As people age, losing excess body fat becomes increasingly difficult without also sacrificing muscle mass. This combination can contribute to frailty, reduced mobility, diabetes, and other age-related health problems. A new animal study suggests that the balance of specific amino acids—not simply the amount of protein consumed—may play an important role in promoting healthier aging.
The study, published in Cell Metabolism, found that an experimental low-protein diet with carefully adjusted levels of the amino acid methionine helped older mice lose body fat while largely preserving lean muscle. The research was conducted by scientists at the University of Southern California.
A Different Approach to Dietary Protein
Previous research has shown that restricting certain amino acids can trigger beneficial metabolic changes, but these diets often cause substantial loss of both fat and muscle.
To address this problem, the researchers developed a low-protein diet based primarily on plant and fish ingredients while carefully controlling methionine intake.
Methionine is an essential amino acid that must be obtained through food. It is found in protein-rich foods including meat, fish, eggs, dairy products, legumes, nuts, and seeds.
The researchers aimed to determine whether maintaining methionine within a narrow range could preserve muscle while still encouraging fat loss and improving metabolic health.
How the Study Was Conducted
The experiment involved mice that were 20 months old, approximately equivalent to humans in their early 60s.
The animals were randomly assigned to one of four diets: a standard laboratory diet, a Western-style diet high in fat and sugar, a low-carbohydrate ketogenic diet, or the experimental low-protein diet with carefully adjusted methionine levels.
All animals had unrestricted access to food throughout the study.
Compared with the other dietary groups, mice receiving the experimental diet lost significantly more body fat while largely maintaining lean body mass.
They also showed reduced frailty and better regulation of blood sugar.
Among female mice, the experimental diet was associated with a modest increase in lifespan compared with the Western diet, although survival was similar to mice eating the standard laboratory diet.
Hormonal Changes Favored Fat Burning
Lead author Maura Fanti said the diet appeared to alter several hormones involved in metabolism.
The mice developed higher levels of growth hormone and GLP-1, a hormone involved in appetite regulation and glucose control, alongside lower levels of insulin-like growth factor 1 (IGF-1), which has been linked to growth and aging.
The researchers also observed a substantial increase in fibroblast growth factor 21 (FGF21), a hormone produced primarily by the liver during periods of nutritional stress such as fasting or amino acid restriction.
FGF21 helps shift the body’s metabolism toward burning fat for energy.
To determine whether this hormone was responsible for the observed benefits, the researchers repeated part of the experiment using genetically modified mice incapable of producing FGF21.
In these animals, the diet no longer produced meaningful fat loss or improvements in insulin sensitivity. Instead, they developed signs of insulin resistance, suggesting that FGF21 plays a critical role in mediating the beneficial metabolic effects.
Finding the Right Methionine Balance
The study showed that the benefits depended on maintaining methionine within a relatively narrow range.
When researchers increased methionine intake beyond that level, the metabolic improvements largely disappeared, and the mice no longer showed the same combination of fat loss and muscle preservation.
The findings build on previous animal studies linking excessive methionine intake—particularly from animal-derived protein—with accelerated aging and metabolic dysfunction.
However, the researchers emphasize that completely eliminating methionine would also be undesirable because the amino acid remains essential for muscle maintenance and many normal biological processes.
According to the study, the favorable effects appeared only within a carefully controlled balance of overall protein intake and methionine levels.
The authors also note that this experimental diet differs substantially from common dietary patterns such as the Mediterranean diet or standard high-protein eating plans. It was specifically designed to investigate how precise changes in amino acid composition influence metabolism during aging.
What the Findings Could Mean for Humans
To explore whether similar patterns might exist in people, the researchers analyzed dietary and health information from more than 200,000 adults.
Individuals reporting the highest intake of animal protein were more likely to have obesity, while type 2 diabetes was nearly twice as common compared with those reporting the lowest animal-protein consumption.
However, these human findings came from cross-sectional data collected at a single point in time.
Because of this, the analysis cannot determine whether higher animal-protein intake directly causes obesity or diabetes. Other factors—including physical activity, socioeconomic status, and overall dietary patterns—may also contribute to the observed associations.
Senior author Professor Valter Longo said that combining controlled laboratory experiments with large human datasets provides stronger evidence than either approach alone, but emphasized that randomized clinical trials will be necessary to determine whether similar hormonal and metabolic responses occur in people.
Caution Before Changing Your Diet
The researchers stress that the study should not be interpreted as advice to drastically reduce protein intake or begin taking methionine supplements.
The positive effects observed in mice depended on a carefully designed experimental diet with precisely controlled amino acid composition.
Outside of a supervised setting, significant changes in protein intake could increase the risk of muscle loss, nutritional deficiencies, or other unintended health consequences, particularly among older adults.
Based on current evidence, Longo recommends that most adults consume approximately 0.37 grams of protein per pound of body weight each day, with at least half of that protein coming from plant sources. Older adults may require approximately 10–20% more protein to help preserve muscle mass, especially if they are physically active or living with chronic illness.
The researchers conclude that future clinical trials will be needed to determine whether carefully adjusting methionine and other amino acids can safely reproduce the benefits observed in mice and whether such dietary approaches are practical for long-term use in humans.
