Sure, you could eat whatever you wanted in high school and stay thin as a rail. But unfortunately you're not 17 anymore, and even if you work out 'round the clock, you can't transform your body if you constantly give in to cravings, high-fat foods and sweet treats. The truth is, flat abs are made in the kitchen, and no amount of cardio and crunches can sculpt a sleek physique if you maintain an unhealthy diet.
"Consume excess calories and you have to counterbalance them," says Sara Haas, RDN, spokesperson for the Academy of Nutrition and Dietetics. "And it's hard to get enough exercise in to undo the calories you'll get in a double cheeseburger with French fries and a milk shake."