Protein has become one of the most heavily promoted nutrients in the American food supply. It’s being added to cereals, snack foods, coffee and even water, often with the suggestion that more is inherently better.
A new review challenges that assumption. After examining more than 350 papers, researchers concluded that limiting protein or certain amino acids can extend lifespan in laboratory animals and produce some favorable metabolic changes in people. But researchers have not shown that eating less protein helps humans live longer, and the right amount likely depends on age, health and physical activity.
The review, published in Cell Press Blue, brings together animal experiments, short-term human trials and research on the biological effects of protein restriction. It was not a clinical trial testing whether people who eat less protein experience less disease or live longer.
“It’s absolutely crystal clear that there are benefits of protein to muscle growth and exercise response of active individuals,” said corresponding author Dudley Lamming of the University of Wisconsin-Madison. “But because most people are relatively sedentary, many people are likely consuming more protein than they actually need, which probably has negative health consequences.”
That remains a scientific argument rather than a settled conclusion. Protein is essential for building and maintaining muscle, organs, enzymes and other tissues. The review raises questions about whether continually pushing intake higher benefits everyone, particularly sedentary adults who already meet their nutritional needs.
Research has long found that restricting calories can extend lifespan in animals, although maintaining a substantially lower-calorie diet is difficult and may carry risks. Scientists have therefore investigated whether limiting particular nutrients could produce some of the same effects without reducing total calories.
In flies and rodents, diets containing less protein have extended lifespan in some experiments. Research also suggests that reducing certain amino acids, including methionine, isoleucine and valine, can affect metabolism and biological pathways involved in growth and aging.
Those findings cannot be assumed to apply directly to people. Human aging unfolds over decades and is influenced by disease, exercise, medications, socioeconomic conditions and many other factors that cannot be fully reproduced in laboratory animals.
Human trials of protein restriction generally have been much shorter. Some have found reductions in body weight and fat mass or improvements in fasting blood sugar, even when participants did not eat fewer calories.
Researchers also have identified hormonal and cellular changes that may help explain why lower-protein diets extend lifespan in some animals. But those biological changes do not demonstrate that protein restriction slows aging, prevents age-related disease or extends life in people.
The effects also are unlikely to depend on protein quantity alone. Different protein sources contain different mixtures of amino acids and come with other nutrients. Beans, fish, yogurt, nuts, processed meats and protein powders may all provide protein, but they do not contribute the same things to an overall diet.
Physical activity may further change how the body uses dietary protein. Resistance exercise stimulates muscle repair and growth, allowing amino acids to support muscle tissue. A sedentary person consuming the same amount may not receive the same benefit.
Lamming said athletes can consume large amounts of protein without necessarily developing the metabolic problems linked to high-protein diets in some studies. He proposed that exercise may help direct the protein toward maintaining and building muscle.
“Recent recommendations have encouraged people to eat more protein, but they’ve also encouraged people to exercise more,” Lamming said. “We probably need to personalize protein recommendations based not just on age, but also on how physically active people are.”
Age is especially important. Adults gradually lose muscle mass and strength as they get older, and inadequate protein can accelerate that process. Muscle loss can contribute to frailty, falls, disability and difficulty recovering from illness.
Studies and expert groups commonly recommend at least 1 to 1.2 grams of protein per kilogram of body weight each day for healthy older adults, with potentially higher amounts for people experiencing illness, malnutrition or frailty. The longstanding U.S. recommended dietary allowance of 0.8 grams per kilogram is intended to meet the basic needs of most healthy adults, but researchers continue to debate whether that amount is optimal for maintaining muscle later in life.
For a 150-pound person, 0.8 grams per kilogram is approximately 54 grams of protein per day. A target of 1.2 grams per kilogram would be about 82 grams.
That does not mean every older adult needs a high-protein diet or protein supplements. Total calorie intake, kidney function, medical conditions, food preferences and exercise habits all matter. But older adults should not reduce protein in pursuit of longevity without considering the possible consequences for muscle and strength.
Pregnant people, children, athletes, people recovering from surgery or illness and those at risk of malnutrition also may have higher or more specific protein needs.
For healthy, sedentary adults who already eat enough protein, the review raises a different question: Do protein-enhanced versions of ordinary foods provide a meaningful benefit, or do they simply add more of a nutrient that is already plentiful?
The paper cannot provide a universal target. It also does not establish that eliminating animal protein, avoiding individual amino acids or following a low-protein diet will slow human aging.
Instead, it pushes back against the idea that protein recommendations should always move in one direction. The available evidence suggests that both too little and potentially too much could matter, with the balance changing according to a person’s age, activity and health.
“These studies show that the amount of protein sedentary people are eating today may have negative health consequences, at least at the population level,” Lamming said.
Long-term human trials are still needed to determine whether moderate protein restriction prevents disease or improves healthy lifespan and whether any benefits outweigh possible losses in muscle and strength. Until then, the review offers a reason to question protein marketing, not a reason for everyone to start eating less.
The work was supported by the National Institute on Aging, the Wisconsin Partnership Program and the University of Wisconsin-Madison.