The Metabolic rate that allowed you to burn through supersize burritos in your 20s is slowing-- dropping by 1 percent every 4 years. And even if the number on your scale isn't rising, it's likely you're getting fatter. In a study published in the American Journal of Clinical Nutrition, scientists found that men who managed to maintain their weight for 40 years still gained 3 pounds of fat each decade-- while losing 3 pounds of muscle. The likely reason: after you pass 30 your testosterone levels decrease by up to 1 percent a year. This means it becomes harder for you to build , or even maintain, metabolism boosting muscle. See the connection?