Exercise, even if relatively mild, apparently helps the body destroy dangerous blood clots, a discovery that may explain why people who work out have less heart disease and fewer strokes, Duke University researchers say. A Duke study team found a dramatic increase in the release of proteins that attack blood clots when people are in good physical shape. Dr. R. Sanders Williams, who directed the research, said his group may have found an "important mechanism" in lowering an individual's susceptibility to heart disease, but more study is needed. "It would be overstating the case to say we have found a vitally important effect of physical conditioning that clearly prevents heart attacks," he said. The study involved 69 healthy adults who had not been exercising regularly. They spent 10 weeks in light workouts, 10-minute stretching-exercise sessions three times a week and then walking half or three-quarters of an hour on a treadmill. Researchers used an inflatable cuff to cut off circulation in the subjects' arms, mimicking a blood clot. They found that after 10 weeks, the lining of the volunteers' blood vessels released greatly increased quantities of proteins called plasminogen activators. Plasminogen dissolves fibrin, a stringy protein that helps form clots. Blood clots are an important cause of heart attacks and strokes. The results appeared in last week's New England Journal of Medicine.