Researchers at USC, Harvard and the University of Toronto reported in a June 2026 Cell Metabolism study that the longevity diet, a low-protein, mostly plant and fish eating pattern developed in the lab of gerontologist Valter Longo, worked far better in aging mice when it was supplemented with methionine.
Methionine is an essential amino acid the body uses to build proteins, and a low-protein diet naturally reduces the supply.
In 20-month-old mice, the low-protein, methionine-supplemented version of the diet extended healthspan, cut fat mass and reduced frailty, and produced better cardiometabolic markers including higher levels of the appetite and blood sugar hormone GLP-1.
The same paper also analyzed dietary and health data from more than 200,000 people. That human data supported the plant-focused pattern broadly, but it did not test methionine supplementation in people. Clinical trials in humans are the next planned step.
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What the mice ate
The team, led by senior author Valter Longo of the USC Leonard Davis School of Gerontology, compared four feeding patterns in older mice: a standard diet, a Western high-fat and high-sugar diet, a ketogenic diet, and the low-protein diet topped up with methionine, which the researchers labeled LDMM.
Mice on the LDMM diet ended up leaner and healthier without eating less. Longo said the animals on the supplemented diet "could eat more food than any other group and as much calories as any other group and yet lose fat without losing lean body mass."
The dose mattered a lot. According to Longo, "Too little methionine caused frailty, but too much methionine abolished the benefits." In other words, the benefits appear in a narrow window where the amino acid is present but restricted.
The GLP-1 connection
Alongside the changes in body composition, the mice on the supplemented diet showed shifts in several metabolic hormones, including GLP-1. GLP-1 is the same gut hormone class targeted by drugs such as semaglutide, the active ingredient in Novo Nordisk's Ozempic and Wegovy, which help regulate appetite and blood sugar.
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Maura Fanti, the study's first author and a research associate in Longo's lab, said in the university's release that "amino acid composition, not just overall protein quantity, may be the target of strategic metabolic interventions." The finding suggests that the type of protein on your plate, not only how much of it there is, may shape how the body ages.
What the human data does and does not show
The human portion of the paper was observational, drawn from more than 200,000 people. Those consuming the highest levels of animal protein showed a higher prevalence of obesity and had twice the rate of diabetes compared with people eating little or no animal protein. That is an association, not proof that switching diets will produce the same effect in a given individual.
Crucially, none of the human data directly tests methionine supplementation. It supports the broader plant and fish pattern the longevity diet is built on, but the specific amino acid tweak that drove the mouse results is still an animal finding. Anyone considering major changes to protein intake, particularly older adults or people on medication, should talk with their doctor first.
Longo holds equity in L-Nutra, a medical foods company that has licensed related USC intellectual property, a tie the university disclosed with the study. The next step, the researchers said, is controlled clinical trials in people.
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