"It's a lot easier not to develop problems than it is to cure them," says Weitzman. One-third of American teens are overweight or obese, which dramatically increases their risk for heart disease, Type 2 diabetes, asthma and other chronic ailments, including depression. While growing teens need extra calories, they should get them from nutritious sources — not high-fat, high-calorie, high-sugar foods like cookies, soda, candy and fast food — and they shouldn't consume more calories than they expend. "On average, if you eat one to two cookies a day more than the energy you need, you'll gain a pound a month," says Weitzman, adding that maintaining a healthy diet is a whole-family affair. After all, kids are not typically the ones doing the grocery shopping. "You can't have foods in the house and ask only one person not to eat them," Weitzman says.