Trained Runners Perform Better On Diet Moderately High In Fat Than On High-Carbohydrate, Low-Fat Regimen, Study Shows

BUFFALO, N.Y. -- Highly trained runners hoping to improve their performance by slashing fat from their diets may be heading down the wrong nutritional path, a small pilot study by University at Buffalo researchers implies.

"Our data are consistent with a number of investigations that have shown muscular adaptations to a high-fat diet which result in increased endurance," said John J. Leddy, M.D., associate director of UB’s Sports Medicine Institute and a co-author of the study, which was reported in Medicine and Science in Sports and Exercise.

"Furthermore, our findings present evidence that severely restricting dietary fat may be detrimental to endurance performance."

"We would not advise fat intake in excess of 30 percent of total daily calories, but athletes who limit fat consumption to well below this level, especially without adequately compensating with other energy sources, may adversely affect performance," he stated.

Historically, researchers have agreed that a high-carbohydrate, low-fat diet is preferred for maximum endurance during moderate-to-high-intensity exercise. Many of the studies supporting this view have used untrained or moderately trained subjects, Leddy said. Those results may not apply to trained runners because trained athletes metabolize fats more efficiently that untrained persons, he noted.

Previous studies using a high-fat diet often severely restricted carbohydrates as well, Leddy said, while carbohydrate levels remained at 50 percent of total calories even on the increased-fat diet in this study.

To compare performance levels, UB researchers placed subjects on three different diets for one week each, and conducted exercise testing at the end of each week. The three diets were:

Six male members of the UB track team took part in the study. The athletes were tested on treadmills after each week’s diet to determine maximum oxygen consumption -- a measure of the efficiency of one’s oxygen-transport system and an indicator of aerobic capacity -- and endurance.

To eliminate any carry-over effect from the carbohydrate regimen, the runners consumed and were tested on the higher-fat diet before the high-carbohydrate diet.

Results showed the athletes ran 20 percent longer on the high-fat diet than on the carbohydrate diet, and 32 percent longer on the high-fat diet than on their normal diets.

Maximum oxygen consumption was 11.4 percent higher on the high-fat diet than on the high-carbohydrate diet, results showed.

Leddy said the study design was actually biased against the fat diet.

"One of the purposes of the study design was to deplete muscle glycogen to observe the effect of presumably higher intramuscular fat. If the study design caused muscle glycogen depletion sufficient to negatively affect performance, it should have affected the higher-fat runners more than the high-carbohydrate runners because they started with lower muscle glycogen."

"In fact, the opposite occurred," he said. "The only difference between these two groups was the higher amount of dietary fat consumed over a one-week period. The total daily calories, training regimen and training response were identical."

This small study shows that athletes who severely restrict the amount of fat in their diets may be sacrificing some endurance, Leddy said. He suggested that researchers do further studies on the role of fat as an energy source during heavy exercise.

Authors of the study, in addition to Leddy, were Deborah M. Muoio, now at the University of North Carolina; Peter J. Horvath, Ph.D., and Atif B. Awad, Ph.D., of the UB Department of Nutrition, and David R. Pendergast, Ph.D., of the UB Department of Physiology.