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60 is the new 30: What a new study says about metabolism and aging

By:  Jacqueline Stenson

Metabolism in adulthood does not slow as commonly believed, study finds…

A new international study counters the common belief that our metabolism inevitably declines during our adult lives. Well, not until we’re in our 60s, anyway.

Researchers found that metabolism peaks around age 1, when babies burn calories 50 percent faster than adults, and then gradually declines roughly 3 percent a year until around age 20. From there, metabolism plateaus until about age 60, when it starts to slowly decline again, by less than 1 percent annually, according to findings published Thursday in the journal Science.

To tease out the specific impact of age on metabolism, the researchers adjusted for factors such as body size (bigger bodies burn more calories overall than smaller ones) and fat-free muscle mass (muscles burn more calories than fat).

“Metabolic rate is really stable all through adult life, 20 to 60 years old,” said study author Herman Pontzer, an associate professor of evolutionary anthropology at Duke University and author of “Burn,” a new book about metabolism. “There’s no effect of menopause that we can see, for example. And you know, people will say, ‘Well when I hit 30 years old, my metabolism fell apart.’ We don’t see any evidence for that, actually.”

Pontzer and colleagues studied a database of more than 6,400 people, ages 8 days to 95 years, from 29 countries worldwide who had participated in “doubly labeled water” tests. With this method, individuals drink water in which some of the hydrogen and oxygen have been replaced with isotopes of these elements that can be traced in urine samples.

“By calculating how much hydrogen you lose per day, and how much oxygen you lose per day, we can calculate how much carbon dioxide your body produces every day,” Pontzer explained. “And that’s a very precise measurement of how many calories you burn every day, because you can’t burn calories without making carbon dioxide.”

The researchers analyzed average total daily energy expenditures, which include the calories we burn doing everything from breathing and digesting food to thinking and moving our bodies.

“There’s nothing sort of more fundamental and basic than how our bodies burn energy, because that represents how all our cells are busy all day doing their various tasks, and we didn’t have a good sense of how that changes over the course of a lifespan,” Pontzer said. “You need really big data sets to be able to answer that question. And this was the first time that we had the ability to do this with a really big data set that would allow us to pull apart the effects of body size and age and gender and all these things on our energy expenditures over the day.”