Metabolism and age: energy use holds steady from 20 to 60 and then declines, in a study of 6,421 people

Almost everyone has heard that metabolism slows down in the thirties, and almost everyone who has gained weight in middle age has blamed it. The idea is repeated so often that it sounds like settled physiology. For a long time, though, nobody had measured how much energy people actually burn across a whole lifetime in large enough numbers to check.

In 2021 an international team led by Herman Pontzer of Duke University did, pooling measurements gathered since the 1980s by laboratories around the world. Their paper in Science found that the body’s energy use does change with age, in four distinct stages, and that the change people complain about is missing. Between the ages of 20 and 60, metabolism was flat.

What metabolism means, and how daily energy expenditure is measured

In everyday speech metabolism means how fast a person burns calories. Researchers separate that into parts. Basal expenditure is the energy used at complete rest to keep organs running, and it accounts for only a portion of the day’s total, usually half to two thirds. The rest goes on moving about, digesting food and everything else a person does.

Measuring the total outside a laboratory is hard, and the method that made it possible is called doubly labeled water. Duke’s account of the study describes it as a urine test that involves having a person drink water in which the hydrogen and oxygen in the water molecules have been replaced with naturally occurring “heavy” forms, and then measuring how quickly they’re flushed out. The difference between the two rates reveals how much carbon dioxide the body produced, and so how much fuel it used, while the person goes about ordinary life.

Because the technique is expensive, individual studies have been small. Pontzer’s group combined them into a database of doubly labeled water measurements for subjects aged eight days to 95 years, 6,421 people in 29 countries.

Everything that follows depends on one adjustment. Bigger people use more energy, and the tissue that uses most of it is fat-free mass: muscle, organs and bone. In this dataset body size accounted for most (83%) of the variation in daily expenditure. To see what age does, the researchers compared each person’s energy use with what would be expected for someone of that size and composition. A result of 100% means exactly as expected for an adult, and 120% means a fifth more.

The four life stages of metabolism

Read against that yardstick, a human life divides into four periods.

Life stage	Energy use for body size, compared with adults
Birth to age 1	Starts at adult level, climbs to about 50% higher
Age 1 to 20	Falls by about 3% a year to adult level
Age 20 to 60	Stable
After 60	Falls by about 0.7% a year, to about 26% lower in the nineties

Newborns in their first month burned energy at about the adult rate for their size. Then the rate soared. For subjects between 9 and 15 months, expenditure was roughly half as high again as an adult’s, more than growth alone could explain. “Something is happening inside a baby’s cells to make them more active, and we don’t know what those processes are yet,” Pontzer said when the study was published.

From the first birthday the rate drifted down for two decades. Teenagers eat a great deal because they are growing and are large, but for their size they are no more energy-hungry than the trend predicts. No pubertal increases in adjusted total or basal expenditure were evident in the data. “We really thought puberty would be different and it’s not,” Pontzer said.

Then came forty years of nothing. The paper states that total and basal expenditure and fat free mass were all stable from age 20 to 60, and that sex made no difference once body composition was taken into account. Pregnancy did not alter the picture: adjusted total and basal expenditures were stable even during pregnancy, with the extra calories a pregnant woman burns matching what her added mass would predict.

The decline began around 60 and was gradual. By their nineties, people’s size-adjusted energy use was about a quarter below that of middle-aged adults.

Does metabolism slow in midlife or at menopause?

On this evidence, no. The decades in which people most often notice weight creeping up are the decades in which energy expenditure, for a given build, was most constant. Duke’s summary put it directly: several factors could explain thickening waistlines in midlife, but the results suggest that a changing metabolism isn’t one of them.

The phrase “for a given build” matters. The stability applies to energy use relative to fat-free mass. Someone who loses muscle through their forties will use fewer calories in total, because there is less tissue to fuel, even though each kilogram of what remains works as hard as before.

Menopause is the midlife event most often blamed for weight gain, and the Science paper did not analyze it separately. A long-running American project, the Study of Women’s Health Across the Nation, did. Gail Greendale of the University of California, Los Angeles, and colleagues tracked body scans in women before and after their final period. Around the transition, the rate of fat gain doubled, and lean mass declined. Yet weight climbed linearly during premenopause without acceleration when menopause arrived. Body composition shifted while the scales kept the same pace.

Those two results fit together. If the amount of fuel burned per kilogram of lean tissue holds steady, then weight gain in the middle decades has to come from the other terms in the equation: how much is eaten, how much lean tissue is kept, and how much a person moves.

Why does metabolism fall after 60?

Part of the answer is simple loss of tissue. Fat-free mass shrinks in later life, and with it the energy needed to maintain it.

The more interesting part is that the fall exceeded what lost tissue could explain. “We controlled for muscle mass,” Pontzer said. “It’s because their cells are slowing down.” The authors tested this by estimating what each person’s organs ought to burn at rest, given their size. In children and teenagers the measured rate was about 30% above that estimate. In people over 60 it was about 20% below. The tissues themselves appear to work at different rates at different ages.

An earlier review had reached the same conclusion from smaller studies. Todd Manini of the University of Florida wrote in a review of the field that the fall in resting metabolic rate with age appears to be due to a reduction in organ mass and specific metabolic rates of individual tissues, alongside a separate decline in physical activity.

What changes inside the cells is unknown. One reassuring detail in the Science paper is that the slowdown did not show up as fat. The authors note that declining metabolic rates in older adults could increase the risk of weight gain, but neither fat mass nor percentage increased in this period, which suggests that appetite falls in step with need.

How much does metabolism differ from person to person?

The four stages describe averages, and the spread around them is wide. The authors write that there is considerable metabolic variation among individuals, with energy use differing by more than 20% in either direction between people of the same age, sex and body composition. Two 45-year-olds of identical build can differ by several hundred calories a day.

Metabolism can also shift within a person for reasons other than age. Sustained weight loss lowers it by more than the lost tissue would predict, a response known as metabolic adaptation. The best-known illustration is small: of the 16 contestants from a television weight-loss competition originally investigated, 14 participated in this follow-up study by Kevin Hall’s group at the US National Institutes of Health. Six years on, their resting metabolism was still several hundred calories a day below what their body composition predicted. The team concluded that metabolic adaptation persists over time. Fourteen people who lost extreme amounts of weight under extreme conditions are not a guide to ordinary dieting.

Whole populations may be drifting too. Using the same international database, John Speakman, a co-author of the Science paper, and colleagues reported in 2023 that total energy expenditure adjusted for body composition and age declined since the late 1980s, and that the fall came from resting metabolism, with energy spent on activity slightly higher than before.

What the energy expenditure study leaves open

The study took a snapshot of many people, each measured once. It compared 30-year-olds with 60-year-olds; it did not follow anyone from 30 to 60. People born in different decades grew up with different diets, infections and activity levels, and some of what looks like aging could be differences between generations.

The team also assembled its data from many laboratories over many years, with volunteers who were not chosen to represent their countries. Estimates are firmer for total expenditure than for basal expenditure, which was measured in fewer than a third of participants, and the authors acknowledge the relatively small number of basal measures in the years from 45 to 65, which is exactly the period of most public interest.

And the study describes without explaining. It shows when size-adjusted metabolism changes and cannot show what drives the change, or whether anything a person does alters the timing.

How the metabolism results bear on nutrition across the lifespan

The authors expected the practical value to lie at the two ends of life. They wrote that the changes they mapped “should help shape nutrition and health strategies across the life span”. An infant’s very high energy needs may help explain why undernutrition in the first year does such lasting harm. In old age, a slower metabolism could affect how much food is needed and how quickly drugs are cleared, both of which the authors flag as subjects for further research.

For the decades in between, the main contribution is to remove an explanation that was never well supported. “What’s weird is that the timing of our ‘metabolic life stages’ doesn’t seem to match those typical milestones,” Pontzer said.

On the best measurements available, metabolism for a given build does not slow in the thirties, forties or fifties, though the evidence comes from comparing different people of different ages.