Your metabolism slows with age, but not as early as you think

If you’re in your 40s or 50s and have been blaming that recent weight gain on a slowing metabolism, you might need to find a new excuse.

Researchers at Duke University have measured life’s metabolic highs and lows from birth to old age, with some surprising findings.

Their results suggest your metabolism — the rate at which you burn calories — actually peaks much earlier in life and starts its decline later than previously thought.

“There are lots of physiological changes that come with growing up and getting older,” study co-author Herman Pontzer, associate professor of evolutionary anthropology at Duke, told Duke Today. “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.”

Most large studies have measured only the calories needed to stay alive — for breathing, digestion and pumping blood. But that’s only about 50% to 70% of our total calories burned each day, the researchers said.

For their study, Pontzer and an international team analyzed the average calories burned each day by nearly 7,000 people age 1 week to 95 years in 29 countries.

Although many people think their metabolism is highest during their teens and 20s, the scientist found that, pound for pound, infants had the highest metabolic rates.

Energy needs shoot up during the first 12 months of life, such that by their first birthday, 1-year-olds burn calories 50% faster for their body size than an adult.

“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,” Pontzer said.

After infancy, the researchers found, metabolism slows by about 3% each year until our 20s, when it levels off into a new normal.

“We really thought puberty would be different, and it’s not,” Pontzer said, adding they found no increase in daily calorie needs during adolescence.

Once metabolism levels off in our 20s, it remains stable until after age 60, when it starts to decline again.

“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 said. “You really need a big data set like this to get at those questions.”

You can read the full study in the journal Science.