Maintain/improve your muscle mass and burn body fat with consistent weight training (including bodyweight exercises ). Muscle never changes into body fat. If you become chronically inactive, your muscles will lose mass (atrophy). And, your fat cells were still there although they had shrunk. So, now the fat cells come back with a vengeance!
Also, your metabolism begins to slow down in your thirties. Your metabolism will slow down even more if you are losing muscle mass. In your early thirties, you can begin to lose as much as one-half pound of muscle each year. Inactivity will make this problem even worse. Remember, muscle mass speeds up your metabolism because your body has to work harder to maintain muscle mass. So, you begin to lose muscle mass and gain body fat
Muscle mass can be maintained/increased (and body fat decreased) throughout your life through exercise and weight training. Thirty-five to 50 daily calories are burned by one pound of muscle. Weight training also helps to keep your bones strong ( preventing osteoporosis ).