Slowing metabolism cannot be blamed for middle-age spread, research concludes

The research found babies have the highest metabolic rate, at 50 percent higher than the average adult
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Metabolic rate does not slow in middle age and is not impacted by pregnancy or menopause, a global study has found.

The research, involving 6,400 people aged between eight-days-old and 95, found the metabolism remains “rock solid” from the age of 20 to 60.

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People across 24 countries took part in the extensive study, which found the body’s ability to rebuild and repair drops significantly in the post-60 age group.

Research findings

The study, published in the journal Science, found four phases of metabolic life:

Researchers have concluded that metabolism, the overarching name used to describe the chemical reactions which keep us alive and revitalise our cells, has four stages throughout life:

  • It works at its highest rate from birth until around one year old. This means babies have the highest energy expenditure in comparison to their overall weight. 
  • Expenditure relative body weight then declines slowly as we reach our twenties. 
  • It then levels out to remain consistent until we are around 60 years old. 
  • Then, a yearly decline begins until the metabolism is working at 26 percent lower at age 90 than it was thirty years previous. 

Middle-age weight gain

Scientists determined that regardless of how body mass is made up, be it lean muscle tissue or predominantly body fat, the bigger the body the more energy it needs to sustain its current condition.

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So the researchers tweaked their measurements, adjusting for body size, to compare people’s metabolism "pound for pound".

"It is a picture we’ve never really seen before and there is a lot of surprises in it," one of the researchers, Prof John Speakman, from the University of Aberdeen, said.

"The most surprising thing for me is there is no change throughout adulthood - if you are experiencing mid-life spread you can no longer blame it on a declining metabolic rate."

Prof Tom Sanders, from King’s College London, said: "Interestingly, they found very little differences in total energy expenditure between early adult life and middle age - a time when most adults in developed countries put on weight.

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"These findings would support the view that the obesity epidemic is fuelled by excess food energy intake and not a decline in energy expenditure."

No increase during puberty

Contrary to popular belief, the metabolism does not peak during puberty either. This means the increased appetite and lack of weight gain during our teenage years is most likely due to increased activity levels.

The research also suggests that pregnancy does not require large spikes in caloric intake, as the metabolism does not rise while carrying a child. A slight increase in calories, roughly between 200 and 500 more than a women’s average of 2000, may still be needed to feed the growing foetus but this is not related to the mother experiencing a heightened metabolism.

"When people talk about metabolism, they think diet and exercise - but it is deeper than that, we are actually watching your body, your cells, at work," Prof Herman Pontzer, from Duke University, told BBC News.

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He stressed the importance of nutrition in babies and early childhood, as malnutrition can have profound effects on development.

"They are incredibly busy at one year old and when we see declines with age, we are seeing your cells stop working."

How the research was conducted

People’s metabolism was measured using doubly labelled water.

Made from heavier forms of the hydrogen and oxygen atoms that make up water, this can be tracked as it leaves the body.

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The research could go some way in determining how medicine is used to fight illness and diseases, including cancer.

Prof Pontzer claimed the advances in understanding the metabolism could reveal whether cancer rates change as the body’s metabolism slows and this could change how drugs are administered at different ages.

Drs Rozalyn Anderson and Timothy Rhoads, from the University of Wisconsin, said the "unprecedented" study had already led to "important new insights into human metabolism".

And it "cannot be a coincidence" diseases of old age kicked in as the metabolism fell.

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