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Combine diet and exercise for best results.

Even though I’m a dietitian, my clients frequently ask me about exercise as part of their weight loss plan.

Since managing weight effectively depends on calorie balance, it makes sense that we talk about not only diet (calories in), but exercise (calories out), too. Most simply want to know how much, or sometimes how little, exercise they need to do in order to lose weight—or to keep off weight that they’ve already lost. So, here are some of the key things I tell them about exercise and body weight.

To lose a pound in a week’s time, strictly through exercise, you’d need to burn up an extra 500 calories a day above and beyond your current activity level. That’s no small task. You’d need to hike uphill for an hour with a 10-pound backpack or swim laps for 90 minutes without stopping. Trying to lose weight only through increased activity, or only by cutting your calories, won’t be nearly as effective as a combination of diet and exercise.

Your metabolic rate represents the number of calories your body burns just to keep basic processes going, and it’s a big part of your ‘calories out.’ But your metabolic rate can dip a little when you cut back on your eating. So, even though your ‘calories in’ may be lower, your ‘calories out’ can drop, too, and leave you more or less in calorie balance.

When people think exercise, they usually think aerobic exercise, like biking, swimming or jogging. But strength training is important, too. In part because it helps to build lean body mass, which can bump up your metabolic rate and help offset the drop in calorie burn that takes place when you cut your calories.

People tend to overestimate the calorie cost of the exercise they do—and underestimate the number of calories they eat. This helps explain the frustration many people feel when they’re sure they’re doing ‘everything right,’ but the scale just won’t budge.

Once you’ve lost it, regular activity is critical when it comes to keeping weight off. But it takes more than a leisurely stroll around the block. Members of the National Weight Loss Registry—people who have lost at least 30 pounds and kept it off for at least a year—burn an average of about 2800 calories a week in exercise. That’s the equivalent of about 90 minutes of exercise, like a brisk four-mile walk every day.

When people hear that they might need an hour or more of exercise a day to keep their weight under control, it can be a little daunting. But don’t let the numbers discourage you. Do what you can, do it regularly, and try to go a little farther or work out a little harder each time.