Metabolism doesn't slow down at 40 — it does at 60

Herman Pontzer and 79 researchers measured, for Science in 2021, the daily energy expenditure of 6,421 people aged 8 days to 95 years using doubly labelled water —the gold standard, not surveys—. They blew up a fixed number: adult metabolic life is flat from 20 to 60, and only then does it fall.

Four age bands:

1. 0-1 year: per-kilo expenditure spikes to 150 % of adult levels, but it's noise, because the baby triples its mass in months. 2. 1-20 years: slow decline to the 100 % adult baseline. There is no special «teenage peak». 3. 20-60 years: per-kilo expenditure is flat. The slowdown people feel at 40 does not show up on the metabolic scale: it comes from eating more, moving less and losing muscle. 4. From 60 onward: the real decline begins, 0.7 % per year, and by 90 per-kilo expenditure is 26 % lower.

Clinical takeaway you can use tomorrow: if your clothes feel tight at 45, the culprit is not your metabolism. It's Saturday dinner and the Tuesday elevator.