People with high cholesterol need to exercise to get the full benefits of a healthful diet, a study concludes.

The study looked at men and women who had moderately high LDL cholesterol and low HDL. LDL increases the risk of heart attacks, while HDL lowers it.The 377 volunteers were already following healthful diets when they were placed on the National Cholesterol Education Program's Step 2 diet, the stricter of its recommended low-fat diets. Some were also assigned to an aerobics program.

After one year, they managed to cut their dietary fat intake by about 8 percent. But without exercise, there was little change in their LDL levels. However, when exercise was added, the women's LDL fell by 15 percent and the men's by 20 percent.

"These findings demonstrate the importance of an exercise program when diet alone has not adequately reduced LDL cholesterol levels," the researchers wrote.

The program had no significant effect on their HDL levels or on their ratios of total to HDL cholesterol, which many experts believe is an important measure of heart disease risk.

The study was conducted by Dr. Marcia L. Stefanick of Stanford University. It was published in Wednesday's issue of the New England Journal of Medicine.