We’re often told our metabolism speeds up at puberty and slows down in middle age, particularly with menopause, and that men have faster metabolisms than women. None of these claims is based on real science.
My colleagues and I have begun to fill that gap in scientific understanding. In 2014 John Speakman, a researcher in metabolism with laboratories at the University of Aberdeen in Scotland and the Chinese Academy of Sciences in Shenzhen, organized an international effort to develop a large metabolic database. Crucially, this database would focus on total daily energy expenditure measured using the doubly labeled water method, an isotope-tracking technique that measures the carbon dioxide produced by the body (and thus the calories burned) over one to two weeks. Doubly labeled water is the gold standard for measuring daily energy expenditures, but it’s expensive, and you need a specialized lab for the isotope analyses. So even though this technique has been around for decades, studies are typically small. Led by Speakman, my lab joined a dozen others around the world in pooling decades of data. We ended up with more than 6,400 measurements of people ranging from babies just eight days old to men and women in their 90s.
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. [Continue reading…]