Want a guaranteed way to lose weight? Put down those fad diet books and pick up your calculator.

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Experts at Harvard Medical School say losing weight is a numbers game, and the prime number is 3,500. Cutting 3,500 calories from your diet, or ramping up exercise to burn 3,500 calories, means you'll lose one pound of fat. The time it takes depends on how many calories you cut from your diet every day and how much you exercise.

The government's daily guideline for "moderately active" adults 51 and older is 2,300 calories for men and 1,800 for women, so cutting 3,500 calories from a diet takes time. But it can be painless. For example, a 12-ounce can of regular soda has about 150 calories. If you cut out a can a day, that equals 54,750 fewer calories in a year, meaning you'd lose about 15 pounds. Imagine what happens if you cut out pies, cakes and cookies.