Protein-rich products – from cereal to crisps to coffee – are all over supermarket shelves, but mounting evidence in non-human animals suggests that restricting our intake could slow ageing and extend lifespan. So, does that mean eating too much protein is harmful?

“We have this impression that dietary protein is sort of a universally good thing,” says at the University of Wisconsin-Madison. But according to the latest review of the evidence, “increasing dietary protein, beyond what’s needed, for sedentary individuals or animals leads to metabolic problems and can lead to shorter lifespan as well”, he says.

But others point out that nutritional studies in animals like mice and rats don’t readily translate to people, and a high protein intake has been linked to benefits in humans.

Lamming and his colleagues sifted through more than 300 studies, mostly in sedentary rodents, that explored how restricting protein intake to the minimum required to prevent deficiency affects ageing and lifespan.

In these studies, rodents generally either ate the equivalent of the minimum protein requirement recommended by US guidelines – 0.8 grams of protein per kilogram of body weight – or about double that. The latter is similar to the intake recently recommended by US dietary guidelines for optimal health, 1.2 to 1.6 grams of protein per kilogram of body weight.

In one study, rats that ate around the minimum protein intake lived nearly 120 days, or more than 50 per cent, longer than those that ate about double that.

This may be because protein restriction increases levels of the hormone FGF21, says Lamming. In rodents, this acts on the brain, fat tissue and the liver to improve blood sugar levels and reduce the number of senescent cells. These cells, which accumulate with age, have stopped dividing and cause chronic inflammation.

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Restricting protein intake has also been shown to dampen the activity of a protein called mTORC1 in mice. mTORC1 ordinarily stimulates cell growth, but reducing its activity boosts cellular repair. “It essentially switches cells from a proliferative state, where they’re growing and engaging a lot of metabolism, to one where they’re dividing more slowly, if at all, and focusing on repair,” says Lamming. Studies suggest this in mice, he says.

All this suggests that the recent trend to up our protein intake is probably fine among people who exercise regularly, because they may need more protein to repair muscle damage, but eating more than is needed could cause harm in sedentary individuals, says Lamming.

But at the University of Illinois Urbana-Champaign points out that rodents vary hugely from people when it comes to protein metabolism. For instance, mTORC1 is activated by feeding, and while people tend to a have a few big meals a day, rodents graze more regularly. Protein restriction may therefore affect mTORC1 pathways differently between the different species, he says. We are also more exposed to factors that accelerate ageing, like infections and extreme heat, than lab animals, says Layman.

What’s more, the review does not include most of the studies in people that suggest that upping our protein intake beyond the minimum , he says.

Dudley says no studies were intentionally omitted. “We were focused on protein restriction, not the many studies looking at higher levels of protein intake,” he says. Beyond building muscle and preventing frailty, there’s mixed evidence on whether a higher protein intake helps with ageing, says Dudley.

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For instance, one study linked this to a reduced risk of age-related muscle loss, or sarcopenia, among . But the rates of sarcopenia were so low that the link is unreliable, says at the University of Sheffield in the UK.

Still, some studies showed that restricting intake of certain amino acids – such as isoleucine and valine, which are highest in meat, fish and dairy – in mice has a similar effect to limiting overall protein intake, says at Nottingham University, UK.

Atherton says he is planning a study to investigate whether drugs that deplete specific amino acids in people improve outcomes like blood sugar control, with results expected in the next few years.  

Chidren are an impressionable lot. In our youth, we are like sponges, picking up habits and neural pathways that will resonate for the rest of our lives. It is no wonder, then, that countries including the UK, Portugal and Mexico have enacted policies to restrict the advertising of unhealthy food to children, in a bid to tackle soaring childhood obesity levels. But if this makes you think that adults are off the hook, then think again.

As we explore, eating appropriately for your age matters no matter how old you are. If you are middle-aged, you may have noticed the warning signs: a horrible hangover after drinking an amount of alcohol that would have left you unaffected decades earlier, or getting increasingly jittery after drinking coffee. These are indicators of more global changes occurring in the body, making us less and less able to tolerate a subpar diet. Breaking bad diet habits – a glass of wine after the kids have gone to bed, or a sneaky croissant from a coffee shop because you skipped breakfast – as we age is just as important as instilling healthy eating in children.

Breaking bad diet habits as we age is as important as instilling healthy eating in children

What’s more, once we reach the age of 60, it becomes ever easier to fall into a “nutrient gap” – when a slowdown in our metabolic rate means that we eat less, but age-related physiological changes mean that we actually need more vitamins and minerals from every bite of food.

That’s the bad news. Recognising these changes in our metabolism and responding accordingly – you know the drill: eating more vegetables, legumes, whole grains and fish, and less ultra-processed food high in sugar, salt and saturated fat – can help set you up for a healthy old age.

But don’t wait until you are already in your latter decades to take action. A number of studies show that, if we tune up our diet in middle age, this can have a big impact on our health and wellbeing years down the line, while a younger biological age also helps you reap the benefits of a good diet. Ditch the junk and your older self will thank you.