But a new study published in August 2021 in Science suggests that neither of these maxims are true. Metabolism, the rate at which the body burns energy, has long been thought to decline during middle age as people gradually lose muscle mass. For the study, researchers assessed metabolism by measuring total energy expenditure, which includes energy burned at rest to perform basic functions like digesting food as well as energy burned during physical activity, using a process known as the doubly labeled water method. The method measures the amount of carbon dioxide people exhale during daily activities to determine how many calories they burn. After collecting this data from 6,421 people ranging in age from 8 days to 95 years along with height, weight, and body fat percentage, researchers calculated average metabolic rates for men, women, and people in different age groups. The data indicated that men and women had similar metabolic rates after accounting for body size and muscle mass. There also were differences in metabolism based on four distinct periods in participants’ lives:
From infancy to 1 year old, metabolic rate surged until it was about 50 percent higher than it will be during adulthood.From 1 to 20 years old, metabolism decreased by almost 3 percent a year.From 20 to 60, metabolism didn’t change.After age 60, metabolism decreased by 0.7 percent annually
“These data suggest that the `middle age spread’ that we all know about anecdotally or personally is not due to a change in intrinsic metabolism as had been long thought,” says Rozalyn Anderson, PhD, coauthor of an editorial accompanying the study and a professor at the school of medicine and public health at the University of Wisconsin in Madison. “It is far more likely now that changes in behavior are at the root of it.” This is good news, Dr. Anderson adds, because it suggests that adopting healthy eating and exercise habits can go a long way toward helping us avoid weight gain as we age. It also means that people who do gain weight in middle age can’t just blame it on being sluggish due to a slowing metabolism, says lead study author Herman Pontzer, PhD, associate professor of evolutionary anthropology and global health at Duke University in Durham, North Carolina, and author of Burn, a book about human metabolism. Dr. Pontzer, who is in his forties, relates to feeling like middle age is accompanied by reduced energy levels. He says he’s felt that way himself. RELATED: 7 Essential Facts About Metabolism and Weight Loss “But those changes can’t be due to slowing metabolism,” Pontzer says. “The data are really clear that total energy expenditure — the calories we burn every day — is very stable from 20 to 60 years old.” One limitation of the study, however, is that researchers only measured metabolism at a single point in time. The conclusion that metabolism doesn’t change from age 20 to 60 isn’t based on following people over four decades and objectively measuring metabolism at several points in time. Instead, it’s based on average metabolic readings from a single assessment of all the individuals within this age range in the study. “Experts in the field of the biology of human growth have taught us decades ago that you need longitudinal data,” says Claude Bouchard, PhD, chair in genetics and nutrition at the Pennington Biomedical Research Center in Baton Rouge, Louisiana. RELATED: Body Burns the Most Calories During This Time of Day, Study Finds Another limitation is that the analysis didn’t account for decreases in muscle mass that happen over time or differences in muscle mass between men and women that might result in a slower metabolic rate, says Samuel Klein, MD, director of the center for human nutrition at Washington University School of Medicine in St. Louis. The study measured fat-free mass, which is mostly muscle but also includes bone and connective tissue. While the study found no decline in the amount of energy expended for each pound of fat-free mass in middle age, people do lose some muscle during this time in their lives, Dr. Klein says. With less muscle and less fat-free mass, total energy expenditure will still go down. “Therefore, even though energy expenditure in relation to fat-free mass remained constant from age 20 to 60, this does not mean that daily energy expenditure does not decrease because of a decrease in muscle mass,” Klein says. Similarly, men typically have more fat-free mass than women so will have a higher total energy expenditure, even though men and women both burn similar amounts of energy for each pound of fat-free mass that they have, Klein says. RELATED: 9 Hard Truths About Weight Loss The study didn’t look at participants’ eating and exercise habits, which can both impact metabolism and weight over time. One study published in The New England Journal of Medicine followed more than 120,000 adults over three decades. Overall, people gained an average of 3.35 pounds every four years. They gained 1.69 pounds more with increased consumption of potato chips and 1 pound more with each daily serving of sugar. They gained 1.76 fewer pounds when they increased physical activity and also gained less weight when they consumed more healthy foods like fruits, vegetables, and whole grains. High-intensity aerobic exercise and strength training, both of which can help build lean muscle mass, may be one way to boost metabolism at any age, says Anderson. However, because not everyone has the same resting metabolic rate, not everyone will see the same results from similar exercise efforts, even when they’re the same size. “We all know that diet and exercise are the key to health,” Anderson says. “It is pretty clear that one size does not fit all.”
