Protein has become the supermarket’s golden goose. It’s being packed into everything from cereal and chips to coffee, snack bars and even water, usually with the implication that more protein means a healthier product.

But more isn’t automatically better.

A major new review drawing on more than 350 scientific papers argues that the optimal amount of protein may depend heavily on who is eating it. For athletes and many older adults, extra protein can be useful. For relatively sedentary people who already get enough, however, animal studies suggest that consistently high protein intake may keep nutrient-sensitive growth pathways switched on when dialing them back could offer metabolic benefits.

“We have this impression that dietary protein is sort of a universally good thing,” said Dudley Lamming of the University of Wisconsin-Madison according to New Scientist.

Lamming and his colleague reviewed decades of work on protein restriction and aging, spanning more than 350 papers and studies in organisms ranging from microbes to rodents and humans.

A recurring pattern emerged. Across numerous animal experiments, diets containing less protein (while still providing enough to prevent deficiency) improved measures of metabolic health. Animals were often leaner, controlled blood sugar better, and, in some experiments, lived longer.

That finding cuts against the way protein is often sold. Protein is essential, yes; without it, bodies cannot maintain muscle, repair tissue or function properly. But once basic needs are met, does extra protein still help? Or does it depend on who is eating it, how old they are, and whether their muscles are actually being used?

“It’s absolutely crystal clear that there are benefits of protein to muscle growth and exercise response of active individuals,” Lamming said in a University of Wisconsin-Madison news release. “But because most people are relatively sedentary, many people are likely consuming more protein than they actually need, which probably has negative health consequences.”

The biology may come down partly to amino acids, the building blocks from which proteins are made.

The review highlights several in particular, including methionine, isoleucine and valine. All are essential amino acids, meaning humans must obtain them from food.

Protein feeds muscle, but it also sends instructions. A protein-rich meal tells cells that nutrients are available and that the body can afford to grow.

But there could be a trade-off. When nutrient-sensitive growth pathways remain highly active, some processes involved in stress resistance and cellular recycling may receive less emphasis, which could have detrimental effects.

Experiments in animals suggest that reducing some of these amino acids can produce surprisingly large metabolic effects. Lamming’s lab has previously found, for example, that restricting isoleucine can improve metabolic health and extend lifespan in mice.

The animal finding is convincing. But in humans, the story gets much more complicated.

Some human studies show clear benefits from higher protein intake, especially for older adults. More protein can help preserve muscle, improve recovery after exercise, and reduce the risk of frailty and sarcopenia, the age-related loss of muscle.

Most convincing evidence that protein restriction extends lifespan comes from animals, not people.

Human studies can tell researchers whether lower-protein diets change body weight, blood sugar, hormones or other metabolic markers over weeks or months. They cannot yet tell us whether cutting protein makes humans live longer.

Humans and laboratory rodents also eat differently, move differently and metabolize nutrients differently. Even within mice, Lamming’s group has found that the response to protein restriction can vary substantially by sex and genetic background.

Protein source matters too. A serving of lentils, a piece of salmon and an ultra-processed protein bar may contain similar amounts of protein while delivering very different combinations of fiber, fats, micronutrients, sodium and other compounds.

So, the useful takeaway from the new review isn’t that everyone should start restricting protein.

Kristin Kirkpatrick, a dietitian at the Cleveland Clinic Department of Wellness & Preventive Medicine, warned against assuming too much. “I want to make sure that my patients don’t look at this study and just blindly eat less protein but to look at this to balance what type of protein they are consuming and their personalized needs based on age, activity level, chronic disease risk, or existing conditions,” she told Medical News Today.

The science strongly supports getting enough protein. It also supports consuming more under certain circumstances, particularly when preserving or building muscle matters.

What remains far less certain is whether sedentary people who already meet their needs gain anything from continually pushing that number higher.

The protein boom has made “high protein” almost synonymous with “healthy.” Biology, as usual, is considerably less interested in slogans.

The study was published in the journal Cell Press Blue.