Filling your diet with junk food may lead to weight gain. It takes an extra 3,500 calories in your diet to gain 1 pound of body weight. For example, consuming 500 calories from junk food each day can lead to 1 pound of weight gain in just a week. If you find yourself at the vending machine in the afternoon, pack healthy snacks the night before and bring them to work. Snack on whole fruit, wheat crackers and baby carrots with fat-free dressing instead. These snacks provide fewer calories than junk food, so you are less likely to gain weight.