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

independent– The trope that metabolism rapidly declines as you hit your 30s has been questioned following an unprecedented, international study.

While metabolism does indeed decline throughout the years, researchers found it plateaus for around half of the average lifetime, only beginning to decline from the age of 60, a lot later than first considered.

The study – published in the journal Science – looked at 6,400 people between the ages of eight days and 95 years across 29 countries, and deduced there were four metabolic stages throughout life.

Metabolism peaks in the first – from birth to the age of one – which sees infants burn calories 50 per cent faster than an adult.

It then decreases slightly, at three per cent every year until the age of 20, with no spike at all during puberty.

The third stage, from the age of 20 to 60, metabolism remains steady with no change at all between the four decades.

It is only from the age of 60, the fourth stage, does metabolism begin to slow by less than one per cent a year. By the age of 90, it would be 26 per cent lower than mid-life.

Researchers used data from those who has participated in ‘doubly labelled water’ test which see individuals drink water that contains isotopes of hydrogen and oxygen.

The isotopes can be traced in urine samples which gives an indication of metabolism.

Lead author Herman Pontzer, associate professor of evolutionary anthropology at Duke University, explained: “By calculating how much hydrogen you lose per day, and how much oxygen you lose per day, we can calculate how much carbon dioxide you body produces every day.

“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.”

Researchers analysed all participants’ daily expenditures – which include everything from breathing and digesting food, to physical exercise.

They found the popular belief that metabolism began steadily declining from 30s or 40s may not hold water, with metabolism not declining again until after age 60, the paper said.

The slowdown is gradual at just 0.7 per cent a year, but someone in their 90s needs 26 per cent fewer calories per day than someone in midlife, it added.

Co-author John Speakman, professor at the University of Aberdeen, said: “Previously there was a suggestion that metabolism might slow in your 30s, and that was then thought to [cause] susceptibility to middle-age spread.

“We found no evidence to support that. So if you are piling the weight [on] and your waistline is expanding during your 30s and 40s, it’s probably because you are eating more food, then expending less energy.”

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