It’s simple, we are often told: All you have to do to maintain a healthy weight is ensure that the number of calories you ingest stays the same as the number of calories you expend. If you take in more calories, or energy, than you use, you gain weight; if the output is greater than the input, you lose it. But while we’re often conscious of burning calories when we’re working out, 55 to 70 percent of what we eat and drink actually goes toward fueling all the invisible chemical reactions that take place in our body to keep us alive. “We think about metabolism as just being about exercise, but it’s so much more than that,” says Herman Pontzer, an associate professor of evolutionary anthropology at Duke University. “It’s literally the running total of how busy your cells are throughout the day.” Figuring out your total energy expenditure tells you how many calories you need to stay alive. But it also tells you “how the body is functioning,” Pontzer says. “There is no more direct measure of that than energy expenditure.”
Though scientists have been studying metabolism for at least a century, they have not been able to measure it precisely enough — in real-world conditions, in enough people, across a broad-enough age range — to see how it changes throughout the human life span. It is clear that the bigger someone is, the more cells they have, and thus the more total calories they burn per day. But it has been much harder to assess whether variables like age, sex, lifestyle and illness influence our rate of energy expenditure. This lack of data led to assumptions rooted in personal experience: for instance, that significant hormonal changes like those that take place during puberty and menopause cause our metabolism to speed up or slow down, prompting us to burn more or fewer calories per day; or that men have inherently faster metabolisms than women, because they seem able to shed pounds more easily; or that our energy expenditure slows in midlife, initiating gradual and inevitable weight gain. “I’m in my 40s; I feel different than I did in my 20s — I buy it, too,” Pontzer says. “All that intuition was never backed up by data. It just seemed so sure.”
Last month, however, a paper published in Science by Pontzer and more than 80 co-authors revealed that much of what we thought we knew about metabolism was wrong. Using previously collected data from more than 6,400 subjects who ranged in age from 8 days to 95 years, and adjusting for body size and the amount of fat and muscle present, they found that our metabolism generally goes through four distinct life phases. Newborns’ metabolism resembles that of adults. Then, when they are about a month old, their metabolic rate starts rapidly increasing, until between 9 and 15 months, it is more than 50 percent higher than an adult’s — the equivalent of a grown-up burning around 4,000 calories a day. (The U.S. Department of Health and Human Services estimates that, on average, adult women need between 1,600 and 2,400 calories a day and adult men between 2,000 and 3,000 calories.) At that point, between age 1 and 2, energy expenditure starts to decline and keeps falling until roughly age 20. From there, it holds steady for the next 40 years, even during pregnancy and menopause; you burn calories as efficiently at 55 as you do at 25. At around age 60, energy expenditure begins to drop again and continues to do so until the end of our lives. Men, the researchers observed, do not have innately faster metabolisms than women; rather, they tend to burn more calories per day for their size because they typically have a higher proportion of muscle, which uses more energy than fat does.