A modified Mediterranean-style diet with low protein and adequate methionine helped mice live ier lives while reducing body fat and frailty.
New Delhi, India Jul 10, 2026 ALN: A mostly plant and fish-based diet with low protein and carefully balanced levels of an amino acid found in foods like eggs, meat, and dairy helped mice stay ier as they aged, according to new research from the University of Southern California. The diet increased lifespan, reduced body fat, and lowered frailty in older mice. This study adds to a growing body of evidence suggesting that dietary patterns can significantly influence outcomes and longevity.
The findings, published in Cell Metabolism, were supported by an analysis of dietary data from more than 200,000 people conducted by researchers from USC, the University of Toronto, and Harvard University. People who followed more plant-focused eating patterns also showed lower rates of obesity and Type 2 diabetes. This correlation highlights the potential for dietary interventions to mitigate the risk of chronic diseases and promote ier aging.
Together, the mouse and human findings suggest that a Mediterranean-inspired "longevity diet" centered on mostly vegan or vegetarian foods with fish may offer important benefits. According to senior author Valter Longo, the diet provides low but adequate amounts of methionine and other essential amino acids. Methionine is an amino acid that plays a crucial role in various bodily functions, including metabolism and cellular repair. The careful balance of this amino acid in the diet appears to be key to promoting and longevity.
Longo has spent years studying how nutrition influences aging and disease. His previous work has focused on the traditional low-protein, plant-centered Mediterranean diets commonly found in parts of southern Europe, where people often live exceptionally long lives. These diets are rich in fruits, vegetables, whole grains, nuts, and y fats, particularly olive oil, while being lower in red meat and processed foods.
Although these populations tend to have some of the world's longest life expectancies, they also experience relatively high levels of frailty in old age. Frailty is characterized by decreased strength, endurance, and overall physiological function, making older adults more vulnerable to adverse outcomes. Because plant foods naturally contain lower levels of essential amino acids than animal products, Longo developed a modified longevity diet that adds a small amount of methionine to determine whether it could reduce frailty while preserving the advantages of a plant-focused eating pattern.
To test the idea, researchers fed 20-month-old mice one of four diets: a standard diet; a Western diet high in fats and sugars; a low-carbohydrate ketogenic diet; or a low-protein, methionine-supplemented longevity diet (LDMM). The age of the mice is roughly equivalent to that of older adults in humans, making the results of this study particularly relevant for understanding dietary impacts on aging.
The mice eating the LDMM consistently performed best. They experienced a longer lifespan (the portion of life spent in good ), carried less body fat, and showed fewer signs of frailty than mice on the other diets. This suggests that dietary composition, specifically the balance of amino acids, may play a more significant role in outcomes than previously thought.
"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." This statement underscores the emerging understanding that not all proteins are created equal, and their specific components can have varying effects on .
The researchers also found several biological markers associated with improved cardiometabolic in mice fed the LDMM. Among them were increased levels of GLP-1 and other signaling molecules involved in regulating metabolism and aging across many species. GLP-1, or glucagon-like peptide-1, is a hormone that plays a crucial role in insulin secretion and appetite regulation, further linking dietary choices to metabolic .
Longo highlighted another surprising result. Mice eating the LDMM consumed more food than any of the other groups and took in as many calories as the other diets, yet still lost body fat while maintaining lean muscle mass. Those benefits appeared only when methionine levels remained low but sufficient. This finding challenges conventional wisdom that weight loss is solely a function of calorie restriction, suggesting that the quality and composition of the diet are equally, if not more, important.
The analysis of more than 200,000 people revealed a similar pattern. Participants who consumed the most animal protein, and therefore the highest levels of methionine and other essential amino acids, had higher rates of obesity and were twice as likely to have Type 2 diabetes compared with people who ate little or no animal protein. This correlation raises important questions about dietary recommendations and the potential need for a shift towards more plant-based diets in public guidelines.
According to Longo, these differences remained even though people with the highest animal protein intake generally consumed fewer calories and otherwise had ier diets. "This challenges the dogma that calorie reduction is necessary to lose weight, but it also tells us that we need to have a 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." This insight could lead to more tailored dietary approaches in the future, focusing on the specific needs of individuals rather than a one-size-fits-all model.
The research team says the next step is to test the LDMM in a controlled clinical trial involving people to determine whether the same benefits can be achieved in humans. Conducting such trials will be crucial to validate the findings from the mouse studies and to explore the practical applications of the longevity diet in human populations.
The study was led by researchers from the USC Leonard Davis School of Gerontology in collaboration with scientists from the University of Toronto, Harvard University, the University of Campinas, the Keck School of Medicine of USC, and Children's Hospital Los Angeles. This collaboration reflects the interdisciplinary nature of the research, combining expertise in gerontology, nutrition, and metabolic .
Funding was provided by National Institute on Aging grant AG084485, National Institute of grant GR1045540, and the USC Edna Jones Chair Fund. These funding sources underscore the importance of governmental and institutional support for research aimed at understanding aging and .
Valter Longo has an equity interest in L-Nutra, a company that develops medical foods. Longo, Todd Morgan, and Sebastian Brandhorst have filed patents related to the Fasting-Mimicking Diet through the University of Southern California, which has licensed related intellectual property to L-Nutra and may receive royalty payments. Longo and Maura Fanti are also inventors on a U.S. provisional patent application covering aspects of the methods and findings described in the study. These potential conflicts of interest highlight the need for transparency in research, particularly when it comes to dietary interventions that could have significant public implications.
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