Q: I've heard that metabolism slows in your 30s and keeps going downhill. Does exercise prevent it?
A: Yes, to a certain degree. Your muscle mass naturally peaks at age 25, and from then on it drops 4 percent per decade if you're physically active. If you're sedentary, you'll lose about 1 percent of your muscle mass a year, says Betsy Keller, an exercise physiologist in Ithaca, New York. "Exercise increases your body's production of growth hormone, which will rev up your metabolism and help keep pounds at bay." Notable drops in your metabolism--which may be due to a decrease in estrogen--don't occur until your 40s and 50s. So if you've added pounds in your 30s, you're likely not exercising enough. To keep your engine from slowing, do three to five cardio workouts and three total-body strength-training sessions each week.