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Metabolism’s Role in Age-Related Weight Gain

It turns out that for most people, age-related weight gain is due in large part to a dramatic decrease in calories burned. While lower levels of physical activity play a large role in the decreased energy expenditure, an age-related decline in metabolic rate is also to blame.

A study evaluating total energy expenditure (TEE) – the sum of calories burned from the basal metabolic rate (metabolism), the energy required to digest and absorb food, and physical activity – confirmed what most people already know: energy expenditure decreases with age.

Basal metabolic rate, which accounts for about 50 to 70 percent of TEE, is thought to decrease about one to two percent per decade. That is, after a person reaches 20 years old, daily energy expenditure decreases about 150 calories per decade. The decline is probably due to decreased muscle mass (which is highly metabolically-active) and increased fat mass (which is relatively metabolically-inactive).

Some studies have also found that, even when controlling for fat-free mass, basal metabolic rate is five percent lower in older compared with younger adults. It is not clear why, but some researchers speculate that it may be due to an unavoidable loss of very metabolically-active organ tissue, or a decreased metabolic rate within muscle tissues. Decline seems to be most rapid after 40 years old in men and 50 years old in women.

In sum, the number of calories burned per day decreases with age. This reality is widely accepted and is even built in to formulas that estimate resting energy expenditure. The age-related decline in energy expenditure is largely due to decreased metabolic rate — which results from decreased lean mass and increased fat mass — and decreased levels of physical activity.