Exercise is a key to good health, but you don't have to train like a marathoner to do your heart some good. Brief, frequent segments of moderate activity can significantly reduce your risk of coronary artery disease, said Dr. Jere Mitchell, director of the Harry S. Moss Heart Center and the Pauline and Adolph Weinberger Laboratories for Cardiopulmonary Research. In a presentation to the Junior Group of the Friends of the Center for Human Nutrition, Dr. Mitchell, holder of the S. Roger and Carolyn P. Horchow Chair in Cardiac Research, in Honor of Jere H. Mitchell, M.D., and the Frank M. Ryburn Chair in Heart Research, answered the question: "How Much Exercise of What Kind is Enough?"

If becoming physically fit is your goal, stick with traditional guidelines: Exercise three to five times a week at 60 percent to 85 percent of your maximal heart rate (calculated by subtracting your age from 220) for 20 to 40 minutes continuously. Also perform strengthening exercises twice a week. But you can fit enough exercise into your daily life to reduce your risk of coronary-artery disease without ever putting on your gym shorts. Just being "physically active" can benefit your heart, too. "There's a difference between physical fitness and physical activity," Dr. Mitchell said. "Physical activity is determined by questionnaires--how much you say you do. Physical fitness refers to the measurement of maximal oxygen uptake on a treadmill. You can be very physically active but not attain a high level of physical fitness because of your genetic background.

"There was a recent study from Copenhagen looking at both. It found that physically inactive men got coronary heart disease even if they were physically fit. Physically active men did not get coronary artery disease even if they were not physically fit. They got all the health benefit just by being physically active." Last year the Centers for disease Control and Prevention gathered top sports-medicine researchers to determine what the guidelines for physical activity should be. "What's being recommended now is quite different in that you don't have to do all your exercising at one time," he said. "You can do a series of exercises for 8 to 10 minutes each that add up to 30 to 40 minutes. That scheme has the same health benefit as exercising for 30 continuous minutes."

Try 10 minutes of calisthenics in the morning, a 10-minute brisk walk at lunch and 10 minutes more of dynamic exercise in the evening. Dynamic exercises are vigorous activities like jumping jacks and should last at least eight minutes. "If you want to spread your segments over the entire day and never have a traditional exercise break--park you car further away from work and walk up the stairs," Dr. Mitchell said. "If you can get exercise in your daily routine, you don't need to set aside 30 minutes a day." Exercise can also lower your cholesterol, said Dr. Scott Grundy, director of the Center for Human Nutrition and holder of the Distinguished Chair in Human Nutrition.

"People who exercise a lot have lower total cholesterol--30 points lower than the average American," he said. "The best benefit is in triglycerides and HDL (high-density lipoproteins) rather than in total cholesterol. Cholesterol goes up 50 points between the ages of 20 and 50 for the average American. I think most of the rise is due to becoming overweight and lack of activity." Dr. Mitchell noted that even if you start exercising in your 50s and 60s you still get some of this protective effect. Exercise also promotes metabolic fitness, Dr. Grundy said. "We're finding that exercise changes metabolism in muscles, so you use more carbohydrates and less fat." If you walk a mile day, you can burn 100 calories a day, Dr. Grundy said. That may not seem like much, but 100 calories a day in a month is 3,000 calories, which is almost 1 pound; over a year that's more than 10 pounds. Paraphrasing a popular advertising slogan, Dr. Mitchell said, "You don't have to be a great athlete to get the protective effects of exercise. The important thing is to just do it."