For years, the go-to explanation for creeping midlife weight has been a slowing metabolism.
A 2021 study in Science, drawing on data from 6,421 people aged eight days to 95 years across 29 countries, found that daily energy expenditure is remarkably steady from age 20 to 60.
Only after 60 does it begin to fall, and then by only about 0.7% per year, according to the analysis led by Duke evolutionary anthropologist Herman Pontzer.
If your metabolism has not slowed, what has? Research points to two other drivers of the extra pounds that show up in the 30s and 40s.
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One is a gradual decline in daily physical activity. The other is the slow loss of muscle that begins in early adulthood, quietly shrinking the amount of metabolically active tissue your body carries.
The four life stages of metabolism
Pontzer and his co-authors used doubly labeled water, considered the gold standard for measuring total daily energy expenditure, to map calorie burn across the lifespan.
Energy use accelerates in infants, whose metabolism runs about 50% faster than an adult's by their first birthday.
It then slows through childhood and adolescence, plateaus in the early 20s, and holds steady across the decades most people assume are the problem. By the 90s, daily calorie needs are roughly 26% lower than in midlife.
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The picture, published in Science in August 2021, contradicts the widely held idea that metabolism starts sliding in the 30s and 40s. Even pregnancy did not change the pattern once body composition was accounted for.
What actually drives midlife weight gain
The more likely culprits are lifestyle and body composition.
Adults tend to move less as work, commutes and family life crowd out activity, while muscle mass falls quietly in the background.
According to NIH News in Health, the body naturally starts losing muscle mass around age 30, at a rate of roughly 3 to 5 percent per decade, in a process that over time can develop into sarcopenia.
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Muscle is metabolically expensive to maintain, so losing it means burning fewer calories at rest.
Small daily imbalances between food intake and energy use add up over the years. Chronic illness, poor diet and the hormonal changes of menopause can accelerate the loss.
The National Institute on Aging notes that strength training is an effective way to preserve and rebuild muscle in adults, improve balance and support independence with age.
What the guidelines actually recommend
The Physical Activity Guidelines for Americans, issued by the U.S. Department of Health and Human Services, recommend that adults do at least 150 to 300 minutes of moderate-intensity aerobic activity, like brisk walking, each week, plus muscle-strengthening activity on at least 2 days per week.
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That target lines up with what the metabolism research points to.
If the engine is not slowing, the fix is not chasing a mythical boost but replacing the movement and muscle that quietly went missing.
If you have symptoms of unexplained weight gain, persistent fatigue or noticeable muscle weakness, talk with your doctor before starting a new exercise program.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.