The old thought was that vigorous exercise could be dangerous to people at risk for heart disease. Emerging evidence suggests that the more vigorous the workout, the more value to your heart—though short, 15-minute spurts of exercise may be as beneficial as one marathon session.

One cardiovascular death per year may be preventable for every 145 people with diabetes who are persuaded to walk at least two hours a week, according to researchers at the Centers for Disease Control and Prevention in Atlanta.

Exercise keeps you ticking
David Cullen's brother Steve died of a heart attack in 1995 at age 40. In 2002 two more of his brothers died of the same fate, one day apart.

Cullen, a state representative from Milwaukee, doesn't expect to die young. At 5'11", he weighs only 165 pounds and has low cholesterol. He credits his good health to running six to eight miles each day.

How exercise clears arteries
Exercise helps dilate the body's blood vessels and enables blood to circulate more freely, said Byung-il William Choi, MD, a professor of cardiovascular medicine at the Medical College of Wisconsin in Milwaukee.

In one study Harvard researchers found up to a 20% reduction of heart-disease risk for those who most frequently got vigorous exercise. This category included running or jogging, swimming laps, playing tennis, or doing aerobics.

Walking three miles or more a week resulted in a 10% reduction in risk. Other moderate exercises include walking, golf, and yard work.

"The benefits of physical activity seem to be independent of other coronary factors," Howard D. Sesso, ScD, of the department of epidemiology at the Harvard School of Public Health, said when the study was published.

]How much exercise is enough?
The amount of exercise it takes to help prevent heart disease is a matter of debate. Some experts urge people to exercise more frequently and moderately; others push for longer, more vigorous workouts. "The sad fact is, most Americans are sedentary," says Matthew Sorrentino, MD, a cardiologist at the University of Chicago. "One-quarter don't exercise at all. So getting them to move at all is an improvement."

Some studies show that regular exercise—30 to 60 minutes a day—lowers blood sugar and blood pressure, boosts HDL (good cholesterol), and can reduce the protein that contributes to blood clots.

A study of more than 44,000 men found that even moderate exercise can shield against heart disease. Compared with men who got little or no exercise, those who walked briskly for at least 30 minutes each day were about 20% less likely to develop heart disease.

Lifting weights—a type of exercise once dismissed by cardiologists for people with high blood pressure— also reduced risk by about 25%.

Running at least an hour each week cut the risk by 40%.

Exercise helps people lose weight, though a surprising study by researchers at the Cooper Institute for Aerobics Research in Dallas found it is better for your heart for you to be fit than thin. Unfit, lean men had a higher risk of death from cardiovascular disease than men who were fit and obese.

Exercise is also associated with other healthy behaviors, like not overeating and not smoking, Dr. Choi says.

David Cullen, 47, has found that his choice to exercise prompts him to make other healthy choices. "The more I run, the less likely I am to want junk food," he says.

Never too late to start
A German study found that people who exercised regularly during their lifetimes were 60% less likely to be diagnosed with coronary heart disease than sedentary people.

But those who became physically active only after the age of 40 were around 55% less likely to be diagnosed with heart disease than those who had been inactive all their lives.