How often have you heard those around you complain that they “can’t eat like they used to” without gaining weight? It could be their metabolism at play...
Perhaps you’re noticing your own energy take a dip as you get older, or finding that your appetite is different from when you were young. That’s because, just like our joints, skin and hair, our metabolism can change as we age.
“Our metabolism is the name for the chemical reactions that take place within each cell as the body breaks down food and converts it into energy,” explains registered associate nutritionist Eli Brecher.
Our metabolisms are often described as fast or slow. “This refers to your metabolic rate, which is the speed at which your body burns calories to meet its basic needs and keep your body alive,” explains Brecher. A slow metabolism means your body doesn’t require much energy to keep your organs pumping, brain thinking and breathing consistent.
Whether or not our metabolic rate changes as we age has long been contested. Some studies have suggested our metabolic rate begins to slow when we are just teenagers, but the most recent and robust piece of research on metabolism ageing, published in the Science journal in 2021, found that metabolism reaches its peak earlier than thought, and slows down much later.
The researchers looked at the metabolisms of 6,421 participants aged between eight days old and 95 years old from 29 countries to find our metabolism peaks at around eight and a half months old. Babies aged nine to 15 months even expended 50% more energy compared to adults when adjusting for size differences. This is because growing and developing are energy-expending processes.
“The study shows that after the age of one, our metabolism then slowly decreases by around 3% a year until we reach the age of 20 when it plateaus until around the age of 60. Only then does it start to decline again, by up to 1% per year,” says Brecher.
Given that the study proved we have pretty much the same metabolism throughout our 20s, 30s, 40s and 50s, it throws out the theory of middle-aged metabolic decline being to blame for weight gain and energy changes. But Jessica Stansfield, Junior CX Nutrition Manager at Huel, notes that changes in metabolism are very individual. “The age at which our metabolism slows down can depend on factors such as body composition and diet,” she says.
Our metabolic rate can change for three main reasons, according to Brecher. “First is a loss of muscle mass, which naturally occurs with age. Secondly, hormonal changes and gender can determine our metabolism, and finally, your genes also impact how our body processes food and energy,” explains Brecher.
Let’s start with muscle loss. Studies, like a 2018 paper from Ageing Research Reviews, note that we can lose around 1% of muscle mass a year from middle age, “and in severe instances can lead to a loss of around 50% by the 8–9th decade of life”. “Maintaining muscle burns more calories than fat, so our metabolic rate will slow as we lose muscle,” says Brecher.
However, one unavoidable change for women is menopause, when the changes in sex hormone levels alter our metabolism. A 2022 paper published in The Lancet reported that post-menopausal women had poorer metabolic responses after eating, resulting in raised blood glucose and insulin resistance compared to pre-menopausal women.
The study also found that women who took hormone replacement therapy after menopause had better blood sugar and blood fat responses to food, suggesting that declining levels of oestrogen could be to blame for the slowing metabolism in those who are naturally menopausal.
Men’s sex hormones also decline with age, particularly testosterone. A study from the University of Sheffield reported that “there is strong evidence that low testosterone levels [...] have a high prevalence in men with metabolic syndrome and/or type 2 diabetes”.
But your metabolism isn’t the only thing that can cause changes to weight and energy levels. Our daily energy expenditure, or the amount of calories we burn, is determined by your metabolic rate plus any form of movement we do, like walking, cleaning, cooking and working out.
According to a report from 2020, physical activity levels decrease by 40-80% as we age. Keeping up physical activity is harder as we get older, especially if we have busy lives and start to develop aches and pains, but keeping moving can support a higher daily expenditure that can keep weight steady.
As you can tell, maintaining your metabolism isn’t just about how you look and feel, but it can also be an important part of staying healthy into older age. ‘Metabolic syndrome’ is even the name given to diabetes, high blood pressure and obesity - conditions which are caused by changes to how we process food and burn energy.
With that in mind, there has been lots of buzz around the concept of the ‘metabolic age’. “Metabolic age looks at an individual's metabolic rate in comparison to the average for their age group. The idea is that the lower the calculated metabolic age, the better the metabolism,” says Stansfield.
In research, metabolic age is calculated using software that takes into account body composition, waist circumference and resting blood. You can find online calculators that use equations including the Katch-Mcardle calculation used to determine metabolic rate. But should you?
Some research has found that a lower metabolic age is linked with lower blood pressure and lower body fat. However, a paper from Current Developments in Nutrition reported that “much more research is needed to confirm the formula and concept of metabolic age”.
“Your metabolic age might offer some general insight in terms of a comparison point for others of your chronological age, but there haven't been many peer-reviewed studies done on metabolic age, so it’s not considered an accurate point of data,” says Brecher.
While everyone will eventually face a slightly slowing metabolism, we can prevent sharp declines by maintaining the right diet and exercise as we age, says Stansfield.
Brecher agrees, saying that “building and maintaining muscle mass through strength training (ideally lifting weights) can help prevent muscle decline as we age and increase metabolism.” Despite the common thinking that hypertrophy is much more difficult when you’re older, a 2019 paper from Frontiers in Physiology found that people who are both new and experienced at resistance training can continue gaining muscle mass well into their 70s to ward off metabolic declines. And remember, the more active you are, the higher your energy requirements.
“Consuming plenty of good quality protein may also be helpful,” adds Brecher. “Your body burns more energy digesting protein than digesting carbohydrates or fat, so a diet that includes lots of protein can help increase your metabolic rate as well as support muscle mass.”
Sleep is also crucial. A 2022 study of sleep habits and metabolic function in older adults found that sleep restriction affects our metabolisms and hormonal function, which then affects the quality of life and health of older adults in the long term.
The researchers recommended that older adults stick with an early-to-bed and early-to-rise pattern, ideally sleeping before 10 pm and no later than 6 am, while also ensuring a good sleep quality to protect elements of metabolism like glycemic control and blood pressure.
Finally, research is starting to point out the role of our gut health in keeping us young. “The gut microbiome affects how you process the food that you eat and certain gut microbes have been linked with better metabolic control,” explains Brecher. Eating a variety of plants, including probiotics and prebiotics, is the best way to support a happy gut - and potentially your metabolism.
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