Strict changes in diet and lifestyle can not only prevent heart attacks, but can reverse the clogging of the arteries, according to a small but pioneering study. The study showed that a vegetarian diet, moderate exercise and an hour a day of yoga and meditation could produce a reversal of atherosclerosis, a blockage of the arteries that can lead to a heart attack, in men and women who were strict in following the daily regimen. Experts say this is the first study to report that such blockage can be reversed without using cholesterol-lowering drugs or surgery. The study, which was conducted by Dr. Dean Ornish, director of the Preventive Medicine Research Institute in Sausalito, Calif., was presented at the meeting of the American Heart Association in New Orleans. This is a tremendously important study in the control of heart disease. It's the very first study indicating regression of coronary heart disease without pharmaceutical intervention. The results also suggest that the current medical guidelines for changes in the habits of people with severe heart disease do not go far enough. Previous studies have shown that exercise and diet changes can slow the progression of heart disease, but not reverse it. While the study did not determine what percentage of improvement could be attributed to the lifestyle changes alone, the researchers noted that stress-control methods have been shown to ease recovery from a variety of disorders, including hypertension. But some experts are skeptical of the need for stress-management methods, which are not currently among standard recommendations for those with severe heart disease. Some experts on cardiac rehabilitation question whether most people with heart disease could follow such strict changes in their habits.