Many people believe eating more protein is always healthier. High-protein diets are often promoted for weight loss, muscle building and healthy aging.
However, a new study suggests that the type of protein and its amino acids may be just as important as the total amount consumed.
Researchers from the University of Southern California, together with colleagues from the University of Toronto and Harvard University, have reported that a mostly plant-based eating pattern with limited protein and a carefully controlled amount of the amino acid methionine produced impressive health benefits in mice.
They also found similar patterns when they examined dietary data from more than 200,000 people.
The research, published in Cell Metabolism, focused on methionine, an essential amino acid found mainly in meat, eggs, dairy products and fish. The body cannot make methionine on its own, so it must come from food. However, scientists wondered whether eating too much might reduce some of the health benefits seen in traditional Mediterranean-style diets.
The researchers designed a special longevity diet based largely on plant foods with fish included in moderation. The diet contained relatively little protein but enough methionine to meet the body’s basic needs.
Older mice were divided into four groups and fed different diets, including a Western diet, a ketogenic diet, a standard diet and the new longevity diet. The mice eating the longevity diet lived a healthier later life. They accumulated less body fat, remained physically stronger and had lower levels of frailty than mice eating the other diets.
Laboratory tests also showed healthier metabolic changes. Hormones involved in blood sugar control and energy balance, including GLP-1, increased. One unexpected finding was that the mice could eat as many calories as the others while still reducing body fat without losing lean muscle.
To see whether similar patterns appeared in people, the researchers analyzed information collected from more than 200,000 adults. Those eating the highest amounts of animal protein had higher methionine intake and were more likely to have obesity and type 2 diabetes than people following more plant-focused diets.
Although these findings are exciting, they should be interpreted carefully. The mouse experiments cannot prove humans will respond in the same way. In addition, the human data came from observational studies, which cannot establish cause and effect because many lifestyle factors may influence the results.
Professor Valter Longo said the research challenges the common belief that calorie reduction alone explains weight loss and healthy aging. Instead, the balance of specific amino acids may play an important role.
However, he also emphasized that too little methionine caused problems such as frailty, showing that the goal is balance rather than complete restriction.
The next step will be controlled clinical trials to test whether this longevity diet produces similar health benefits in people. If future studies confirm these results, doctors may one day recommend not only how much protein to eat but also which protein sources provide the healthiest balance of amino acids.
Overall, this study adds to growing evidence that plant-rich Mediterranean-style diets may promote healthier aging. It also reminds us that nutrition is more complex than simply counting calories or grams of protein.
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The study was published in Cell Metabolism.
