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Another gem of a site is 4myheart.com which recently told it simply:
3500 CALORIES = 1 POUND OF BODY WEIGHT
For every 3500 calories you burn, you lose 1 pound, and for every 3500 calories you take in, you gain 1 pound. To lose a pound in 1 week you eat 500 less calories a day, or eat 250 less calories and exercise to burn 250 more. Foods high in fat, such as cheese, have the most calories, so eating lower fat foods is an easy way to cut calories. Remember, calories that aren’t burned get stored as body fat. All physical activity burns calories. The more vigorous an activity, the more calories burned