When you put on muscle mass, your body requires more energy to maintain it…more than it does to hold onto fat. So, when your muscles grow, even when you’re at rest, the calories are being burned at a greater rate. While this doesn’t mean you can eat whatever you want and not gain a pound, it does prove that muscle is important in terms of staying fit and allowing the body to function more effectively. So go on and pump some iron along with your cardio workout. The lean mass will lead to calorie burn long after the workout is complete.