The short version
- A 2021 study in Science measured total daily energy expenditure by doubly labelled water in 6,421 people from 29 countries, aged eight days to 95 years.
- Size-adjusted daily energy expenditure was stable from age 20 to 60, including during pregnancy, with no decline at 30 or 40.
- Segmented regression placed the start of the age-related decline at 63.0 years, after which adjusted expenditure fell by roughly 0.7% per year.
- Fat-free mass and body size accounted for 83% of the variation in daily energy expenditure between people, which is why body composition rather than age drives most individual differences.
- Accelerometer data from a representative US sample found fewer than 5% of adults met 30 minutes of daily activity, far below what people reported about themselves.
It is one of the most repeated sentences in adult life, said with a shrug at a birthday dinner: well, everything slows down after thirty. Almost everyone believes it. It is also, as far as the best measurement we have can tell, not true.
In 2021, a consortium of researchers led by Herman Pontzer published a study in Science that did the thing nobody had managed before: it measured real-world daily energy expenditure across an entire human lifespan, in one consistent way. The database held 6,421 people from 29 countries, 64% of them female, aged from eight days old to 95.
The method matters, because it is the reason this paper carries weight. Doubly labelled water is not a formula on a treadmill screen. You drink water in which the hydrogen and oxygen have been swapped for heavier isotopes, and researchers then track how quickly each leaves your body in urine. The difference between the two elimination rates tells them how much carbon dioxide you produced, and therefore how much energy you actually burned — living your normal life, for a week or two, with no lab and no guessing.
What they found
First, the boring, load-bearing result: daily expenditure tracked fat-free mass in a power-law relationship, and body size alone accounted for 83% of all the variation between people. Size is the story. Everything else in this paper is what is left after you adjust for it.
Adjusted that way, four life stages appear. Newborns start out roughly at adult levels and then climb steeply, running about 50% above adult values by around one year old. From age one, adjusted expenditure declines slowly — about 2.8% per year — until it flattens out at a breakpoint the authors put at 20.5 years.
Then nothing happens for forty years. In the paper's own words, size-adjusted total and basal expenditure “remains stable in adulthood (20 to 60 years), even during pregnancy.” No dip at 30. No shelf at 40. Segmented regression placed the start of the real decline at 63.0 years (95% confidence interval 60.1 to 65.9), after which adjusted expenditure falls by about 0.7% per year — leaving people in their nineties around 26% below middle-aged adults.
So why does everybody feel it?
Because two real things do change, and neither of them is the engine.
The first is what you are made of. Janssen and colleagues used whole-body MRI on 468 men and women aged 18 to 88 and found that relative skeletal muscle mass starts falling in the third decade, but a noticeable drop in absolute muscle mass did not appear until the end of the fifth decade, driven mostly by the lower body. Less muscle means a smaller engine burning fewer calories. But that is a change in your body composition, not a change in your metabolic rate — and it is the one item on this list you can directly do something about.
The second is what you do. Troiano and colleagues put accelerometers on a nationally representative US sample rather than asking people to estimate. Physical activity declined steadily with age, and fewer than 5% of adults met the recommended 30 minutes a day when it was measured objectively — dramatically fewer than self-report suggested. Their conclusion is worth sitting with: “Great care must be taken when interpreting self-reported physical activity.” We are collectively wrong about how much we move, in the direction that flatters us.
What this paper does not say
We are not going to overclaim on the back of one very good study, so here are its edges.
- It measured energy out, not energy in. Appetite, portion drift, alcohol, and the way food shows up in an office are not in this dataset, and they are the other half of the equation.
- It reports group averages. Stability across thousands of people does not promise that your own expenditure has been flat, and it does not rule out a medical cause for a change you have noticed.
- It did not analyse menopausal status as a variable. Anyone citing this paper to tell you what happens to metabolism through menopause is citing something it did not measure.
- Stability of the rate is entirely compatible with weight gain, because the rate is not what changed. Your habits and your body composition did.
What we'd actually tell you
This is the article where we have nothing to sell you, and we would rather say that out loud than pretend otherwise. There is no product in our catalogue that raises your metabolic rate, and if you find a brand claiming one does, notice that they are arguing with a 6,421-person study and losing.
The levers are the ones that were always the levers. Keep or rebuild muscle, because fat-free mass is the single strongest predictor of what you burn in a day and it is the part that genuinely erodes. Measure your activity rather than estimating it, because the accelerometer data says your estimate is generous. And look honestly at intake, since that is the variable this study deliberately did not cover.
One more thing. If your weight or energy has changed noticeably and quickly, and you cannot point at a change in what you eat or how you move, that is a reason to see a doctor rather than to buy something. Thyroid function, a new medication, sleep apnoea and a handful of other ordinary explanations are all worth ruling out first, and none of them are on a supplement label.
Good questions
Does your metabolism slow down after 30?
No. The largest measurement ever made of real-world daily energy expenditure, published in Science in 2021, found size-adjusted expenditure stays flat from age 20 to 60. The decline the researchers could detect began at 63 and then ran at about 0.7% a year. Nothing distinctive happens at 30, or at 40.
Then why am I gaining weight in my forties?
Most likely because of what changed around the rate rather than the rate itself. Muscle mass erodes from midlife, measured activity drops steadily with age, and intake tends to creep. The 2021 study measured energy burned, not energy eaten, so it cannot rule out that last one. If the change was sudden or large, see a doctor before you change your diet.
Is there a supplement that speeds up your metabolism?
Not one we sell, and not one we would sell. Daily energy expenditure is driven overwhelmingly by how much fat-free mass you carry, which accounted for 83% of the variation between people in the 2021 dataset. No capsule meaningfully changes that. Anything marketed as a metabolism booster is asking you to believe a number that has been measured and did not move.
Does building muscle actually raise your metabolism?
It raises fat-free mass, which is the strongest single predictor of daily energy expenditure, so yes in direction. Be realistic about size, though: a few added kilograms of muscle is a modest daily number, not a transformation. The larger practical benefit of training is what it does to how much you move and what your body can still do at 70.
At what age does metabolism actually decline?
Around 63, according to segmented regression in the 2021 Science analysis, with a confidence interval running from about 60 to 66. After that, size-adjusted expenditure falls roughly 0.7% per year, so someone in their nineties sits about 26% below a middle-aged adult. That is a gradual slope late in life, not a cliff in your thirties.
Does menopause slow your metabolism down?
This study cannot answer that, and we are not going to pretend it can. It found stability across ages 20 to 60 in a mixed-sex sample but did not analyse menopausal status as a variable. Anyone quoting the 6,421-person paper to make a claim about menopause is citing something it did not measure. Ask your doctor about your own situation.
Sources
- Pontzer H, Yamada Y, Sagayama H, et al.; IAEA DLW Database Consortium. Daily energy expenditure through the human life course. Science, 2021. View study
- Janssen I, Heymsfield SB, Wang ZM, Ross R. Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr. J Appl Physiol, 2000. View study
- Troiano RP, Berrigan D, Dodd KW, Mâsse LC, Tilert T, McDowell M. Physical activity in the United States measured by accelerometer. Med Sci Sports Exerc, 2008. View study
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