Think more protein is better? Scientists say you may want to rethink that

"It's absolutely crystal clear that there are benefits of protein to muscle growth and exercise response of active individuals. But because most people are relatively sedentary, many people are likely consuming more protein than they actually need, which probably has negative health consequences," Dudley Lamming, the paper's corresponding author, University of Wisconsin-Madison, was quoted as saying by News Medical.

For decades, scientists have known that eating less calories can extend lifespan in many organisms and reduce the risk of age-related diseases like cancer. But maintaining a calorie-restricted diet is difficult for most people, News Medical reported quoting the review

Previous studies have found that eating less protein can extend the lifespan of flies and rodents without reducing their calorie consumption.

Several recent clinical trials in humans have also shown that reducing protein intake can reduce weight and fat mass and improve fasting blood sugar in humans, even though protein-restricted individuals tend to eat more calories.

But at the same time, some studies have found that eating more protein could promote weight loss and reduce age-related muscle loss in older adults when paired with exercise. Those findings prompted US authorities to update its dietary guidelines this year, recommending an increase in daily protein intake to 1.2–1.6 grams per kilogram of body weight (0.5–0.7 grams per pound), nearly doubling the previous recommendations.

With Americans consuming more protein than ever and older adults encouraged to boost their intake, Lamming and his colleague set off to review decades of research on protein restriction and ageing.

Drawing on more than 350 papers, the team outlined potential mechanisms that consistently emerge across studies and may explain how protein-restricted diets could improve health and promote longevity. These mechanisms suggest that protein restriction slows ageing by improving metabolism, changing how cells respond to nutrients, reducing cellular damage, and preserving healthy cell function.

One of the key players is a hormone called fibroblast growth factor 21 (FGF21), which rises when protein intake is low. FGF21 can increase the body's energy expenditure, improve blood sugar control, and reduce inflammation. Studies in mice have shown that animals with higher FGF21 levels lived longer than normal mice, and the benefit was more pronounced in male mice than female mice. Eating less protein raises FGF21 levels in humans as well.

The review also highlights several amino acids, the building blocks of protein, that appear to drive many of these effects, including methionine, isoleucine, and valine. Studies show that consuming too much of these amino acids could trigger biological processes that promote growth, increasing the risk of obesity, inflammation, and other age-related diseases.

"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.

He adds that while some people, like pregnant women and some older adults, have higher protein needs, for most sedentary adults, protein-fortified food may not provide the health benefits people expect.

Lamming notes that athletes often consume large amounts of protein without developing metabolic diseases. He suspects that regular exercise, perhaps by using protein to build strong healthy muscles, helps protect them from the negative health effects associated with a protein-rich diet.

"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."