A low-fat diet rich in vegetables, fruits, whole grains and beans has twice the cholesterol-lowering power of a conventional low-fat diet -- even when the two diets have the same amount of calories and fat, researchers said.

Writing in the Annals of Internal Medicine, they said the study suggests that low-fat diets may often fail to lower cholesterol because they contain the wrong nutrients.

"The effect of diet on lowering cholesterol has been really minimized and undermined by a lot of clinicians and researchers saying, 'Yes, it has an effect but it's really trivial: It would be better to put you on drugs to control your cholesterol,"' said Christopher Gardner of Stanford University, who led the study.

"But we think part of the reason was that we weren't really giving diet a fair shake. We were so focused on the negative -- just what to avoid -- and not what to include."

Gardner and colleagues tested 120 adults aged 30 to 65. All had moderately high low-density lipoprotein (LDL or "bad" cholesterol), with levels of 130 to 190. A desirable level is 100.

Of the volunteers, 61 ate a conventional low-fat diet, which included frozen waffles, turkey bologna sandwiches, frozen pizza and similar foods. The other 59 ate a plant-based diet including whole-grain cereals, dark lettuces, bean burritos and vegetable soups.

Both diets contained identical amounts of total and saturated fat, protein, carbohydrate and cholesterol. Calories were carefully controlled to keep each volunteers' weight constant.

After a month of eating in a special dining hall, both groups had lower cholesterol. The conventional diet lowered LDL cholesterol by, on average, 4.6 percent.

The plant-based diet lowered LDL by more than twice as much, by 9.4 percent, the researchers reported.

Gardner said the plant-based diet followed American Heart Association guidelines. These include advice to eat at least five servings of vegetables and fruits every day and at least six daily servings of grains, especially whole grains.

New guidelines from the U.S. Department of Agriculture make similar recommendations stressing whole plant foods and minimizing meat, fats and sugar.

"Include more whole grains and vegetables and beans and colors -- not iceberg lettuce, but red bell peppers and carrots and broccoli and red cabbage and the really colorful foods," he said in a statement.