Why does your metabolic rate drop as you age?
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.
• Firstly, 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
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.
About The Author
 Christian Finn holds a master's degree in exercise science, is a certified personal trainer and has been featured on BBC TV and radio, as well as in Men's Health, Men's Fitness, Muscle & Fitness, Fit Pro, Zest and other popular fitness magazines.
If you're stuck
in a rut with your current exercise and diet plan... fed up with
only losing a pound here and there... or still skinny after months
(or even years) of trying to build muscle and gain weight... click here now for instant access to his step-by-step muscle-building and fat-burning workout routines.
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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
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