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This ‘Longevity Diet’ Raised Growth Hormone and GLP-1 in Mice, and Cut Their Body Fat
In A Nutshell
- Aging mice on a low-protein, methionine-supplemented “longevity diet” lost more body fat than mice on four other diets, without losing muscle, even though they ate more than the Western and keto groups.
- The diet raised growth hormone, GLP-1, and FGF21 while lowering IGF-1, and FGF21 turned out to be required for the fat-loss effect in follow-up experiments.
- The methionine dose mattered: too little raised concerns about frailty, and too much erased most of the diet’s benefits.
- A separate analysis of more than 200,000 people found that those eating the most animal protein had roughly double the prevalence of type 2 diabetes, despite otherwise healthier habits.
Losing weight is supposed to come down to one rule: eat less than the body burns. A new mouse study suggests that rule misses something. Older mice on one diet ate more than mice on the Western and keto diets, yet still lost the most body fat without losing muscle. They also beat their peers on strength and balance tests. The difference was not how much they ate, but what.
Published in Cell Metabolism, researchers at the University of Southern California built that menu around the traditional diets of Mediterranean and Okinawan populations: mostly vegetables, legumes, and potatoes cooked in olive oil, with a little fish. Study lead Valter Longo described the plan simply as “largely vegan or vegetarian, but with fish added.” The twist was a small, carefully measured dose of methionine, an amino acid found in meat, eggs, and dairy, added back in after an earlier version made the mice lose weight too quickly, raising concerns about frailty. Too much methionine, on the other hand, erased many of the diet’s benefits.
That same paper adds a real-world echo. An analysis of health records from more than 200,000 American men and women found that people who ate the most animal protein, and therefore the most methionine, had roughly double the prevalence of type 2 diabetes compared with those who ate the least, even though the high-protein group reported healthier habits overall, including less soda and alcohol.
The ‘Longevity Diet’ Cut Fat and Frailty Without Cutting Muscle
Researchers enrolled several hundred mice at 20 months old, about the equivalent of a 60- to 65-year-old person, and split them across five diets for up to six months: standard mouse chow, a Western diet heavy on fries and processed fat, a ketogenic diet built from ostensibly healthy foods like nuts and avocado, a periodic fasting plan, and the new methionine-supplemented longevity diet, which the team calls LDMM.
Mice on LDMM lost more fat than every other group without losing muscle. They also beat nearly every group on grip strength and balance tests, and showed the lowest frailty scores among aged females. Blood tests backed this up: LDMM mice had far better blood sugar control and insulin sensitivity than mice on the standard, Western, or keto diets. The Western and keto groups, by contrast, gained fat and developed higher cholesterol and frailty, even though the keto diet was built from foods often marketed as healthy.
Three Hormones May Help Explain the Longevity Diet’s Effects
Blood and liver samples offered a clue. LDMM mice had higher growth hormone, higher GLP-1 (the same hormone family targeted by drugs like Ozempic), and sharply higher FGF21, a liver hormone linked to fat burning, alongside lower IGF-1, a growth-promoting hormone tied to aging in other studies. To test whether FGF21 was doing the actual work, researchers ran the diet on mice bred without it. Those mice barely lost any weight and developed signs of insulin resistance, confirming FGF21 as a necessary piece of the puzzle. A separate experiment in mice missing growth hormone receptors suggested that growth hormone signaling helps push the liver to make more FGF21. Fanti said the results point beyond simple protein counting: “It points to the idea that amino acid composition, not just overall protein quantity, may be the target of strategic metabolic interventions.”
When researchers restored methionine to normal levels, the fat-loss effect, several hormone changes, and blood sugar improvements reversed, pointing straight at methionine dose as the switch controlling much of this system. GLP-1 was the exception, staying elevated even after methionine was restored. Longo said the finding challenges a core assumption in weight science: “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.”
The Diet’s Effect on Actual Lifespan Was More Modest
Among female mice, LDMM increased median lifespan by about 5.6 percent compared with the Western diet and 7.2 percent compared with the fasting plan. Against the standard diet, though, LDMM improved healthspan markers like frailty and metabolism without mice actually living meaningfully longer. Researchers note the diet started fairly late in the mice’s lives, around the human equivalent of retirement age, which may have limited its effect on lifespan.
None of this proves methionine alone drives the diabetes gap in the human data, since that part of the study is a one-time snapshot, not a years-long track record. Still, the mouse experiment and the 200,000-person snapshot point in a similar direction, which makes the finding worth following. A menu built around vegetables, legumes, and a little fish produced fat loss and stronger muscles without the mice eating less, which says more about food composition than about calorie counting.
Disclaimer: This article summarizes findings from a mouse study and an observational analysis of human health data. Mice are not humans, and no clinical trial has yet tested this diet in people. Nothing here should be taken as medical or nutritional advice. Anyone considering a significant change to protein or amino acid intake should talk with a doctor or registered dietitian first.
Paper Notes
Limitations
This lifespan portion of the experiment began when the mice were already 20 months old, equivalent to a person in their early sixties, at a point when researchers acknowledge dietary interventions tend to have reduced power to extend maximum lifespan. Starting the same diet earlier in life might produce a stronger longevity effect. The human epidemiological analysis was cross-sectional, meaning it captured diet and health data at a single point in time rather than following the same people over years, so the findings show an association between animal protein intake and conditions like diabetes rather than proof that methionine itself causes them. Several of the mechanistic follow-up experiments used small numbers of mice, which the authors flag as a reason for additional confirmation.
Funding and Disclosures
This work was supported by grants from the National Institutes of Health and the National Institute on Aging, along with the USC Edna Jones Chair Fund. Senior author Valter Longo holds equity in L-Nutra, a company that develops medical foods, and he, along with two co-authors, has filed patents related to the fasting-mimicking diet referenced in the study; the University of Southern California has licensed related intellectual property to L-Nutra and may receive royalty payments as part of that agreement. Longo and first author Maura Fanti are also listed as inventors on a separate pending patent application covering methods and findings described in this paper.
Publication Details
Fanti, M., Brandhorst, S., Navarrete, G., et al. (2026). “Methionine-supplemented longevity diet increases growth hormone, GLP-1, and FGF21; reduces frailty; and promotes healthspan.” Published in Cell Metabolism, Volume 38, August 4, 2026. DOI: 10.1016/j.cmet.2026.05.015. Lead contact: Valter D. Longo, University of Southern California Longevity Institute ([email protected]).
