Don’t Blame Aging for a ‘Slowed Metabolism’

This post originally appeared as my weekly newsletter, which you can sign up for HERE.

It is commonly stated and generally well-accepted that metabolism declines with age. As one leaves their early 20s and 30s and enters middle-age, the body “slows down,” — burning less calories than it once did. Coincidentally, weight and fat mass start to creep up as one progresses throughout life, and the slowing down of metabolism is typically to blame.

New data suggest that many of our prior conceptions about metabolism and aging may be wrong. In this study, researchers analyzed a database of diverse individuals (individuals from many countries) from which they gathered data on total daily energy expenditure (TEE) to determine the trajectory of metabolism throughout the life course of a human. How does metabolism compare at birth (or very shortly after) to when you’re in your early to late 90s?

Study: Daily energy expenditure through the human life course

The findings were (perhaps) quite surprising. The paper describes four distinct “phases” of metabolism that seem to emerge throughout life. As neonates (birth to 1 year of age), our metabolism is higher than any other point in life (after adjusting for body mass, of course) — almost 50% higher than adult levels. This makes sense, as this is a period of rapid growth and development. Metabolism begins to decline thereafter until about age 20 — the “juvenile phase.” The decline in total energy expenditure of about 2.8% per year continues until about age 20.5, after which it reaches a plateau.

Adulthood, age 20 – 60, is a period where total energy expenditure is stable, along with fat-free mass, up until age 63. Finally, after age 63, a period known as older adulthood, we see a drop in energy expenditure consistent with a drop in fat-free mass. For context, in someone who is 90 years old, energy expenditure is around 26% lower than that of a middle-aged adult.

What do these findings mean? Well, for one, they tell us that a “slowed metabolism” may NOT be the reason for the mid-life weight gain and other health problems. In fact, this study did not find any evidence that weight or fat mass increased between the ages of 20 and 60, nor were there seemingly any differences between men and women.

As to what causes these declines in energy expenditure, two likely candidates exist — reductions in physical activity/exercise throughout life and reduced tissue metabolism (our organs use less energy with age.) Both situations are likely true and the good news (and perhaps bad news) is that one of these is largely under our control. To “fight” the age-related decline in energy expenditure, you can be aware of your activity levels and maintain or even increase them with age — this will also help stave off age-related losses in muscle mass.

This may also explain why as adults, and especially as older adults, people tend to eat less. Perhaps the body is fine-tuned to recognize reductions in energy need, and responds accordingly by reducing appetite. This may also be why those who fail to rein in their appetite with age do seem to experience an age-related increase in weight and fat.

I hope you found this discussion interesting. See you next Friday!

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