Every year after age 25, our body's composition begins to shift quite dramatically. We gain, on average, about one pound of body weight each year and lose a third to a half pound of muscle. As a result, our resting metabolism decreases approximately 0.5 percent annually. So unless you downshift your caloric intake as your metabolism slows down, you'll experience frustrating weight gain, which can then inhibit optimal energy metabolism.
Although losing a fraction of muscle mass each year may seem minuscule, it adds up to be quite significant -- translating to about a 1 to 2 percent loss of strength each year. With this loss of muscle strength, we tend to spontaneously become less active because daily activities become more difficult and exhausting to perform. We, in effect, lose energy more easily just like an old car that hasn't been serviced in a while will use up more gas than a new, efficient model.