If you’re past a certain age, there’s a strong chance you have either thought it, or heard someone say it.
It may have happened when you felt particularly tired and achy, or more likely if you have put on a few unwelcome pounds from your 30s onwards.
You’ll know the seven words, or a variation of them: “It’s because my metabolism is slowing down.”
First things first, you are not alone – it’s certainly something I’ve uttered in response to the latest reading on my scales, without thinking twice about what it meant. No doubt I thought those changes indicated only one thing: the slow grind towards my eventual physical breakdown and inevitable death had begun.
But, while it’s true that your metabolism does indeed slow down as you age, this happens much later than you might expect – and it turns out we have much more control over our metabolism than you would think.
The four ages of metabolism
It’s understandable that we blame our metabolism for the problems that tend to emerge as we get older.
Your metabolism is, in short, the complex set of chemical processes your body uses to convert food into energy – powering everything from temperature control to tissue repair. When it slows, the effects can be significant: more body fat, less muscle, slower recovery, a greater sensitivity to cold.
But, when our bodies change in middle age, it turns out metabolism is largely innocent. The real culprit, scientists say, is us.
A 2021 study published in the journal Science assessed more than 6,400 people from 29 countries, aged between eight days and 95 years old. It looked at how much energy people expended in their daily lives, making adjustments for differences in people’s body weight.
Scientists asked participants to consume ‘doubly labelled water’, made up of heavier oxygen and hydrogen atoms than regular old H2O, which can be tracked as they leave the body.
The study concluded there were four main metabolic stages in our lives.
In the first year of life, the study found, metabolism is almost in overdrive: infants up to the age of one were found to expend energy at a rate 50 per cent higher than adults. That’s partly due to the vast amount of energy required to fuel infants’ growing brains.
From then, up to our early 20s, metabolism was found to slow down by about three per cent a year. So far, so normal. But then, intriguingly, the study found that energy expenditure remained fairly constant from our 20s all the way through to our 60s – even during pregnancy.
Then, from our 60s onwards, it declined at a slightly higher rate, until – in our 90s – energy expenditure is 26 per cent lower than it is in middle age.
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‘It’s not my fault, it’s my biology working against me’
If our metabolism doesn’t slow until our 60s, then why do so many of us believe the myth of earlier metabolic decline?
“I suppose it’s something that is in people’s psyche, isn’t it? This idea that you lose a certain amount of energy per decade, or you’ve got this gradual decline from the peak of your early 20s, when everything starts to slow down,” says Dr Adam Collins, an associate professor of nutrition at the University of Surrey.
“It’s quite nice to think of something as physiologically driven, because it removes you from any responsibility: ‘It’s not my fault, it’s my biology working against me.’ But when you age, other things will happen across that life course which will make a difference.”
Collins conjures someone in their 20s who is single, goes to the gym and watches their diet without overindulging in alcohol too often.
“You are a bit bulletproof in your 20s,” he says. “But then you hook up with someone, and you start to settle down in your 30s, maybe in your 40s, and that lifestyle changes.
“You become less physically active, less conscious of your lifestyle. You become more sedentary, you may indulge a bit more regularly, you might have children, and then work also comes into play a bit more.”
All of this goes to say: the biggest driver of change is not your metabolism, but your lifestyle – or more specifically, how that lifestyle affects your body.
“Our metabolism, what is commonly referred to as our resting metabolic rate, is largely determined by skeletal muscle mass,” says David Broom, a professor of health and physical activity at York St John University.
The more muscle mass we have, the better we are at burning fuel, and the more we are physically capable of doing.
“As a person ages naturally, we do potentially see some skeletal muscle mass loss,” Broom says, adding that this can be down to a surfeit of factors, from reduced physical activity to smaller appetites, which lead to less protein in the diet. “And therefore, as expected, our metabolism would slow down.”
What this means is that the physical changes that feel like the inevitable passage of time – and might easily be blamed on our metabolism – are often still in our control.
“Your metabolism will slow down if you don’t do the things to try and counteract those changes, to increase and maintain your skeletal muscle mass,” Broom says.
“Being quite blunt about it, and there are exceptions, if you find yourself putting on body mass, that is typically because you are overeating or you are not being active enough.”
A good suggestion, Broom says, is to follow the UK government’s guidelines for physical activity in adults.
People over the age of 19 are recommended to undertake at least 150 minutes of activity of moderate intensity per week (involving increased breathing in which you are able to talk) or at least 75 minutes of vigorous intensity activity (involving breathing quickly, with difficulty talking), as well as strength building two days a week, such as by doing yoga or carrying heavy bags.
Broom says: “Even in old age, it’s really important not to slow down, even to actually speed up, to keep active, to maintain your metabolism, effectively.”
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