Growth hormone is a naturally occurring substance secreted by the brain that helps guide growth in childhood and maintain muscle mass during adulthood.

FORBES: Sweet syringe of youth

Not only are you healthier as a result, your comprehensive exercise program, which includes cardiovascular and weight-bearing exercise, is preventing further weight gain and allowing you to maintain muscle mass so your metabolism does not drop significantly as you get older.

CNN: I'm exercising; why can't I lose weight?

The science is increasingly clear: Older adults need both strength training, to help replenish and maintain the muscle mass and (for women especially) the bone density our bodies lose each year as we age, and cardio to help guard against the risks of clogged arteries in the heart and brain.

WSJ: Exercise in Retirement: Cardio vs. Weight Training

Finally, you might want to consider boosting your strength training routine to increase muscle mass which, over time, can increase (or maintain) metabolism.

CNN: What's the best way to 'kick-start' my metabolism?