Physiology

Does your metabolism really crash in your 30s and 40s?

Large studies suggest metabolism stays remarkably stable through most of adulthood — what changes more is activity and muscle.

Many people blame weight gain in their 30s and 40s on a slowing metabolism. A large international study published in 2021, which analyzed thousands of measurements of daily energy expenditure using the doubly labeled water method, challenged this idea. After adjusting for body size and composition, it found that energy expenditure was highest relative to size in infancy, gradually declined through childhood and adolescence, then stayed remarkably stable from about age 20 to 60, before declining slowly after that.

If metabolism per kilogram is fairly stable through middle age, other factors likely explain gradual weight gain. Daily movement often drops as careers, commutes and family responsibilities increase, with less walking and more sitting. Many adults also lose muscle slowly when they do not strength train, and muscle is metabolically active tissue that affects how much energy the body uses and how it handles carbohydrates. Sleep, stress, alcohol and eating patterns change too, and small daily energy surpluses add up over years.

The encouraging part is that most of these factors are within your control. Strength training two or more times per week helps preserve and build muscle at any age, and staying active through daily walking and other movement keeps total energy expenditure higher. Paying attention to protein, portion sizes and sleep matters more than worrying about a mysterious metabolic slowdown. After about age 60, the natural decline makes these habits even more important for keeping strength, mobility and independence.

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