Why Does Your Metabolic Rate Drop as You Age?

Editor's Note:  Conventional wisdom says your metabolic rate drops because you lose a little muscle each year. However, Christian explains that muscle mass is not the only thing that affects it.

Your basal metabolic rate - the number of calories your body burns each day just to stay alive - tends to drop as you age.

According to conventional wisdom, this is because you lose a small amount of muscle each year.

However, many people don't realize that muscle mass is not the only thing that affects your basal metabolic rate.

In fact, studies show that your metabolic rate declines with age independently of muscle loss. In other words, young physically active men tend to have a higher metabolic rate than their older counterparts, even if they have the same amount of muscle.

Why?

The age-related decline in metabolic rate - even when muscle mass is taken into account - is because of two reasons.

First, there's a strong link between exercise volume and your metabolic rate. In other words, the more exercise you do, the higher your metabolic rate. The fact that people tend to exercise less as they age is partly responsible for the drop in metabolic rate.

Second, metabolic rate is also linked to total calorie intake. This means that the more you eat, the higher your metabolic rate. A reduced metabolic rate in older physically active men could be due to the fact they eat less than their younger counterparts.

Some evidence for this comes from a research group based at the University of Colorado [1]. They compared a group of young and older inactive men with physically active men of a similar age.

Even when differences in muscle mass between young and old subjects were taken into account, basal metabolic rate was lower with age in both the inactive and physically active subjects. The older men burned around 64-68 calories per hour, compared to 72-77 calories per hour in the younger subjects.

However, when researchers compared older and younger subjects doing the same amount of exercise and/or eating the same number of calories, there was no difference in basal metabolic rate.

The same holds true in women as well as men. Research comparing premenopausal and postmenopausal women shows that basal metabolic rate, again adjusted for age-related muscle loss, was roughly 10% lower in postmenopausal women who do no exercise [2].

But there was no difference in metabolic rate between premenopausal and postmenopausal women who exercise regularly.

Energy Flux

The link between adjusted basal metabolic rate, exercise volume, and energy intake involves a concept known as energy flux, which refers to the flow of calories (or energy) through your body.

A high energy flux means that you're eating a large number of calories, but balancing that with an equally high volume of exercise. A good example of someone with a high energy flux would be a cyclist taking part in the Tour de France.

Despite the fact these athletes eat thousands of calories each day, they're so lean simply because they do so much exercise.

Someone following an extremely low-calorie diet while doing little or no exercise is a perfect example of low energy flux.

A lot of people use this approach to lose weight. And it does work, if only for a short time. If you cut your calorie intake too much for long, several things will happen.

• You'll end up losing muscle as well as fat. Half of the weight lost on a very low-calorie diet comes from muscle.
• You won't have the energy to exercise. The result is that your metabolic rate will slow down, and weight loss will get harder.

The bottom line is that a drop in your metabolic rate is not an inevitable consequence of aging, and has a lot more to do with the way you live your life.

You can minimize the age-related drop in metabolic rate by maintaining a high energy flux - balancing a nutrient-dense diet with regular exercise.

Not only will this help you stay lean, it's a great way to provide your body with more of the vitamins, minerals and other nutrients it needs to stay healthy.

(Note: If you've been putting off starting an exercise program because you think you're too old or just too far gone, grab a copy of Fit Over 40: Role Models For Excellence At Any Age. You’ll find more than 50 inspiring real life success stories which prove that you can boost your metabolism, gain muscle and lose all the weight you want regardless of how old or out of shape you are!)
 

Read other articles by Christian Finn

About the Author

Christian Finn is a Certified Personal Trainer and holds a masters degree with distinction in exercise science. He's lectured at a number of universities and private training organizations around the United Kingdom on fitness training, weight loss and the effective use of nutritional supplements. He writes extensively on the subject and his articles have been published in numerous magazines, leading industry journals and websites worldwide, including Men's Health, Men's Health Muscle, Fit Pro (April/May 2001), CAM magazine (February 2003), Image (January 1997), Zest (March 2004), and Body Life magazine (March/April 1997). He was also featured in the July 2004 issue of Muscle & Fitness (UK edition). His website, TheFactsAboutFitness.com, is dedicated to providing its members up-to-date, unbiased information and research on the world of fitness.

References

1. van Pelt, R.E., Dinneno, F.A., Seals, D.R., & Jones, P.P. (2001). Age-related decline in RMR in physically active men: relation to exercise volume and energy intake. American Journal of Physiology, E281, 633-639

2. van Pelt, R.E., Jones, P.P., Davy, K.P., Desouza, C.A., Tanaka, H., Davy, B.M., & Seals, D.R. (1997). Regular exercise and the age-related decline in resting metabolic rate in women. Journal of Clinical Endocrinology and Metabolism, 82, 3208-3212