The New Science on How We Burn Calories
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.
What metabolic changes have you noticed as you've aged?