Mostly no, not in the years people usually blame. In a study of 6,421 people aged 8 days to 95 years, daily energy use adjusted for body size stayed flat from about 20 to 60 and only began to fall after 60, slowly1.
That changes what "my metabolism slowed down at 35" can mean. This article walks through what was measured, what "adjusted" hides and reveals, and what it means for a calorie target.
What the study measured
Metabolism here means all the energy your body uses in a day: at rest, digesting, moving, everything. Researchers call it total daily energy expenditure. The study measured it with doubly labelled water, a drink of water whose hydrogen and oxygen carry harmless, traceable tags. How fast the tags leave the body shows how much energy it used over one to two weeks of normal life, outside a lab1.
The team pooled measurements from 29 countries into one database: 6,421 people, 64% of them female, from newborns to 95-year-olds1. In 2,008 of them, resting energy use was also measured by indirect calorimetry (breathing into a hood that reads oxygen used)1.
| What | Number |
|---|---|
| People | 6,421 (64% female) |
| Countries | 29 |
| Ages | 8 days to 95 years |
| Method | doubly labelled water (1–2 weeks of free living) |
| Resting measures | 2,008 people |
| Share of spread explained by body size and make-up | 83% |
In plain terms: this is real-life energy use in thousands of people, not a formula and not a lab session.
Why "adjusted" is the key word
Bigger bodies use more energy. Size, above all lean (fat-free) mass, explained 83% of the differences between people1. So a 70-year-old using less energy than a 30-year-old could simply be smaller.
To see whether age matters on its own, the authors adjusted each person's expenditure for their fat-free mass and fat mass. On their scale, 100% means "what an adult of this size and make-up is expected to use"; 120% would be 20% above that1.
Daily energy use across life, adjusted for body size · % of adult level
The four stages
The adjusted curve has four parts1:
- Infancy. Newborns in their first month sit near the adult level (about 99%). By around one year they are roughly 50% above it, the highest point in life.
- Childhood and adolescence. Adjusted energy use eases down, reaching the adult level at about 20 (break point 20.5 years). There was no bump at puberty.
- Adulthood, 20 to 60. Flat. Total and resting energy use and fat-free mass were all stable from 20 to 60, and adjusted energy use stayed stable even through pregnancy. Men and women did not differ once size and make-up were counted.
- After 60. The decline starts at a break point of 63.0 years (95% confidence interval 60.1 to 65.9) and runs at about 0.7% a year. People in their nineties used about 26% less than middle-aged adults of the same size and make-up.
A confidence interval is the range the true value most likely sits in. Here it says the turn happens somewhere in the early sixties, not at 40.
In plain terms: once body size is accounted for, the engine idles at the same speed from 20 to 60.
Why it drops after 60
The authors could not explain the late decline by smaller bodies alone: it was "not only a function of" less fat-free and fat mass1. Their model matched the data only when two things fell together: daily physical activity, and the rate at which tissues and organs work per gram1.
That is a model fitted to a snapshot, not a trial. Each person was measured once, so the study compares different people at different ages rather than following the same people as they age. Treat the "why" as the authors' best explanation, not a measured cause.
What this means for "my metabolism slowed down in my 30s"
The study does not say nothing changes between 20 and 60. It says the energy used per unit of body stays the same. Two things can still move:
- Body make-up. If lean mass falls, total energy use falls with it, because lean tissue drives most of the total.
- How much you move. Daily activity is part of the total. A new desk job or fewer steps changes the number, at any age.
Neither is "metabolism slowing" in the sense people usually mean. Both are things the study adjusted for, and both are things you can see and change.
What calorie equations assume about age
Most calorie calculators, including the one in Zyra Nutrition, estimate resting energy use with the Mifflin-St Jeor equation. It subtracts 5 kcal a day for every year of age at a fixed weight and height (see how TDEE is calculated). From 20 to 60 that is 200 kcal a day lower for the same body.
That is not a contradiction of the study. The equation knows only weight, height, sex and age, not lean mass, so its age term partly stands in for the lean mass people in its original sample tended to lose over the years. If your lean mass holds, the study suggests your adjusted expenditure holds too. This is our reading of the two, not a result either paper tested.
A 2023 study measured daily energy use with doubly labelled water in 21 adults aged 80 to 90 and compared four equations2. Mifflin-St Jeor came out close on average but low, and one older equation overestimated by more.
Equation minus measured daily energy use, adults aged 80 to 90 · kcal/day
The averages hide the spread: the individual errors ran to more than 1,000 kJ (about 250 kcal) a day in either direction2. That is why an app should correct its estimate from your own logs and weight trend rather than trust the formula forever (see how apps estimate energy expenditure).
What the studies found
| Study | Who was studied | Key result | Year |
|---|---|---|---|
| Pontzer et al., *Science*1 | 6,421 people, 8 days to 95 years, 29 countries; doubly labelled water | Adjusted daily energy use stable from 20 to 60; falls about 0.7% a year after about 63 | 2021 |
| Nguo, Truby, Porter, *Ann Nutr Metab*2 | 21 adults aged 80 to 90; doubly labelled water vs four equations | Mifflin-St Jeor underestimated daily energy use by about 85 kcal on average; individual error was large | 2023 |
Limits worth knowing
- One snapshot, not a follow-up. Different people at different ages, measured once each.
- Averages, not you. Even at the same size, sex and age, individuals varied by more than 20% around the prediction1.
- Supplementary tables not read. We read the full main text and figure captions of the 2021 paper; claims here stay inside them.
- The over-80 study is small. Twenty-one people, and we read its abstract only.
- Funding. The database is supported by the International Atomic Energy Agency and two companies; the authors state the funders played no role1.
Latest research
The most recent doubly labelled water study we found that speaks to age is the 2023 over-80 study above2: in the oldest adults, measured energy use and the common equations can disagree by hundreds of calories for an individual. We found no newer life-course dataset that overturns the 2021 curve. When one appears, this page will be updated and its date will change.
The short version
- Adjusted for body size, daily energy use is flat from about 20 to 60.
- It starts to fall around 60 to 65, slowly, about 0.7% a year.
- What changes before 60 is usually lean mass or how much you move, not the rate per unit of body.
- Calorie equations are starting points; correct them with your own data.
Educational overview only — not medical advice.
