The arrival of middle age cannot be attributed to a decline in metabolism, according to an unprecedented study of the body’s energy use.
The study, of 6,400 people, ages 8 to 95, conducted in 29 countries, suggests that metabolism remains “rock solid” during middle age.
It peaks at one year of age, is stable from 20 to 60, and then inevitably declines.
Researchers say the results bring new and surprising discoveries about the body.
Metabolism is every drop of chemical needed to keep our bodies working. And the bigger the body—whether in terms of developed muscles or lots of abdominal fat—the more energy it takes to move it.
So the researchers adjusted their measurements, according to body size, to compare people’s metabolism “pound for pound.”
The study, published in the journal Science, found four phases of metabolic life:
– From birth to 1 year, when the metabolism leaves the same level as the mother and reaches the highest point of a lifetime, 50% above the adult population
– A mild deceleration occurs until age 20 years, with no increase during all puberty changes
– No change from 20 to 60 years old
– A permanent decline, with annual drops that, around the age of 90, leave metabolism 26% below middle age
“It’s a picture we’ve never seen before and there are 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 during adulthood—if you are experiencing a midlife crisis, you can no longer blame the decline in metabolic rate.”
Other surprises also came from what the study did not find.
There was no metabolic increase during puberty or pregnancy and no deceleration around menopause.
High metabolism in the early years of life also emphasizes how important this moment is in human development and why childhood malnutrition can have lifelong consequences.
“When people talk about metabolism, they think about diet and exercise — but it goes deeper than that. We’re actually watching your body, your cells, in action,” says Duke University Professor Herman Pontzer. to BBC News.
“They’re incredibly busy at one year of age, and when we see declines with age, we’re seeing their cells stop functioning.”
People’s metabolism was measured using so-called double-labeled water.
Made from the heavier forms of the hydrogen and oxygen atoms that make up water, it can be tracked as it leaves the body.
But double-labeled water is incredibly expensive, so it took researchers working together in 29 countries to collect data on 6,400 people.
Researchers say that a complete understanding of changes in metabolism can have impacts on medicine.
Pontzer says this could help reveal whether cancers spread differently as metabolism changes and whether drug doses need to be adjusted during different phases.
It is also 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 brought “important new discoveries about human metabolism.”
And that “it can’t be a coincidence” that old-age diseases arise at the time when metabolism declines.
Professor Tom Sanders of King’s College London in the UK says: “Interestingly, few differences were found in total energy expenditure between early adulthood and middle age — a period when most adults across countries developed gains weight.”
“These findings may support the idea that the obesity epidemic is a result of excess dietary energy intake rather than a decline in energy expenditure.”
Soren Brage of Cambridge University, also in the UK, says the total amount of energy used was “notoriously difficult to measure”.
“We urgently need to turn our attention not only to the global energy crisis defined by the burning of fossil fuels. But also to the energy crisis that is caused by not burning enough calories in our own bodies.”