DURHAM, N.C. — Our metabolisms would seem to slow down once we hit adulthood and grind to a halt by middle age. However, a new study finds that’s not actually the case. In fact, researchers from Duke University say human metabolism doesn’t start slowing until the age of 60.
After an early rush as we grow up, researchers find that our metabolisms slow and stabilize in our 20s before finally declining in our 60s. Many people commonly think of their teens and 20s as the time their metabolism hits its peak, but the truth is even that comes much earlier.
In fact, metabolism is at its fastest during the first few years of life. The study finds humans burn energy roughly three percent slower every year after infancy, until reaching adulthood. Through our 20s, 30s, 40s, and 50s, metabolism is actually the most stable.
Human metabolism won’t start to fully decline again until after age 60. The slowdown is gradual, at only 0.7 percent per year. At that rate, a person in their 90s needs 26 percent fewer calories each day than someone in midlife.
Infant metabolism is the fastest it will ever be
An international team made their surprising discovery after analyzing data on more than 6,600 people ranging from one week-old to age 95 as they went about their daily lives in 29 countries worldwide.
“There are lots of physiological changes that come with growing up and getting older,” says Duke associate professor of evolutionary anthropology Dr. Herman Pontzer in a university release. “Think puberty, menopause, other phases of life. What’s weird is that the timing of our ‘metabolic life stages’ doesn’t seem to match those typical milestones.”
“All of this points to the conclusion that tissue metabolism, the work that the cells are doing, is changing over the course of the lifespan in ways we haven’t fully appreciated before,” Pontzer adds. “You really need a big data set like this to get at those questions.”
Energy needs shoot up during the first 12 months of life, such that by their first birthday, a one-year-old burns calories 50 percent faster than an adult does. That’s not just because, in their first year, infants are busy tripling their birth weight.
“Of course they’re growing, but even once you control for that, their energy expenditures are rocketing up higher than you’d expect for their body size and composition,” Dr. Pontzer explains.
An infant’s gas-guzzling metabolism may partly explain why children who don’t get enough to eat during this developmental window are less likely to survive and grow up to be healthy adults.
“Something is happening inside a baby’s cells to make them more active, and we don’t know what those processes are yet,” the Duke researcher continues.
Myths about our teen years
Despite the teen years being a time of dramatic growth spurts, the researchers didn’t see any uptick in daily calorie needs during adolescence after they took body size into account.
“We really thought puberty would be different and it’s not,” Dr. Pontzer notes.
Midlife delivered another surprise for the team. Perhaps you’ve been told that it’s all downhill after 30 when it comes to your weight. Lost muscle mass as we get older may be partly to blame since muscle burns more calories than fat. Despite that, Dr. Pontzer says that’s not the whole picture.
“We controlled for muscle mass,” Pontzer explains. “It’s because their cells are slowing down.”
Previously, most large-scale studies measured how much energy the body uses to perform basic vital functions such as breathing, digesting, pumping blood – in other words, the calories you need just to stay alive. However, that amounts to only 50 percent to 70 percent of the calories we burn each day.
This doesn’t take into account the energy we spend doing everything else like washing the dishes, walking the dog, breaking a sweat at the gym, or even just thinking or fidgeting.
Measuring energy expenditure
To come up with a number for total daily energy expenditure, the researchers relied on the “doubly labelled water” method.
This technique involves a urine test after having a person drink water in which the hydrogen and oxygen in the water molecules have been replaced with naturally occurring “heavy” forms, and then measuring how quickly they’re flushed out.
Scientists have used the method – considered the gold standard for measuring daily energy expenditure during normal daily life outside of the lab – to measure energy expenditure in humans since the 1980s. Studies, however, have been limited in size and scope due to cost.
So, for this project, multiple labs decided to share their data and gather their measurements in a single database, to see if they could tease out truths that weren’t revealed or were only hinted at in previous work.
The findings appear in the journal Science.
South West News Service writer William Janes contributed to this report.