Americans are increasingly urged to consume more protein than ever before, prompting food companies to add it to everything from cereals and coffee to bottled water. However, an analysis of more than 350 studies suggests that for most people, reducing protein intake may be more beneficial than increasing it. The findings were published in Cell Press Blue.
Health authorities have recently raised the target protein intake to 0.5–0.7 grams per 0.5 kg of body weight — a notable increase from the previous benchmark of 0.4 grams. Food manufacturers have followed this trend, boosting protein content across a growing range of products.
In 2024, 61 % of US consumers reported eating more protein, up from 48 % five years earlier.
Bailey Knopf and Dudley Lamming reviewed over 350 papers on protein restriction and ageing. Most of these studies were conducted in animals rather than humans, with flies, worms, and rodents showing the most pronounced life‑extension effects. In these species, low‑protein diets often improve metabolism and increase lifespan.
Reduced calorie intake tends to extend lifespan across many species and also lowers the risk of age‑related diseases, including some cancers. The challenge is that most people struggle to maintain a low‑calorie diet. Low‑protein diets can deliver some of the same benefits even without cutting total calories.
“It’s clear that protein has beneficial effects on muscle growth and the body’s response to exercise in active individuals,” said Dudley Lamming of the University of Wisconsin–Madison. “But because most people lead relatively sedentary lives, many are likely consuming more protein than they truly need — and that probably has negative health consequences at the population level.”
Short‑term human studies show similar patterns: individuals who reduced their protein intake lost weight and decreased body fat, while also improving blood sugar control — even when their overall calorie intake increased.
A key hormone helps explain this effect: FGF21, whose levels rise when protein intake drops. FGF21 boosts energy expenditure, improves blood sugar regulation, and reduces inflammation. Mice engineered to produce extra FGF21 lived longer — about 30 % longer for males and 40 % for females.
“These studies indicate that the amount of protein consumed today by sedentary populations may have negative health implications, at least at the population level,” Lamming noted.
Low‑protein diets don’t just aid weight loss; they also change how cells respond to nutrients. Lower protein levels reduce the activity of the body’s main growth sensor, which stays highly active on a protein‑rich diet. This then allows cells to carry out their own cleanup processes, removing damaged components. This cellular maintenance helps tissues stay functional longer and reduces the buildup of worn‑out cells that drive inflammation.
Positive effects can appear even when the diet starts later in life: older mice switched to lower‑protein diets showed improvements as well.
Protein is made up of amino acids, and certain ones appear to drive these effects. Methionine, isoleucine, and valine stand out. In large amounts, these amino acids activate growth signals linked to obesity, inflammation, and other age‑related conditions.
Restricting methionine alone extends rodent lifespan. This amino acid is abundant in meat, eggs, and fish. Isoleucine and valine are branched‑chain amino acids, commonly taken as supplements. Reducing any of these amino acids improved metabolism in mice, and both extended male lifespan.
The source of protein may also matter. Plant and animal proteins appear to have different impacts: plant protein is associated with a lower risk of frailty in old age, whereas no such link was found for animal protein.
The Okinawan population offers a long‑studied example. Their traditional diet is about 9 % protein, largely from plant sources, and they are well known for healthy ageing. They also tend to consume fewer calories overall, so low protein intake is just one part of the picture.
The situation is nuanced. Some research shows that higher protein intake combined with exercise helps older adults preserve muscle mass — partly explaining the push for increased recommendations. Age‑related muscle loss is a serious issue, and protein clearly helps active people rebuild muscle.
Athletes present a particular puzzle: they consume large amounts of protein and rarely suffer from metabolic disorders. Lamming suggests that exercise protects them by using the protein to build strong, healthy muscles. Regular physical activity may offset the potential harm a high‑protein diet could cause in people with little movement.
“Recent guidelines encouraged people to eat more protein, but they also urged them to exercise more,” Lamming said. “We probably need to personalise protein recommendations, taking into account not just age but also people’s activity levels.”
Some groups genuinely need more protein, not less. These include pregnant women, growing children, some older adults, and people recovering from injuries. For them, low‑protein diets could be counterproductive, as their bodies are building or repairing tissue — a process that requires protein.
It remains unknown how reducing protein intake affects human lifespan. Most longevity data still come from animal studies. This work is a review of prior findings, not a new experiment, and highlights areas researchers should test in humans going forward. Many older adults already consume too little protein due to poor appetite or limited budgets; for them, further reductions could cause real harm.
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