A new, large-scale study upends traditional thinking on midlife weight gain and more

We've all heard the conventional wisdom on metabolism, and it goes something like this: During your 20s your metabolism is firing on all cylinders, zapping enough calories to let you scarf down second helpings and extra slices of late-night pizza with few consequences on the scale. Then midlife and/or menopause come along, jamming up the works on your body's energy-burning engine and leading to middle-aged spread.

While all of this may sound all-too-familiar, a major new study has upended the story about how our metabolism changes as we grow older. That middle-aged slowdown? The idea that calorie burning crests in your 20s — or slumps with menopause? Not true, this new research says. Instead, its evidence shows that age 60 is when metabolism starts to dip — far later than previously believed.

“If you're gaining weight it's easy to say, ‘Oh, that's my metabolism.’ It's almost like a scapegoat. Now that we know it's not metabolism, we can focus on some of those other factors.”

Herman Pontzer, Associate Professor, Duke University

"This is an incredibly important study for people who study metabolism across the lifespan,” says Anthony Ferrante Jr., M.D., a professor of medicine and chief of preventive medicine and nutrition at Columbia University Irving Medical Center. “These data will help frame how people approach metabolic health going forward."

What makes the study so groundbreaking is its scale: It analyzed things like energy expenditure from more than 6,400 people across 29 countries — from 1-week-old newborns to people in their 90s. To capture such information, researchers used something called the “doubly labeled water” method to determine how many calories the subjects burned each day. Considered the scientific gold standard, it involves having people drink water that has been chemically modified to allow scientists to measure how quickly molecules are flushed through the digestive system. The method has been around since the 1980s, but it's complicated and expensive, and as a result most previous studies — unlike this new one — have included only a small number of participants.

What's more, the global team of researchers measured not only how much energy is used to perform basic functions (breathing, healing wounds, pumping blood through your veins), but also the energy used for daily activities like jogging, brushing your teeth — even thinking. To further zoom in on the effects of age and gender, they adjusted the data to account for body size (a smaller person naturally burns fewer calories than a bigger person) and body composition (muscle tissue burns more calories than fat)

What they discovered? In a nutshell:

• There are four metabolic life stages, but they don't necessarily line up with big milestones like puberty, pregnancy or menopause.
• Metabolism peaks much earlier — and starts slowing down much later, around age 60 — than formerly believed.
• Controlling for body fat and muscle percentage, women's metabolisms were essentially the same as men's.
• Within the larger population-based trends, individual metabolic rates varied significantly: Some subjects had rates 25 percent above average for their age, while others had rates 25 percent below average.

But the complete story is full of other interesting details. When infants are first born, for instance, their metabolism mirrors that of their mothers. Then, about a month after they enter the world, their metabolic rate begins to rev up — a lot. Adjusting for weight, a 1-year-old burns calories about 50 percent faster than an adult. In fact, toddlerhood — not the teen years or early adulthood — is when human metabolism peaks. After the initial acceleration, your metabolic rate slides about 3 percent every year until age 20, when it plateaus. From there metabolism holds steady until that magic (average) age of 60, and then declines at a rate of .7 percent a year indefinitely.

That was a surprise to Herman Pontzer, the study's lead author and associate professor of evolutionary anthropology at Duke University. “I'm in my 40s, so I expected to see some evidence to back up my subjective experience that my metabolism is slowing down. It feels that way to me! But it's not really what's happening,” he says.

So what's really behind middle-aged weight gain?

Of course, that doesn't mean that the dreaded middle aged spread is all in our heads, he notes. (Research shows the average U.S. adult gains one to two pounds a year from early to middle adulthood.) It just means there are contributing factors other than metabolism. “Your stress level, your schedule, your hormone levels, your energy levels are different in your 40s or 50s compared to your 20s,” says Pontzer, who is also the author of Burn, a new book about the science of metabolism. “If you're gaining weight it's easy to say, ‘Oh, that's my metabolism.’ It's almost like a scapegoat. Now that we know it's not metabolism, we can focus on some of those other factors.”

Research has also shown that metabolism and weight aren't always as closely linked as you imagine. “It's not about how many calories you burn, it's about whether you're burning more than you're eating,” says Pontzer. “Just because you have a high metabolism doesn't mean you're better at matching your intake to your output."