According to an unprecedented study of the body’s energy expenditure, the arrival in middle age cannot be attributed to a decline in metabolism.
The study of 6,400 people aged 8 to 95, which was carried out in 29 countries, suggests that metabolism remains “rock solid” in middle age.
It reaches its peak at the age of one year, is stable between 20 and 60 years, and then inevitably decreases.
Researchers say the results bring new and surprising discoveries about the body.
Metabolism is every drop of chemical that is needed to keep our bodies going. And the bigger the body – be it in terms of developed muscles or a lot of belly fat – the more energy it takes to move it.
So the researchers adjusted their measurements for height to compare people’s metabolism “pound for pound”.
The study, published in the journal Science, found four stages in metabolic life:
– From birth to 1 year, when the metabolism leaves the mother’s level and reaches the highest point in life, 50% above the adult population
– A slight slowdown occurs by the age of 20, with no increase in any changes in puberty
– No change from 20 to 60 years
– A permanent decrease with annual decreases that leave the metabolism 26% below mean age by around 90 years of age
“It’s a picture we’ve never seen before and it has a lot of surprises in it,” says John Speakman, one of the researchers at the University of Aberdeen in Scotland.
“The most surprising thing to me is that there is no change in adulthood – when you experience a midlife crisis, you can no longer blame the drop in metabolism.”
Other surprises also arose from what the study failed to find.
There was no metabolic increase during puberty or pregnancy, and no slowdown around menopause.
A high metabolism in the first years of life also underlines how important this moment is for human development and why malnutrition in childhood can have lifelong consequences.
“When people talk about metabolism, they think about diet and exercise – but it goes deeper. We watch your body, your cells, in action, ”Duke University professor Herman Pontzer told BBC News.
“They are incredibly busy by the age of a year, and as we wear off as we age, we see their cells stop functioning.”
The metabolism of humans was measured with so-called double-labeled water.
It is made up of the heavier forms of the hydrogen and oxygen atoms that make up water and can be tracked as it leaves the body.
But double-labeled water is incredibly expensive. Therefore, researchers in 29 countries worked together to collect data from 6,400 people.
Researchers say that a complete understanding of changes in metabolism can have implications for medicine.
Pontzer says this could help uncover whether cancer spreads differently with changes in metabolism and whether drug doses need to be adjusted at different stages.
It is also being discussed whether metabolism-modifying drugs can delay old-age diseases.
Rozalyn Anderson and Timothy Rhoads of the University of Wisconsin (USA) say the “unprecedented” study has already provided “important new insights into human metabolism”.
And that “it cannot be a coincidence” that diseases of old age develop when the metabolism is declining.
Professor Tom Sanders of King’s College London in the UK says: “Interestingly, little differences were found in total energy expenditure between early adulthood and middle age – a time when most adults gained weight in all countries.”
“These results could support the idea that the obesity epidemic is due to excessive dietary energy intake rather than a decrease in energy expenditure.”
Soren Brage of Cambridge University, also in the UK, says total energy use is “notoriously difficult to measure”.
“We urgently need to focus our attention not only on the global energy crisis defined by the burning of fossil fuels.