Scientists have advised low consumption of protein-rich foods for longer life, following new research suggesting that excess protein could come at a hidden cost.
Protein has become one of the food industry’s favorite selling points.
Findings by nutritionists hint that the macronutrient is now being added to cereal, coffee, snack bars, desserts, and even water, with the belief that more of it contributes to a healthier body and slows aging.
However, a major scientific review now challenges that assumption. After examining more than 350 papers, researchers concluded that reducing protein, or limiting certain amino acids within it, may improve metabolic health and slow some biological processes associated with aging. In several animal studies, these dietary changes also extended lifespan.
The review, written by Lamming and Bailey Knopf of the University of Wisconsin-Madison and published on July 31 in a Cell Press journal, explores how lower protein intake may influence metabolism, cellular repair, inflammation, and the systems cells use to detect nutrients.
“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,” Lamming said.
Scientists have long known that restricting calories can extend lifespan in organisms ranging from flies to rodents. Calorie restriction also reduces the incidence of several age-related diseases in laboratory animals.
The problem is that consistently eating substantially less food is difficult and can carry risks if the diet does not provide enough essential nutrients.
Protein restriction may trigger some of the same protective responses without requiring a reduction in total calories. Flies and rodents fed less protein have lived longer in multiple experiments, even when they consumed as many or more calories than comparison animals.
Small and relatively short human trials have produced encouraging but incomplete results. Participants placed on lower protein diets have experienced reductions in body weight and fat, along with improvements in insulin sensitivity and fasting blood sugar. In some studies, these changes occurred even though participants ate more calories.
That apparent contradiction may be explained partly by fibroblast growth factor 21 (FGF21), a hormone released in greater amounts when the body detects low protein availability. FGF21 can increase energy expenditure, improve blood sugar regulation, and reduce inflammation.
Mice with elevated FGF21 have lived longer than typical mice, although the effect was greater in males than in females. Protein restriction also raises FGF21 in people, but researchers have not yet shown that this response extends human lifespan.
Lower protein intake may also influence two major nutrient-sensing systems inside cells. One, known as mTORC1, encourages cells to grow and build new material when nutrients are abundant. The other involves GCN2, which helps cells adapt when amino acids are scarce.
Briefly reducing growth signals can allow cells to redirect resources toward maintenance, stress resistance, and the removal of damaged components. Persistently elevated growth signaling, however, has been associated with cellular dysfunction during aging. The review identifies reduced mTORC1 activity and increased cellular stress defenses as two possible reasons protein restriction benefits animals.
The finding noted that not every part of protein appears to have the same effect. Proteins are assembled from amino acids, and the review highlights methionine, isoleucine, valine, and leucine as particularly important.
Animal experiments suggest that restricting individual amino acids can sometimes reproduce many of the metabolic and longevity benefits seen with broader protein restriction. In one mouse study summarised in the review, valine restriction improved metabolic health, reduced signs of cellular aging and frailty, and increased lifespan in males by 23 percent.
These amino acids help activate biological pathways that promote growth. That activity is necessary during childhood, pregnancy, physical training, and tissue repair. But researchers suspect that continuously stimulating growth pathways in sedentary adults may eventually contribute to obesity, inflammation, insulin resistance, and other age-related problems.
“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.
The conclusion conflicts with a powerful trend in nutrition, as Americans, for instance, buy more protein-enriched products, while the latest US dietary guidelines advise adults to aim for 1.2–1.6 grams of protein per kilogram of body weight (0.5–0.7 grams per pound) each day. The previous recommended dietary allowance was 0.8 grams per kilogram (0.36 grams per pound), a level intended to meet the basic needs of most healthy adults.
According to experts, for a person weighing 180 pounds (82 kilograms), the updated target equals roughly 98–131 grams of protein per day. However, the ideal amount may differ considerably depending on age, body size, health, pregnancy, and activity level.
Older adults are one important exception to the idea that less may be better. Aging is often accompanied by sarcopenia, the gradual loss of muscle mass and strength. Consuming additional protein, particularly when combined with resistance exercise, may help older people preserve muscle and remain physically independent.
Pregnant women, people recovering from illness or surgery, and highly active individuals may also need more protein. A restrictive diet could be harmful for these groups if it fails to meet their nutritional needs.
Exercise may be the factor that helps reconcile the conflicting evidence. Athletes routinely consume protein-rich diets without showing the metabolic problems seen in some sedentary populations. Lamming suspects that trained muscles absorb and use more amino acids for repair and growth, preventing the prolonged nutrient signals that could become harmful when protein intake is high but physical demand is low.
“Recent recommendations have encouraged people to eat more protein, but they’ve also encouraged people to exercise more. We probably need to personalize protein recommendations based not just on age but also on how physically active people are,” Lamming said.
Note: This article discusses dietary and protein-restriction research. It is general reporting on a scientific review and is not personalized nutrition advice; individual needs vary by age, health status, pregnancy and activity level, and anyone considering major dietary changes should consult a qualified professional.
