If 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 it is to burn 500 extra calories through exercise. You'd have to walk or run about five miles a day for a week to lose one 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 is starving and, in response, slows its metabolism. When your metabolism slows, you burn fewer calories — even at rest. When you burn fewer calories, two things can happen if you continue eating fewer calories:

you will stop losing weight as quickly as you have been

you'll stop losing weight altogether

If you then increase your calorie consumption, you may actually gain weight more quickly than you had in the past.

The solution is to increase your physical activity, because doing so will counteract the metabolic slowdown caused by reducing calories.

Regular exercise increases the amount of energy you burn while you are exercising. But it also boosts your resting energy expenditure — the rate at which you burn calories 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 resting energy expenditure accounts for 60% to 75% of the calories you burn each day, 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 two miles or riding a bike uphill. Even small, incremental amounts of energy expenditure, like standing up instead of sitting down, can add up.

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.

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