If you're more of an endurance junkie than a powerhouse athlete, take heart: Duration of exercise could have the most positive impact on cholesterol, according to a breakthrough study. The research project is the first of its kind to establish the type of exercise needed to make changes in cholesterol profiles and to examine how and why exercise works.

The study, conducted at Duke University and East Carolina University, involved 111 inactive, overweight men and women, whom researchers divided into four exercise groups. Instead of simply measuring cholesterol levels, the researchers looked at cholesterol particle sizes and found that those who exercised most—the 20-mile-a-week joggers—had the greatest positive changes in their cholesterol particle sizes (New England Journal of Medicine, 2002, vol. 347, no. 19).

Evaluating cholesterol particle size is a revelatory new twist on the study of the correlation between exercise and cholesterol. Until now, the exercise and research communities were baffled about how exercise benefits the heart because exercise has never proven to have a direct impact on total cholesterol levels. It turns out that "bad" cholesterol is carried by smaller, denser protein particles, not the larger, fluffy particles that are less likely to clog arteries. According to the study, the key to developing more large protein particles and decreasing the number of harmful smaller ones is longer stints of exercise. "I believe we now know definitively that exercise does have a great positive effect on the lipoprotein particles that carry cholesterol," says lead researcher Cris A. Slentz, of Duke University.

—Chelsea Vurciaga