Does my metabolism really slow down as I get older?

The picture of a metabolism that gradually seizes up from 30 fits the best measurements poorly. In a large 2021 study, Pontzer and colleagues measured energy expenditure with doubly labelled water in more than 6,400 people, from newborns to 95-year-olds, and found four clear stages. Expenditure per kilo peaks in small children, roughly 50 percent above adult levels around age one, then falls slowly until about 20, and stays flat after that. Once body size is accounted for, metabolism is stable the whole way from 20 to 60, even during pregnancy. Only after 60 does it begin to fall.

So the feeling that the engine slowed at 35 rarely matches what can be measured. What actually changes lies outside the burn rate itself: we move less, lose muscle, and often keep the same portions year after year. In their review, Roberts and Rosenberg point to declining physical activity as an important part of the decline that does arrive with age.

When expenditure does fall after 60, it is not only that the muscles shrink. St-Onge and Gallagher describe how the metabolic rate of the organs and tissues themselves also appears lower in older people, adding to the slow decline beyond the loss of muscle mass.

One caveat: some medical conditions, such as an underactive thyroid, genuinely affect metabolism. If you suspect one, that assessment belongs with a doctor, not a table.

The slow metabolism is a convenient explanation. Weight does creep up in mid-life, and age stands ready as a made-to-order culprit no one can be blamed for. It lifts the fault from everyday choices, and shame is a poor foundation to change anything from. At the same time, an entire market profits from the story: supplements, programs and 'metabolism boosters' sell exactly the feeling that a sluggish engine can be restarted. The real causes are harder to point at, because they happen slowly and invisibly: a little less movement each year, a little less muscle, the same portions as at 25. A single clear villain like 'age' is easier to reach for than the sum of small drifts.

Since the engine doesn't slow in your 30s and 40s, weight gain is workable through the same levers as always: portions, protein and strength work to keep your muscle, and staying active. Start from your actual energy needs based on your current size and activity, rather than assuming a broken metabolism. And if you suspect a medical cause, raise it with a doctor.