Physical Activity How Much Is Enough - How To Lose Weight: Obesity

Content provided by the Faculty of the Harvard Medical School

Physical activity: How much is enough?

If one person cuts back on calories without exercising and another person increases exercise without cutting back on calories, the first person would lose weight more quickly. That's because it's easier to cut 500 calories a day from your diet than it is to burn 500 extra calories through exercise. You'd have to walk or run about 5 miles a day — or 35 miles a week — to lose 1 pound of fat. But if you only cut back on calories, you're more likely to regain the weight you lose. Why? The body reacts to weight loss as if it were starving and, in response, slows its metabolism. When your metabolism slows, you burn fewer calories — even at rest. When you burn fewer calories, three things can happen. If you continue eating fewer calories, you will either stop losing weight as quickly as you have been, or you'll stop losing weight altogether. If you increase your calorie consumption, you may actually gain weight more quickly than you have in the past. The solution is to increase your physical activity because doing so will counteract the metabolic slowdown caused by reducing calories.

A regular schedule of exercise raises not only your energy expenditure while you are exercising but also your resting energy expenditure — that is, the rate at which you burn calories even when the workout is over and you are resting. Resting energy expenditure remains elevated as long as you exercise at least three days a week on a regular basis. Because it accounts for 60%–75% of your daily energy expenditure, any increase in resting energy expenditure is extremely important to your weight-loss effort. The kinds of vigorous activity that can stimulate your metabolism include walking briskly for 2 miles or riding a bike uphill.

For people who have obesity and who have been sedentary, any amount of physical activity is beneficial, including walking, swimming, and water aerobics. But start out slowly, and gradually increase the pace and duration of such activities. Low-intensity activities — such as taking a short walk or raking leaves — won't raise your resting energy expenditure as much as high-intensity activities will, but they have other advantages. For one thing, they help reduce body fat and build muscles — and muscle tissue burns more calories than fat does. Regular weight lifting also builds muscle and has a similar effect.

Another benefit of regular physical activity of any sort is that it temporarily curbs your appetite. Of course, many people joke that after a workout they feel extremely hungry — and promptly indulge in a snack. But because exercise raises resting energy expenditure, people continue to burn calories at a relatively high rate. So a moderate snack after exercising does not erase the benefits of exercise in helping people control their weight.

Last updated:	June 20, 2007