New Human Metabolism Research Upends Conventional Wisdom about How We Burn Calories

New Human Metabolism Research Upends Conventional Wisdom about How We Burn Calories

Herman Pontzer

In 2021, after years of collaborative effort, we published the first comprehensive study investigating the effects of age and body size on daily energy expenditure. As expected, we found that metabolic rates increase with body size: bigger people burn more calories. In particular, fat-free mass (the muscles and other organs) is the single strongest predictor of daily energy expenditure. This makes good sense. Fat cells aren’t as active as those in the liver, brain, or other tissues, and they don’t contribute much to your daily expenditure. More important, with the relation between mass and metabolic rate clearly established from thousands of measurements, we could finally test whether metabolism at each age was faster or slower than we’d expect from size.

The results were a revelation, the first clear road map of metabolism over the human life span. We found that, metabolically, babies are born like tiny adults, reflecting their development as part of their mom’s energy budget. But metabolism skyrockets over the first year of life, so that by their first birthday children are burning 50 percent more energy than we’d expect for their size. Their cells are far busier than adults’ cells, hard at work on growth and development. Earlier studies measuring glucose uptake in the brain during childhood suggest some of this work is neuronal growth and synapse development. Maturation in other systems no doubt contributes as well. Metabolism stays elevated through childhood, slowly decelerating through adolescence to land at adult levels around age 20. Boys decline more slowly than girls, consistent with boys’ slower development, but there’s no bump at puberty in males or females.

Perhaps the biggest surprise was the stability of our metabolism through middle age. Daily energy expenditures hold remarkably steady from age 20 to 60. No middle age slowdown, no change with menopause. The weight gain so many of us experience in adulthood cannot be blamed on a declining metabolism. As a man in my 40s, I had sort of believed the folk wisdom that metabolism slowed as we aged. My body definitely feels different than it did 10 or 20 years ago. But like hunting some metabolic Sasquatch, when you actually look there’s nothing there. Same for the much touted metabolic differences between men and women. Women have lower daily energy expenditures on average, but that is only because women tend to be smaller and carry more of their weight as fat. Compare men and women with the same body weight and body fat percentage, and the metabolic difference disappears.

We did find a decline in metabolism with age, but it doesn’t kick in until we hit 60...