Protein restriction could support healthier aging, but researchers warn against one-size-fits-all advice

A growing body of evidence suggests that eating less protein – and restricting certain amino acids in particular – could improve metabolic health and support healthier aging, challenging the current enthusiasm for ever-higher protein consumption.

• Researchers identified six hallmarks of protein restriction, spanning metabolic health, nutrient sensing, cellular senescence, mitochondrial function, epigenetic changes and healthy aging.
• Restricting methionine, isoleucine and valine produced many of protein restriction’s metabolic and longevity benefits in animal studies, although human evidence remained limited.
• The authors cautioned that protein restriction could be harmful for people with higher requirements, including children, pregnant women and some older adults.

In a new review, What are the best sources of protein? Introducing the BPI Score, published in Frontiers in Sustainable Food Systems, Bailey A. Knopf and Dudley W. Lamming examined decades of research into protein restriction (PR) and amino acid intake, proposing six biological hallmarks through which lower protein consumption could influence aging.

The researchers pointed to improved metabolic health, altered nutrient-sensing pathways, decreased cellular senescence, improved mitochondrial function, epigenetic modification and healthier aging as recurring responses to PR across experimental models.

The conclusions arrive as protein has become one of the food industry’s dominant nutrition claims. The review cited a 2025 report showing that 61% of US consumers increased their protein intake during 2024, compared with 48% in 2019.

That consumer behavior sits alongside dietary recommendations encouraging higher protein intake, particularly among older people. The US Recommended Dietary Allowance remains 0.8g/kg of bodyweight, while the authors noted that intakes of 1.0–1.2g/kg are routinely recommended for people over 65 to help protect against sarcopenia and frailty.

Yet the evidence reviewed by Knopf and Lamming suggested that the relationship between protein and healthy aging is considerably more complicated.

Human association studies have linked high-protein diets with increased risks of diabetes, cancer, cardiovascular mortality and overall mortality, while randomized controlled trials have reported metabolic benefits from reducing protein.

A 2016 trial cited in the review found that people following a low-protein diet for 43 days lost bodyweight and fat mass and reduced fasting blood glucose despite consuming more calories. Another five-week intervention in lean men improved insulin sensitivity and increased energy expenditure.

Short-term protein restriction in people with metabolic syndrome has also been associated with lower adiposity, improved insulin sensitivity, reduced glucose and lipid concentrations and lower inflammation.

The authors stressed, however, that these findings do not establish an optimal protein intake for healthy aging in humans.

Much of the strongest longevity evidence still comes from laboratory animals. Protein restriction has extended lifespan in yeast, flies and rodents, with several studies reporting substantial improvements in mice and rats.

One mechanism repeatedly implicated is fibroblast growth factor 21 (FGF21), a hormone produced in response to nutritional stress. Protein restriction increases FGF21 in mice, rats and humans, while experiments in mice have found that FGF21 is required for several of PR’s effects, including increased energy expenditure and lifespan extension.

The review also highlighted the role of mTORC1, the nutrient-sensing pathway closely associated with cellular growth and protein synthesis. Protein restriction inhibits mTORC1 across tissues including the liver, heart, muscle, adipose tissue and brain, potentially increasing autophagy – the process through which cells remove damaged proteins and organelles.

Rather than total protein alone, however, Knopf and Lamming argued that the amino acid composition of the diet could prove particularly important.

Restricting essential amino acids has reproduced many of the longevity effects of protein restriction in animal models, with methionine and the branched-chain amino acids leucine, isoleucine and valine receiving particular attention.

Of these, isoleucine emerged as one of the strongest candidates.

Reducing isoleucine consistently improved metabolic health across different diets and age groups in mice. A 66% reduction extended lifespan by 33% in male genetically heterogeneous mice while reducing frailty. Isoleucine restriction has also extended lifespan in flies and produced molecular changes associated with younger animals.

Valine restriction produced similar results. A 66% reduction improved metabolic health, decreased senescence and extended male mouse lifespan by 23%.

Methionine restriction has meanwhile extended lifespan across several species and has been associated with lower bodyweight and improvements in mitochondrial function, lipid metabolism and frailty in rodents.

Human evidence remains at a much earlier stage. Eight weeks of sulfur amino acid restriction increased FGF21 while reducing leptin and bodyweight, according to studies cited by the authors.

Two human trials have examined BCAA restriction. Reducing BCAA intake by 75% for seven days lowered circulating BCAAs by around half and improved insulin sensitivity in lean participants. A separate four-week randomized trial using approximately 60% BCAA restriction improved glucose and insulin homeostasis, increased FGF21 and reduced mTORC1 signaling in white adipose tissue.

Leucine presents a more complicated picture. Although it is a powerful activator of mTORC1 and stimulates skeletal muscle anabolism, evidence for restricting it has been inconsistent. The review concluded that changes in leucine alone were unlikely to explain PR’s benefits.

The researchers also challenged the assumption that non-essential amino acids are relatively unimportant.

Glycine supplementation, for example, extended lifespan by 6.2% in male mice and 3.7% in females in one study, while supplementation has also extended lifespan in C. elegans. Proline and serine supplementation have produced longevity effects in several experimental models.

For food and nutrition researchers, the findings point toward a more nuanced question than simply how many grams of protein a product contains. Different amino acids may exert markedly different effects on metabolism and aging, while protein source could also influence outcomes.

The review cited evidence that plant protein, but not animal protein, has been associated with a lower risk of frailty. It also noted that people following strict vegan diets can have lower circulating methionine levels, alongside evidence associating vegan diets with improved glucose homeostasis and lower circulating lipid levels.

But the authors cautioned strongly against interpreting the findings as a recommendation for older adults simply to reduce protein.

Some older people already consume insufficient protein because of reduced appetite, financial constraints or social isolation. Children, pregnant women, people recovering from injuries and those consuming restricted calories can also have higher protein requirements.

Exercise adds another complication. While protein restriction can reduce lean mass, the researchers noted evidence that exercise can largely offset lean-mass losses associated with low-protein diets in humans.

The central research question is therefore moving toward identifying the appropriate amount and composition of protein for different people rather than finding a universally optimal intake.

The authors called for clinical research to determine suitable diets, develop ways to measure their effects on health, frailty and aging, and establish how factors including age, activity and individual physiology alter protein and amino acid requirements.

For an industry currently racing to add protein to everything from snacks to beverages, the review offers a timely complication: when it comes to healthy aging, more protein may not always mean better nutrition.

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