Blaming those extra pounds on a slowing metabolism as you age? Not so fast.
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.”
Take, for instance, the finding that metabolic rate declines in seniors, which might have been expected.
"People thought, 'Well, maybe it's because you're less active, or maybe it's because people tend to lose muscle mass as they get into their 60s, 70s and older,'" he said. "But we can correct for all those things. We can say, 'No, no, no, it's more than that.' It's that our cells are actually changing."
Results did not show that metabolic rates spiked upward during the teen years or pregnancy, as commonly thought, or that there were specific differences between men and women after accounting for body size and composition.
What factors cause weight gain?
Registered dietitian Colleen Tewksbury, a senior research investigator at the University of Pennsylvania and a spokesperson for the Academy of Nutrition and Dietetics, said the new study is surprising.
“Historical convention was really that with different life cycle changes — of puberty, of pregnancy, of menopause — we thought that there was some shift in metabolism and it impacted nutrition status and how we approached things from a nutrition standpoint,” she said. “This high-level rigorous assessment does not show that.”
But if changing metabolism is not playing a role in weight gain at certain points in adult life, there could be other contributing factors, she said.
“There are lots of things that impact weight status and also someone's nutritional status,” Tewksbury said. “It's not as simple as just one food or one lifestyle change or one change from a biological standpoint. It's more likely a much more complex web of lots of different changes happening at once. So that could be changes to food intake. It could be changes in activity levels. It can be where they're living, what they have access to, what are their sleep changes.”
Steven Malin, an associate professor of kinesiology and health and director of the Rutgers Applied Metabolism and Physiology Laboratory, called the study results “illuminating on something that we thought we know a lot about and realize that there's more to be discovered.”
Malin said the findings, for instance, contradict the belief that adults experience a decline in metabolism as they move from their 20s into their 30s and that this may be contributing to the obesity epidemic.
“It's not as if the weight gain is occurring because you don't ‘burn the same calories’ anymore,” he said.
Pontzer said the findings in early life highlight the critical importance of infant nutrition meeting the increasing energy demands of growing babies.
In addition, he said, the study results could have implications for how much medicine people need at various ages, when they could be metabolizing drugs differently.
In a commentary published with the new study, Timothy Rhoads and Rozalyn Anderson, who work in geriatrics at the University of Wisconsin, said the findings also may have implications for the study of age-related diseases.
“The decline from age 60 is thought to reflect a change in tissue-specific metabolism, the energy expended on maintenance,” they wrote. “It cannot be a coincidence that the increase in incidence of noncommunicable diseases and disorders begins in this same time frame.”