August 16, 2021

Don't blame middle-age weight gain on a 'slowing metabolism,' a new study suggests

Daily Briefing

    Most people have long believed that metabolisms slow down around middle age, making it harder to control weight. But a groundbreaking study published in Science suggests we may have been misunderstanding metabolism all along, Gina Kolata writes for the New York Times.

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    Study details and findings

    For the report, more than 80 co-authors combined efforts from a half dozen labs studying metabolic rates over 40 years. According to Kolata, all the labs included in the study used the "gold standard" to examine metabolic rates, "doubly labeled water," which involves measuring calories burned by monitoring how much carbon dioxide an individual breathes out during everyday activities.

    Overall, the researchers had data from nearly 6,500 people, ranging in age from 8 days to 95 years. The data included information on participants' height, weight, and body fat percentage, which enabled the researchers to examine fundamental metabolic rates, Kolata writes.

    Specifically, the researchers were able to control for size and body fat percentage to assess whether or how metabolism differed. They found that metabolism differs for people across four distinct stages of life:

    • Infancy to age 1, when calorie burning peaks at about 50% above the adult rate
    • Age 1 to ~20, when metabolism gradually slows by about 3% per year
    • Age 20 to 60, when metabolism holds constant
    • Post-age 60, when metabolism declines by about 0.7% per year

    And while there were outliers, the researchers found that the general patterns remained true.

    For instance, when controlling for body size and muscle amounts, researchers found no differences between men and women's metabolisms. While researchers expected women's metabolisms to slow with the onset of menopause, the study's principal investigator, Herman Pontzer, an evolutionary anthropologist at Duke University, said, "we just didn't see that."

    Implications and reaction

    According to Kolata, the research findings could reshape the science of human physiology and some medical practices, such as determining appropriate drug doses.

    Pontzer said the study exposed how little the medical community understood about how body size and aging affect metabolism. "These are basic fundamental things you'd think would have been answered 100 years ago," he said.

    Separately, Leanne Redman, an energy balance physiologist at Pennington Biomedical Research Institute, called it a "pivotal paper," saying "it will be in textbooks." She said the four metabolic periods depicted in the paper show that "there isn't a constant rate of energy expenditure per pound."

    And Rozalyn Anderson, a professor of medicine at the University of Wisconsin-Madison, who studies aging and who wrote an accompanying editorial, said she was "blown away" by its findings. "We will have to revise some of our ideas," she added.

    But Samuel Klein, director of the Center for Human Nutrition at the Washington University School of Medicine, who was not involved in the study, said researchers should look at the findings with caution, noting the study provides "a 30,000-foot view of energy metabolism."

    As a result, he said the paper's implications for public health, diet, and nutrition are limited at the moment. "I don't think you can make any new clinical statements" for individual people, he said. (Kolata, New York Times, 8/13)

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