Does metabolism really slow down in your 40s?

Metabolism slows down in your forties. It is one of the most repeated lines in health writing, and it is usually offered as the reason a waistband tightens in midlife. The largest study of human energy use so far found no such drop. Once body size and composition were taken into account, daily energy expenditure stayed level from about age 20 to about age 60.

The study, published in the journal Science in 2021 by Herman Pontzer and a large international group of co-authors, pooled measurements from 6,421 people in 29 countries. The youngest was eight days old and the oldest was 95. Just under two-thirds (64%) were female.

How energy use was measured

Most figures quoted for metabolism come from short tests of a person lying still. This dataset is different. It relies on the doubly labelled water method, which, as the paper notes, made it possible from the 1980s to measure total energy expenditure during ordinary daily life, outside a laboratory. A summary of the research from Duke University describes it as a urine test: the person drinks water in which the hydrogen and oxygen have been replaced with naturally occurring heavy forms of those elements.

What comes out is total daily energy expenditure, which takes in everything from keeping the heart beating to digesting dinner to walking the dog. For 2,008 of the participants the researchers also had basal expenditure, the energy used at rest, measured by a separate technique.

Bigger bodies use more energy, and the paper shows that total expenditure is strongly tied to fat-free mass, the part of the body that is not fat. So the researchers adjusted each person's result for fat-free mass and fat mass. That adjustment is what makes it fair to compare a 25-year-old with a 55-year-old, or a man with a woman.

Four stages rather than a steady slide

The adjusted figures fell into four distinct phases. In the first year of life, energy use for body size climbs steeply. Duke's summary says that by their first birthday infants burn energy about 50% faster for their size than adults do. From there the rate declines slowly through childhood and adolescence, reaching adult levels at around 20.

Then comes the long level stretch. From 20 to 60, the authors report, total expenditure, basal expenditure and fat-free mass were all stable. Sex made no difference once body composition was included in the analysis. Pregnancy did not shift the adjusted figure either: the extra energy a pregnant woman used matched what would be expected from her added body mass. The researchers looked for a surge at puberty and did not find one.

The fourth phase begins later than folklore suggests. The analysis placed the turning point at 63.0 years, with a 95% confidence interval of 60.1 to 65.9 years. After that, adjusted total expenditure fell by about 0.7% a year. Participants in their nineties used roughly 26% less energy than middle-aged adults.

What changes after 60

At around 60, the paper reports, total and basal expenditure begin to decline along with fat-free mass and fat mass. Less lean tissue means less energy used. That is only part of the explanation, because the adjusted figure falls as well, which means the tissue that remains is using less energy than it did. Among participants over 60, measured basal expenditure was about 20% lower than estimates built up from the size of their organs. The authors' model matched what they observed only when it included age-related changes in both physical activity and the metabolism of the tissues themselves.

Pontzer put it more briefly in the Duke summary:

It's because their cells are slowing down.

The paper adds one more observation. Falling energy needs in later life could raise the risk of weight gain, yet in this dataset neither fat mass nor body fat percentage rose after 60. The authors read that as consistent with the idea that the amount people eat tends to follow the amount of energy they use.

Why midlife can still feel different

None of this makes the changes people notice in midlife imaginary. The study measured energy going out. It did not measure what people ate, how they slept or what their working days looked like, and it cannot explain why any one person's weight has shifted.

It also describes averages. Individual results varied by more than 20% in either direction, even after accounting for fat-free mass, fat mass, sex and age. Two people of the same age and build can have quite different energy needs.

And the adjusted figure holds body composition constant, which real life does not. Healthdirect, the government-funded health information service, lists muscle mass among the things that set a person's resting energy use and notes that losing muscle with age lowers it. Victoria's Better Health Channel describes muscle fibres reducing in number and shrinking in size as people get older, and attributes at least half of the age-related changes to muscles, bones and joints to disuse.

Disuse is common, and it starts well before 60. In Australian Bureau of Statistics figures for 2022, 31.3% of people aged 18–24 met the physical activity guidelines for adults. The ABS describes these as 150 to 300 minutes of moderate activity a week, or 75 to 150 minutes of vigorous activity, plus strength or toning work on at least two days. Among people aged 45–54 the share was 17.7%. At 55–64 it was 16.1%.

What to take from it

The research suggests that the passing of years between 20 and 60 does not, by itself, change how much energy a given amount of body tissue uses. What does vary across those decades is how much muscle a person carries and how much they move, and both can shift without much notice.

On muscle, the Better Health Channel states that older people can increase muscle mass and strength through muscle-strengthening activities. On movement, the guideline the ABS reports against for people aged 65 and over is 30 minutes or more of physical activity on most, preferably all, days.

Energy needs are individual. Anyone with unexplained weight change or persistent tiredness, or who has a health condition and wants to start a new exercise routine, is best served by a conversation with their GP, who can look at one person's circumstances instead of a population average.

Sources

1. Pontzer H, Yamada Y, Sagayama H and others, Science (2021), via PubMed Central, Daily energy expenditure through the human life course.
2. Duke University Population Research Institute, Pontzer and Colleagues Explore How Metabolism Changes Throughout the Life Course.
3. Australian Bureau of Statistics, Physical activity, 2022.
4. Healthdirect, Metabolism.
5. Better Health Channel (Victorian Department of Health), Ageing - muscles bones and joints.