Both too much and too little methionine may undermine healthy aging. The findings point toward a nutritional “sweet spot” that researchers now hope to test in people. Credit: Stock

A new study suggests the secret to healthier aging may not be eating less but eating the right balance of protein and amino acids.

Could healthier aging depend less on cutting calories and more on getting the right balance of protein and amino acids?

A new USC study, published in Cell Metabolism, suggests that possibility. Researchers found that older mice fed a mostly plant-based, low-protein diet with fish and a carefully measured amount of methionine lived healthier for longer, became less frail, and lost body fat without losing lean mass.

Methionine is an essential amino acid, meaning the body needs it but cannot make it on its own. It is found in foods such as eggs, meat, dairy, fish, legumes, nuts, and seeds. Too little can be harmful, but this study suggests that too much may also erase some of the benefits of a longevity-style diet.

The diet tested in the study was inspired by eating patterns linked to long-lived populations, including traditional Mediterranean and Okinawan diets. These diets tend to emphasize plants, modest protein intake, and limited animal foods, but they may also leave some people vulnerable to frailty if essential amino acids fall too low.

The Longevity Diet Behind the Research

Valter Longo of the USC Leonard Davis School of Gerontology and his colleagues tested a low-protein, methionine-supplemented longevity diet (LDMM) in 20-month-old mice. The animals were compared with mice fed a standard diet, a Western diet high in fats and sugars, or a low-carbohydrate ketogenic diet.

Mice on the LDMM had a longer healthspan (the portion of life spent in good health), lower fat mass, and less frailty. Even more surprising, they could eat as much as, or more than, mice in the other groups while still losing fat and preserving lean body mass.

“We expected different diets to produce different outcomes, but what really impressed us was how modulating just a single amino acid, methionine, in the longevity diet could produce such dramatic metabolic changes,” said USC Leonard Davis Research Associate Maura Fanti, first author of the new study. “It points to the idea that amino acid composition, not just overall protein quantity, may be the target of strategic metabolic interventions.”

How Methionine Changed Metabolism in Mice

The mice also showed signs of improved metabolic health, including higher levels of GLP-1 and FGF21, signaling molecules involved in appetite, blood sugar regulation, metabolism, and aging. GLP-1 is especially notable because it is the same biological pathway targeted by several widely discussed weight loss and diabetes drugs, although the study did not test those medications.

“There are, of course, differences in how these pathways are regulated between mice and humans, but seeing such coordinated changes across multiple metabolic hormones is genuinely encouraging, and we’re very curious to know whether effects of similar magnitude would be seen in human studies,” she said.

Human Study Links Animal Protein to Health Risks

The human data pointed in a similar direction. The researchers analyzed dietary and health information from more than 200,000 people and found that those who ate the most animal protein, and therefore higher levels of methionine and other essential amino acids, had more obesity and twice the diabetes rate of people who ate little or no animal protein.

That pattern remained even though the high animal protein group reported eating fewer calories and otherwise healthier diets, Longo said. The finding challenges the familiar idea that weight loss and metabolic health are mainly about calorie reduction.

“This challenges the dogma that calorie reduction is necessary to lose weight, but it also tells us that we need to have clear understanding of the mechanisms,” he said. “Too little methionine caused frailty, but too much methionine abolished the benefits of this diet, which was otherwise based on the diet of long-lived populations like the traditional Italian and Okinawa diets. … These results indicate that overall protein intake may be less important than specific amino acid intake.”

What the Findings Mean for People

For now, the findings are not a prescription for people to change their diets or take amino acid supplements. Mouse studies do not always translate to humans, and the human data were observational, meaning they can show associations but cannot prove cause and effect.

The next step the team hopes to pursue is a controlled clinical trial of the LDMM in humans, Longo and Fanti noted.

Reference: “Methionine-supplemented longevity diet increases growth hormone, GLP-1, and FGF21; reduces frailty; and promotes healthspan” by Maura Fanti, Sebastian Brandhorst, Gerardo Navarrete, Arnold Diaz, Giacomo Giuliani, Dolly Chowdhury, Gabriel C. Antunes, Todd E. Morgan, Louis Dubeau, Valentina Villani, Laura Perin, Vasanti S. Malik, Frank B. Hu and Valter D. Longo, 23 June 2026, Cell Metabolism.
DOI: 10.1016/j.cmet.2026.05.015

The study was funded by National Institute on Aging grant AG084485, National Institute of Health grant GR1045540, and the USC Edna Jones Chair Fund.

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