You may have heard that some foods, because they are difficult to digest, will make you lose weight. Dubbed "negative-calorie foods," citrus fruits and celery have both basked in this flattering light in fad diets over the years. The problem is that it's not true. The calories your body burns in fueling the digestive cycle are minuscule compared with the calories in the food itself. Although chewing celery might seem like a strenuous activity, it burns about the same amount of calories as watching grass grow.