Most of us know the conventional wisdom about metabolism. You put on a few pounds from your 20s onward because the metabolism slows down, especially around middle age. But according to a new study, that’s all wrong.

Using data from 6,500 people ranging from 8 days to 95 years old, researchers from Duke University discovered that there are four very distinct periods of life – as far as metabolism goes. The study also found out that there are no real differences between the metabolism of men and women after controlling for other factors.

“There are lots of physiological changes that come with growing up and getting older,” study co-author Herman Pontzer, associate at Duke University, said in a press statement. “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.”

A new approach to metabolism

Pontzer and a team of scientists analyzed the average calories burned by a group of people as they went about their lives in 29 countries. Previous studies measured how much energy the body uses to perform basic vital functions, such as breathing or digesting. But that only represents 50% to 70% of the calories we burn each day. 

To measure the total daily energy expenditure, the researchers used a urine test known as “doubly labeled water”. It involves having 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 are flushed out. 

The technique is considered the most accurate way to measure daily energy expenditure during normal daily life outside of the lab, and its not novel — it has been used since the 1980s but studies have been limited in size and scope due to its cost. That’s why for this new study multiple labs decided to share their data and gather their measurements into a single database. 

“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 key findings

Energy needs increase during the first 12 months of life, making one-year-old burn calories 50% faster for their body size than an adult, the study showed. And that’s not only because, in their first year, but infants also triple their body weight. Something happens inside a baby’s cells that makes them more active, a process yet unclear to researchers. 

Following this surge in infancy, the data showed that metabolism slows down by 3% every year until we reach our 20s when it levels off into a new normal. Teen years are also a time of growth but the researchers didn’t find any increase in daily calorie needs in adolescence after taking the body size into consideration. But the unexpected findings don’t end there.

Midlife was also very surprising. We used to think that after 30s, it was all downhill when it comes to our weight. But while several factors may explain the thickening waistlines that usually emerge during our working years, the study suggests that metabolism isn’t one of them. In fact, energy expenditures during that period seem to be the most stable.

The findings suggest that our metabolism only tends to slow down after 60. The slowdown is gradual, at only 0.7% a year. But a person in their 90s needs 26% fewer calories each day than someone in midlife. The researchers think lost muscle mass as we get older might be partly to blame, as muscle burns more calories than fat.

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

The study was published in the journal Science.