Researchers at the University of Southern California report that a low-protein, mostly plant-and-fish diet extended the healthy portion of life in aging mice. It also cut fat mass and reduced frailty.
The findings were published in Cell Metabolism in June 2026 by a team led by longevity biologist Valter Longo at the USC Leonard Davis School of Gerontology.
The counterintuitive detail is what the mice ate. Animals on the longevity diet actually consumed more food and as many calories as the other groups, yet still lost body fat while keeping lean muscle mass.
The diet is modeled on traditional Mediterranean and Okinawan eating patterns. It is mostly plants and fish, with very little animal protein from meat, eggs and dairy, plus small amounts of the amino acid methionine.
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What the mice showed
The team tested several diets on 20-month-old mice, roughly equivalent to older adults. The version they call LDMM, a low-protein longevity diet with modified methionine, came out on top.
LDMM mice showed longer healthspan, less frailty and better cardiometabolic markers. That included higher levels of GLP-1 and FGF21, hormones involved in blood sugar control and fat metabolism.
Methionine turned out to be a delicate balancing act. "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," Longo said.
First author Maura Fanti, a USC research associate, said the size of the effect from tuning a single amino acid was the surprise.
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"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," she said.
The human signal is only observational
Alongside the animal work, the researchers analyzed dietary and health data from more than 200,000 people.
People eating the most animal protein had higher obesity rates and roughly twice the prevalence of type 2 diabetes compared with those eating little or no animal protein. The pattern held even though the higher-protein group actually took in fewer overall calories.
That is an observational finding, not proof. It cannot show that switching diets would produce the same benefit in a person, only that the eating pattern tracks with better metabolic health at population scale.
What it means, and what it doesn't
The strongest claims here rest on rodents. Pathways such as GLP-1 and IGF-1 signaling do exist in humans, but effect sizes and timing in people cannot be assumed from mice.
The team says the next step is a controlled human clinical trial of the longevity diet. Longo has filed related patents and holds equity in the nutrition company L-Nutra.
"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," Longo said.
For anyone thinking of eating more like the mice, the takeaway is less about a single supplement and more about the overall pattern. A diet built around vegetables, legumes, whole grains and fish, with animal protein kept to a supporting role, already looks a lot like the eating patterns linked to long life in Mediterranean and Okinawan populations.
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Anyone with diabetes, kidney disease or a specific dietary need should check big changes with a doctor first.
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