Not So Fast Blaming Slow Metabolism for Midlife Weight Gain

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Aug. 13, 2021 -- Although a number of things can add to weight gain during midlife, it's unlikely that a slowing metabolism is one of them, new evidence suggests.

A team of global investigators found that total energy needed by the body increases after birth and peaks around 1 year. In fact, after adjusting for weight, 1-year-olds need more calories than adults do. After 1 year, energy requirements drop about 3% a year until age 20, hold steady through midlife, and decrease again after age 60.

"We don't see any evidence for metabolism slowing in middle age. That's surprising, given the common experience people have that their metabolism slows in their 30s, 40s, or with menopause, for example," says lead author Herman Pontzer, PhD.

"Instead, our results indicate that metabolism is really steady from 20 to 60 years," added Pontzer, who is an associate professor of evolutionary anthropology and global health at Duke University in Durham, NC.

The study was published online Thursday in Science.

Total energy spent or calories burned is important to human health and functioning of the body, but little research has looked beyond baseline energy needs or how total requirements to live and to perform day-to-day physical activities change over a lifetime.

To learn more, Pontzer and colleagues looked at total energy requirements in people from 8 days to 95 years old. Participants included 6,421 people across 29 countries. The study included a urine test that measures metabolism based on the speed with which drinking water with specially labelled hydrogen and oxygen molecules clear the body.

"People's intuitions about their metabolic rates don't have any real connection to their actual rate of energy expenditure. I suspect that what people experience and call metabolism is really their energy levels -- their feeling of vitality -- or how much weight they put on," Pontzer said.

"Neither is necessarily related to calories burned per day."

Unexpected Findings

Before this research, Pontzer also pointed to a slow metabolism for his own experience. "I'm in my 40s and have definitely experienced the feeling that my metabolism has slowed since my 20s, so yes the results surprised me," he said.

Another surprise is the "very eye-opening" finding of how dramatically metabolic rates change over the life course, being remarkably elevated in early childhood and declining after roughly age 60, Pontzer said.

The high total energy requirements in 1-year-olds was the biggest surprise, says lead author John R. Speakman, PhD.

Interestingly, pregnant women in the study did not have needs beyond those expected for the added bulk from the growing baby.

'Important New Insights'

The study "provides important new insights into human metabolism," Timothy Rhoads, PhD, and Rozalyn Anderson, PhD, write in an accompanying perspective titled “Taking the long view on metabolism.”

For example, they note, "It seems clear from their data that infants and adolescents form two different metabolic categories. It has been said before, but children are not just small adults."

Separate categories for metabolism in young people have "important implications for recommendations about diet and physical activity, not to mention pharmaceutical dose recommendations for younger persons,” says Rhoads, who is affiliated with the School of Medicine and Public Health at the University of Wisconsin-Madison. Anderson is affiliated with the Geriatric Research, Education, and Clinical Center at the William S. Middleton Memorial Veterans Hospital in Madison.

One Less Thing to Worry About?

The study is ongoing, and the researchers hope the findings will hold true over time. If that's the case, "I think the main message is you don't have to worry about [midlife metabolism changes] too much because it seems our metabolism is pretty much constant over this period," says Speakman, a biologist who leads the Energetics Research Group at the University of Aberdeen in the United Kingdom.

"If you are experiencing middle-aged spread, then it's probably because you are eating more than you used to -- not because you are expending less," he says.

Going forward, the researchers would like to continue studying the four distinct phases of metabolism over a person's lifetime. "It would be great to do a set of longitudinal studies," Pontzer says, "following people through each of the metabolic transitions we see in our paper."

The findings could lead to a new understanding of the biology behind total energy expenditure as people age.

"These changes seem to indicate that our cells and organs are changing how active they are over the lifespan in ways we hadn't previously appreciated," he says.

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