High-fat diets help athletes perform.

Marathon runners and other highly trained endurance athletes appear to perform better after consuming a high-fat diet for several weeks than after eating their customary carbohydrate-rich, low-fat meals, a new study reveals.

Peter J. Horvath and his colleagues at the State University of New York at Buffalo tailored a trio of diets for 25 competitive runners 18 to 53 years old who trained at least 40 miles each week. Each 4-week-long diet aimed to provide the same total energy, but fat contributed just 16 percent of calories in one diet, 30 percent of calories in the second (the recommended amount for most people today), and a full 45 percent of calories in the last. Each athlete cycled through all three diets, but 13 runners failed to comply fully with the third regimen, complaining that it was too fatty, even though they gained no weight.

After completing a 4-week diet, each athlete performed a series of exercise tests. At the end of the two fattier diets, the athletes increased the amount of time they could run at peak capacity by an average of 7 percent-or about 30 seconds. Overall endurance increased 14 percent, and exercise-induced muscle fatigue decreased. Moreover, the study found that runners who consumed the fattiest diet used their stored energy more efficiently than they did when on the lowest-fat diet.

The body takes weeks to adjust to the new diets-changing where fat is stored as well as how accessible it is for fueling exercise. Horvath suspects that a failure to wait for such changes may explain why earlier studies, often just a few days long, missed the performance advantages of higher-fat diets.

Jaya T. Venkatraman, also at Buffalo, found that the athletes' immune systems responded better after the higher-fat diets, increasing their production of germ-fighting white blood cells and reducing their generation of inflammatory agents.