Protein-enriched products now fill grocery store shelves, appearing in everything from cereal and coffee to water. Yet a new review of more than 350 studies on protein restriction and aging suggests that many people may gain more from eating less protein, and that lower intake could sometimes contribute to a longer life.
Published July 31 in the Cell Press journal Cell Press Blue, the review examines how reducing protein may influence aging. The researchers conclude that protein restriction can improve metabolism, alter the way cells react to nutrients, limit cellular damage, and help cells maintain normal function.
“It’s absolutely crystal clear that there are benefits of protein to muscle growth and exercise response of active individuals,” says Dudley Lamming, the paper’s corresponding author, 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.”
An Alternative to Calorie Restriction
Scientists have long known that reducing calorie intake can lengthen lifespan in many organisms and lower the risk of age-related illnesses such as cancer. In practice, however, following a calorie-restricted diet over the long term is extremely difficult for most people.
Protein restriction may offer another route. Earlier studies found that flies and rodents lived longer when they consumed less protein, even when their overall calorie intake did not decline.
Recent human clinical trials have also produced promising results. People who lowered their protein intake lost weight and body fat and showed improvements in fasting blood sugar, despite often consuming more total calories.
Why More Protein Can Still Be Beneficial
The evidence is not one-sided. Other studies indicate that higher protein intake can support weight loss and help older adults preserve muscle, particularly when combined with exercise.
Those findings contributed to updated US dietary guidance this year. The new recommendations call for daily protein consumption of 1.2-1.6 grams per kilogram of body weight (0.5-0.7 grams per pound), nearly twice the previous amount.
As Americans consume more protein than ever and older adults are increasingly advised to raise their intake, Lamming and his colleague reviewed decades of evidence to better understand the relationship between protein, metabolism, and aging.
Their analysis of more than 350 papers identified several recurring biological mechanisms that could explain why protein restriction may improve health and promote longevity. Across the research, lower protein intake was linked to better metabolic function, altered nutrient signaling, reduced cellular damage, and improved maintenance of healthy cells.
A Hormone Linked to Longevity
One important factor is fibroblast growth factor 21 (FGF21), a hormone that increases when protein intake falls. FGF21 can raise energy expenditure, improve blood sugar regulation, and reduce inflammation.
Mouse studies have shown that animals with elevated levels of FGF21 lived longer than typical mice. The effect was stronger in male mice than in female mice. Lower protein intake also increases FGF21 levels in humans.
Certain Amino Acids May Drive Aging
The review also focuses on several amino acids, which are the individual building blocks of protein. Methionine, isoleucine, and valine appear to play especially important roles.
Research suggests that excessive intake of these amino acids may activate biological pathways that encourage growth. When those pathways remain highly active, they may increase the risk of obesity, inflammation, and other conditions associated with aging.
“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 says.
Protein Needs Vary From Person to Person
Some groups clearly require more protein. Pregnant women and certain older adults, for example, may have higher nutritional needs. For many sedentary adults, however, protein-fortified foods may not offer the health advantages they expect.
Athletes often consume large quantities of protein without developing metabolic disease. Lamming suspects that regular physical activity may offer protection by directing protein toward the development and maintenance of strong, healthy muscle.
“Recent recommendations have encouraged people to eat more protein, but they’ve also encouraged people to exercise more,” Lamming says. “We probably need to personalize protein recommendations based not just on age, but also on how physically active people are.”
The findings suggest that protein guidance may be most effective when it takes both age and activity level into account, rather than applying the same recommendation to everyone.
This work was supported by the National Institute on Aging, the Wisconsin Partnership Program, and the University of Wisconsin-Madison.