If you're 40 or older, chances are your waistline is getting bigger every year. Unless you are exercising, your metabolism is slowing down. Most of you who want to lose weight will focus on aerobic exercise. Aerobic exercise is important, but it does little to offset the slower metabolism of middle age. Past age 35, most people start losing muscle mass which slows down your metabolism. Weight training not only tones your muscles, but increases your muscle mass which in turn makes your body burn more calories. To rev up your metabolism, include weight training in your routine twice a week.