As we age, keeping excess body fat in check while preserving muscle becomes increasingly difficult. This raises the risk of frailty, metabolic disease, and loss of independence.Protein is often presented as the obvious answer. Yet healthy aging may depend not only on how much protein we consume, but also on the balance of its amino acids.A new study in aging mice found that an experimental low-protein diet, supplemented with methionine – an essential amino acid obtained through food – helped the animals lose fat without sacrificing lean body mass.The diet also reduced frailty and improved markers of metabolic health, even though the mice could eat as much as they wanted.Developed at the University of Southern California (USC) and described in Cell Metabolism, the experimental diet was low in protein and contained a carefully adjusted amount of methionine.A graphical summary of the study, which involved mouse experiments as well as an analysis of human epidemiological data. (Fanti et al., Cell Metab., 2026)An earlier version of the low-amino-acid diet had caused the mice to lose weight too rapidly. The researchers therefore added a controlled amount of methionine, aiming to prevent excessive weight and muscle loss while preserving the diet's metabolic benefits.Although inspired by Mediterranean-style eating patterns, the experimental diet was not a conventional Mediterranean diet. It was a specially formulated mouse diet based largely on plant and fish protein, with low overall protein and precisely controlled amino acid levels.The team studied 20-month-old mice – roughly comparable to humans in their early 60s – assigned to a standard diet, a Western-style diet high in fat and sugar, a low-carbohydrate ketogenic diet, or the experimental diet.