Q. Please settle a long-running argument between me and my sister. What's more effective for weight loss, diet or exercise?
A. Losing weight can be a challenge. The best approach is to eat less and exercise more. But that's not what you asked me.
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Let's say one person cuts back on calories without exercising and another person increases exercise without cutting back on calories. The first person would probably find it easier to lose weight. That's because it's easier to cut 500 calories a day from your diet than to burn 500 extra calories through exercise.
But there's more to it. If you cut back only on calories, you're more likely to regain the weight you lose. Why? The body reacts to weight loss as if it were starving. In response, the body slows its metabolism. When your metabolism slows, you burn fewer calories -- even at rest. Even 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.
The solution is to increase your physical activity. Doing so will counter the metabolic slowdown caused by reducing calories. You'll burn more calories just by sitting and daydreaming if you have been exercising regularly.
With regular exercise, you don't just burn calories when you're active; you increase the number of calories you burn at rest. This is called resting energy expenditure. Any increase in resting energy expenditure is extremely important for weight loss, because most of us are at rest the greater part of the 24-hour day. In addition, physical activity temporarily curbs appetite in the time immediately following exercise.
Your resting energy expenditure remains elevated as long as you exercise at least three days a week on a regular basis. Vigorous activities that can stimulate your metabolism include walking briskly for two miles or riding a bike uphill.
Moderate-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 muscle -- and muscle tissue burns more calories than fat does. (I've put a table listing several moderate- and vigorous-intensity activities on my website, AskDoctorK.com.)
One last thing about exercise: Many people believe that regular exercise is healthy for you only because it helps you lose weight. Surely, having a healthy weight reduces your risk of many diseases, and regular exercise can help you get down to a healthy weight.
But what many people don't know is that regular exercise powerfully protects you against major diseases such as diabetes, heart disease, stroke and many types of cancer -- even if you don't lose weight. Regular exercise changes the chemistry of your body in ways that improve your health.
So, try to eat healthy and exercise regularly. These are the two most powerful, and natural, things you can do to protect your health.
• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com.