High-protein diets are everywhere, but new research suggests eating less may slow ageing. Here’s what scientists found

Protein has taken over the supermarket. It is added to cereal, crisps, yoghurt, even coffee, sold on the promise that more is better. A large new review of the science suggests the opposite may be true for some people, at least in the lab.

“We have this impression that dietary protein is sort of a universally good thing,” says Dudley Lamming at the University of Wisconsin-Madison. His team’s conclusion cuts against that. For sedentary people and animals, he says, “increasing dietary protein, beyond what’s needed, leads to metabolic problems and can lead to shorter lifespan as well.”

That is a striking claim, so it is worth being clear about what the evidence does and does not show.

Does eating less protein slow ageing?

Lamming and his colleagues went through more than 300 studies, most of them in rodents that were not physically active. The studies compared animals eating close to the minimum protein needed to avoid deficiency against animals eating roughly double that.

The minimum in these experiments tracks the US guideline of 0.8 grams of protein per kilogram of body weight. Double is close to the 1.2 to 1.6 grams per kilogram that recent US dietary advice recommends for better health, and close to what many high-protein diets aim for.

The gap in outcomes was large. In one study, rats kept near the minimum lived almost 120 days longer than those eating about twice as much, an increase of more than 50 per cent in lifespan.

Why would less protein help you live longer?

Two biological mechanisms show up repeatedly in the animal work.

The first is a hormone called FGF21. Restricting protein raises FGF21 levels, and in rodents that hormone acts on the brain, fat tissue and liver to steady blood sugar and clear out senescent cells. Senescent cells build up as animals age, stop dividing and drive the chronic, low-level inflammation linked to age-related disease.

The second is a protein called mTORC1. It normally tells cells to grow, but eating less protein turns its activity down. “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,” Lamming says. In mice, dialling mTORC1 down has been tied to fewer age-related conditions and a longer life.

Should you cut your protein intake?

Not so fast, and Lamming himself draws the line carefully. The concern is about eating more protein than the body needs while sitting still most of the day. People who exercise regularly are a different case, because working muscle needs protein to repair, so the review is not an argument for everyone to eat less.

It is also, on its own terms, a body of evidence built largely on inactive rodents. That is the single biggest reason to treat it as a lead, not a verdict.

Do animal studies apply to humans?

This is where the researchers split, and the disagreement is the honest heart of the story.

Donald Layman at the University of Illinois Urbana-Champaign argues that rodents and people handle protein too differently to map one onto the other. mTORC1 responds to eating, and humans tend to eat a few large meals while rodents graze through the day, so protein restriction may hit those pathways in different ways. People also face ageing pressures that lab animals are shielded from, such as infections and extreme heat.

Layman makes a second point about what the review left out. Most human studies suggesting that extra protein lowers the risk of frailty were not part of it. Lamming says nothing was dropped on purpose: “We were focused on protein restriction, not the many studies looking at higher levels of protein intake.” Beyond building muscle and preventing frailty, he adds, the evidence that more protein helps with ageing is mixed.

What about protein and muscle loss in older age?

The case for higher protein rests partly on muscle. One study of older twins in the UK linked greater protein intake to a lower risk of sarcopenia, the age-related loss of muscle. But Elizabeth Williams at the University of Sheffield cautions that sarcopenia was so rare in that group that the link is unreliable.

There may be a more precise target than protein overall. Philip Atherton at the University of Nottingham notes that in mice, cutting specific amino acids, isoleucine and valine, both highest in meat, fish and dairy, produces effects similar to restricting protein as a whole. He is planning a study to test whether drugs that deplete particular amino acids can improve markers such as blood sugar control in people, with results expected within a few years.

The takeaway

For now, the picture is unsettled. Animal studies point fairly consistently toward a link between eating less protein and slower ageing, driven by FGF21 and mTORC1, while human evidence is thinner and pulls in both directions. The safest reading is that very active people likely benefit from ample protein, while sedentary people loading up well beyond their needs may not be doing themselves the favour the marketing implies. As always with a changing diet, that is a conversation for you and a doctor or dietitian, not a supermarket label.