Eating Fat May Improve Endurance

From the WebMD Archives

Dec. 22, 1999 (Atlanta) -- Usually, when fat and exercise are mentioned together, it has to do with losing extra pounds. However, a new study from Scotland shows that consuming extra fat -- at least by athletes in tiptop shape -- can enhance athletic endurance. "We discovered that raising the circulating levels of FFAs [free fatty acids] immediately prior to exercise can increase the capacity to perform prolonged exercise in endurance-trained individuals," lead author Yannis Pitsiladis, MD, tells WebMD.

According to The Nutrition Bible, by Jean Anderson and Barbara Deskins (William Morrow and Company), FFAs are by-products of fat and are usually formed when the fats in butter, lard, margarine, or cooking oils are digested and transported within the body.

In the study, the researchers sought to clarify the relationship between carbohydrates and fat oxidation during exercise. Their research was based on the understanding that the biggest reason for fatigue during exercise is the depletion of stored carbohydrates by muscles.

"Depletion of muscle glycogen is widely considered to be the most likely candidate limiting prolonged exercise performance," Pitsiladis tells WebMD. "There has been much research interest directed toward finding ways of 'sparing' muscle glycogen in order to delay muscle fatigue."

For years, marathoners and other endurance athletes have practiced "carbohydrate loading" based on this premise. According to Ellen Coleman, RD, when carbohydrates are consumed, the body changes much of it into glucose, the chief source of energy for the body. Glucose that is not needed immediately is stored as glycogen in the liver and muscles for later use. "Although eating carbohydrates 30 to 45 minutes before exercise raises insulin levels and lowers blood glucose, these effects are temporary and will not harm performance," write Coleman and colleagues in the February 1997 issue of The Physician and Sportsmedicine. "In fact, consuming carbohydrates an hour before exercise can improve performance. Carbohydrate feedings 3 to 4 hours before exercise also enhance performance by 'topping off' glycogen stores."

Pitsiladis says that fat may enhance performance even more. Pitsiladis, who is a professor at the Exercise Physiology Center for Exercise Science and Medicine at University of Glasgow in Scotland, says this new study was the first in humans to examine the effects of increased FFA availability on exercise performance.


To study this relationship, the researchers chose six well-conditioned male athletes. The athletes were an average 27 years of age, stood an average of 5 feet 11 inches tall, and weighed an average of 167 pounds. Each participated in two nine-day diet and exercise programs, each divided into three segments.

In the first part, the athletes followed their normal diet and then exercised on stationary bicycles to exhaustion. A work rate was selected that would result in fatigue after approximately 90-100 minutes at about 50? F, and the athletes were given ample amounts of water.

Then after three-and-a half days of consuming a 70% carbohydrate diet, either an experimental 70% carbohydrate or a 90% fat meal was given four hours before the subjects again cycled to exhaustion. After three-and-a-half more days on the high-carbohydrate diet, the athletes were fed the alternate experimental meal; those who had the high-carbohydrate diet before were given the fat meal, and vice-versa. The high-carbohydrate diet was designed to increase muscle glycogen content. The high-fat meal, which included FFA and an injection of heparin, was intended to elevate the levels of circulating fats in the blood.

The high-fat meal and shot of heparin are not something anyone should try on their own, the researchers caution. Heparin is a substance found in the liver, lungs, and other tissues that prevents blood from clotting. It is sometimes injected into a patient during surgery to prevent clotting. "Heparin injection to elevate plasma fatty acid concentration would not represent sound medical practice," write the researchers.

The researchers found that heart rates remained fairly stable throughout all the exercise periods, regardless of diet. However, they found that all study participants exercised for longer periods on the fat trial. Therefore, the researchers conclude that burning fat during exercise prevents the carbohydrates from being used up, and thus wards off exhaustion.

However, consuming more fat did not make the tasks easier, notes Pitsiladis. "We were very surprised to find that five of the six subjects ranked the fat trial as the more difficult trial," he tells WebMD. "We are, therefore, currently examining whether we can overcome this increased perception of effort that normally accompanies this intervention [of consuming more fat before exercising]."


Pitsiladis says the study was not intended to send athletes onto the field after consuming gobs and gobs of fat. Rather, they hoped to add to the understanding of the factors that may contribute -- and limit -- athletic performance.

Vital Information:

  • It is believed that depletion of stored carbohydrates by the muscle is the most likely limiting factor in prolonged exercise performance.
  • A new study shows, however, that well-trained athletes who eat a high-fat meal before exercising can increase their endurance.
  • Researchers suspect that burning off fat delays the depletion of carbohydrates.
WebMD Health News
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