In your mid-40s, with the decreasing estrogen levels in perimenopause, you begin to lose muscle mass, and muscle has a much higher metabolism than fat. When menopause officially arrives, weight gain happens disproportionally in the belly. Tips: 1. Pump iron three times per week. 2. Do interval cardio in the form of HIIT 3 times per week. 3. Do one day of stretching and yoga for rest, recovery, and rejuvenation.