races. They are not without examples.

Some very successful athletes, including the dominant Russian cross-country skiers, have been fueled by high-fat diets. In addition, higher-fat diets are associated with higher testosterone levels in male athletes.

One of the earliest investigations showed that when elite cyclists consumed a diet consisting of 85% fat calories for four weeks, fat burning during exercise was doubled. The cyclists were also able to pedal longer during a performance test at 75% maximal heart rate.

However, the increased endurance was not enough to qualify as a significant difference. Critics also contend that the intensity of the endurance test was well below race pace and would not be a good indicator of how well an athlete might do during competition.

These types of questions have characterized the research focus. Although it has become clear that adaptation to a high-fat diet increases fat metabolism during exercise, the amount of fat an athlete needs to eat, or for how long, remains to be worked out.

Exercise tests have also not been able to consistently demonstrate a clear improvement in performance, largely due to variability in research methods and inability to duplicate race conditions in the lab. Exercise tests rarely reach three hours in length, so they don't address the needs of athletes engaging in very prolonged events -- when an increase in fat metabolism would be most critical.

Despite these limitations, the results are quite promising. Recent research showed that five weeks on a diet consisting of a modest 53% fat, 15% protein and 32% carbohydrate increased the amount of fat stored within the muscle fiber and improved fat metabolism during exercise.

However, the restricted carbohydrate intake resulted in lower levels of carbohydrate in the muscle. The downside of a high-fat diet is that low carbohydrate levels will result in fatigue and poor quality training and make the athlete vulnerable to illness and injury.

Ideally a high-fat diet should be limited to periods of reduced training or low-intensity training.

As little as five days of a 60-70% fat diet results in substantially higher fat metabolism. The increase in fat burning persisted after switching to two days of a 70% carbohydrate diet. The muscles are now fat-primed and stuffed with carbohydrate.

A taper which includes five days of fat loading before the carbohydrate loading shows great potential to improve performance in long events.

Deborah Shulman, Ph.D. is a physiologist specializing in nutrition, health and sports performance. For more information on her and programs, visit or e-mail

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