A medical study that concluded oat bran has no special ability to lower cholesterol levels has come under sharp attack in a series of letters to the New England Journal of Medicine.

The study of 20 volunteers, published in January, was done by a team led by Brigham and Women's Hospital dietitian Janis F. Swain. It concluded that cholesterol counts probably drop with an oat bran diet because the bran simply replaces fatty foods that raise cholesterol levels. Swain's team was able to cut cholesterol levels just as effectively using a diet rich in low-fiber, refined wheat.

But critics now say the Swain team was wrong.

Rebecca Roubenoff and Ronenn Roubenoff of Johns Hopkins University charge that the work "suffers from several fatal flaws" because the amount of oat bran consumed by Swain's volunteers was too low and there were not enough volunteers.

"The authors have managed to confuse the American public further with a poorly designed and underpowered trial that draws erroneous conclusions," they write.

But Swain and Frank M. Sacks, also at Brigham and Women's Hospital, defend their results, saying the Roubenoffs misunderstood the amount of oat bran eaten by the volunteers and saying the Roubenoffs' calculations of how many volunteers were needed for the study were "far from true values."

James W. Anderson of the University of Kentucky, credited with starting the oat bran craze, charged the study was designed to make it hard to see any benefits of oat bran.