The trainer I follow on YouTube ends every session by telling her viewers to prioritize water, rest and protein. Today’s barrage of “more protein” messaging can make eating more of the macronutrient feel like gospel.
Yet the average couch potato may be better off eating less protein, not more, according to a review of more than 350 papers appearing July 31 in Cell Press Blue.
People generally assume that getting too much fat or carbohydrates is bad for health. But with protein, the general thinking is too much doesn’t matter. That may not be true, says aging biologist Dudley Lamming of the University of Wisconsin–Madison.
The issue has to do with where extra amino acids, the building blocks of protein, go, says Cristal Hill, a molecular biologist of aging who was not involved with the study. When someone exercises, the body uses those amino acids to help build muscle, says Hill, of the University of Southern California’s Leonard Davis School of Gerontology in Los Angeles. Without exercise, though, extraneous amino acids accelerate cell turnover. The end result? More molecular and cellular damage that accumulates across one’s lifetime — aka aging.
But research on protein as a nutrient is a paradox. Depending on the study, eating less or more protein helps with weight loss and disease prevention, Hill and colleagues noted in a 2025 Annual Review of Nutrition.
Part of the issue is that nutrition research is messy. For instance, studies following people over time link protein-heavy diets to an increased likelihood of premature death. But is that because too much protein is bad for health or because such individuals were eating unhealthy diets overall?
And, says Lamming, “most of these really rigorous studies showing benefits of high protein diets are done in the context of exercise.” Researchers assume people will move more. Yet most people in the United States struggle to average 20 minutes of exercise per day, the U.S. Centers for Disease Control and Prevention reported last year.
Proponents of high-protein diets push back, pointing to considerable evidence that shows that eating more protein can benefit health by bolstering physical performance and reducing or preventing obesity. That work has largely been conducted in humans; studies suggesting the opposite have been done mainly using other animals.
Do findings in lab rats eating identical chow and living identical, brief lives really translate to humans? “What we need is a handful of high-quality human studies,” says nutritionist Joseph Matthews of the University of Arkansas for Medical Sciences in Little Rock.
Why less protein could be more
The new analysis by Lamming and Wisconsin colleague Bailey Knopf, also an aging biologist, comes in the wake of new U.S. nutrition guidelines that nearly double previous recommendations for daily protein intake to 1.2 to 1.6 grams of protein per kilogram of body weight. Most Americans already eat that much.
Researchers studying protein restriction, meanwhile, tend to define low protein as roughly 0.8 grams of protein per kilogram of body weight, the minimum amount of protein people need to avoid deficiencies that can cause fatigue, difficulties recovering from sickness or injuries or mood changes.
The idea that eating a low-protein diet could benefit health emerged over a decade ago. Multiple teams have demonstrated that mice fed a low-protein diet often eat more than mice on a normal diet, but tend to be thinner and have better fasting blood sugar levels. They also live longer. Scientists like Lamming think this is, in part, because the diet increased the mice’s production of a hormone called FGF21. It helps convert energy-storing white fat cells into beige fat cells that burn off extra energy.
Subsequent research by Lamming’s team and others has shown that restricting certain amino acids, in particular, appears to help govern this process.
The divide between the protein camps has a lot to do with where scientists point their lens, Hill says. Nutritionists tend to look at the whole person. Scientists like herself and Lamming want to drill down into what is happening on a very small scale. At that molecular level, the evidence seems to suggest that protein restriction is better for cells, Hill says.
The protein puzzle is nowhere near solved
No long-term studies of low-protein diets in humans exist yet, Lamming acknowledges. Yet he considers emerging evidence tantalizing. In one study, researchers placed 22 healthy, lean men in their 20s on low-protein diets for five weeks. As with the mouse studies, the men needed to eat more to maintain body weight. Their FGF21 levels also increased, the team reported in March 2025 in Nature Metabolism. Much like the mice, the young men seemed to burn more energy on a low-protein diet than a higher-protein one.
What all this means for dietary recommendations remains a puzzle. Could the weight loss people experience on low-protein diets be due to burning more muscle than fat, for instance? That doesn’t appear to be the case with the healthy young men in this study, the authors note, but it has been cited as an issue in previous studies. Nor do Lamming and most experts recommend a low-protein diet for pregnant women, children, the elderly and certain other groups.
Both high- and low-protein diet camps agree that protein recommendations should be personalized based on individual sex, age, genetics and, of course, exercise levels. What those amounts should be, though, remains an open question.
With more research, we might learn that today’s trendsetters have it backward and that less protein, not more, leads to healthy aging — at least in the sedentary set.
What the review offers, Matthews says, “is sufficient evidence to warrant trialing [low-protein diets] in humans.”
In the meantime, people could just get moving. Exercising a lot could justify eating protein at existing American levels. Dietary guidance typically starts with protein and throws in exercise as an afterthought, says behavioral scientist Chris Macdonald of the University of Cambridge’s Lucy Cavendish College. That’s backward, he says. “We need to start from exercise.”
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