Why does metabolism slow down with age, and what, if anything, can be done to prevent it?

First, let's define "metabolism."  The metabolic rate of a person consists of: 1) the basal metabolic rate (BMR) or the closely related resting metabolic rate (RMR); and 2) the expendable metabolic rate (EMR).  The BMR is defined as the amount of energy utilized by the body at rest (kcal/day).  The EMR is the extra number of calories (kcal/day) that the body utilizes for all extra activities (sitting, standing, eating, working, exercising, etc.) beyond that used while lying still.

       total metabolic rate = basal metabolic rate (BMR) + expendable metabolic rate (EMR)

The BMR is closely related to the body's lean body mass (LBM), which consists of all body tissues that are not fat (bone, organs, muscles, skin, etc.).  The greater the LBM, the higher the BMR.  What happens with age is that muscle mass usually decrease, a process known as "sarcopenia," or muscle wasting.  Studies have shown that muscle mass generally decreases by 35 to 40 percent between the ages of 20 and 80, with the majority of muscle loss after the age of 50.  This is seen in both men and women.

Therefore, BMR does not decrease because of age, per se, but because the muscle mass of the person has decreased with age.  Therefore, as a person ages, the number of calories that are consumed at rest has now decreased.

Secondly, often as a person ages, the EMR decreases as well.  This is because the person has simply become less active.  This can be due to lifestyle changes (less activities, less sports, more sitting at a desk, etc.), and it can be due to orthopedic issues (arthritis, back problems, other health issues) which can make moving painful and/or more difficult.

So, how does one prevent this decrease in metabolism with age?  First, a person could reduce the effects of sarcopenia by engaging in a program of progressive resistance exercise (i.e., weight training).  This is definitely the most effective way to ward off extra fat pounds as one ages, since the BMR is now higher all the tine, whether the body is at rest or is moving.

Secondly, a person could increase the EMR by becoming more active.  This burns off extra calories whenever the body engages in extra activity outside of the resting state.