People with unhealthy cholesterol levels are typically told to adopt a low-fat diet. Yet unless they also do a modest amount of moderate exercise, their cholesterol levels won't improve, an important suggests.

"Diet alone doesn't do what people think it will," says Marcia Stefanick, associate professor of medicine at Stanford University School of Medicine in California. In fact, in some overweight people, she notes, "a low-fat diet alone may actually adversely affect levels of HDL (good) cholesterol."

But one of her studies, published in The New England Journal of Medicine, showed that combining a low-fat diet with regular physical activity significantly improved cholesterol levels in people at increased risk of heart disease. "When you adopt a healthy diet and exercise program together," Stefanick says, "you get this tremendous benefit."

The study followed 197 men, ages 30 to 64, and 180 postmenopausal women, ages 45 to 64, who had unhealthy cholesterol levels -- moderately elevated LDL (bad) cholesterol and low HDL cholesterol. Participants were randomly assigned to one of four groups: an aerobic exercise group that walked or jogged the equivalent of about 10 miles per week, a diet group that was placed on the National Cholesterol Education Program's Step 2 diet (the stricter of its recommended low-fat diets), a diet-plus-exercise group and a control group.

After one year, the diet group reduced their intake of dietary fat and cholesterol, yet showed no reduction in LDL levels. But the diet-plus-exercise group experienced a 15-point reduction in LDL levels among the women and a 20-point reduction among the men.