Eating less protein may ‘slow’ ageing. But here’s why the evidence isn’t clear-cut

A recent review of over 350 studies indicates that consuming less protein could potentially slow down the aging process, but the evidence is not conclusive. The findings mostly come from animal studies, which raises questions about their applicability to humans. Protein is a vital nutrient found in many foods, such as legumes, nuts, fish, eggs, dairy, and lean meat, and it plays a crucial role in maintaining muscle, repairing tissues, and supporting overall health.

According to Australian dietary guidelines, adults should aim for 15-25% of their daily energy intake from protein, with good sources including the aforementioned foods. Most Australians already meet these recommendations. The new study examined evidence from both animal and some human studies to understand how limiting protein and specific amino acids impacts the biological aging process.

In general, the evidence points to the idea that reducing protein intake can alter energy usage, help cells eliminate damaged components, and reduce the accumulation of aging cells, leading to healthier aging and potentially longer lifespan in animal studies. However, the exact reasons behind these benefits remain unclear and could be due to various factors, such as reduced protein intake, lower animal protein consumption, fewer specific amino acids, or following a healthier overall diet.

Most of the compelling evidence for these benefits comes from laboratory animals like mice, rats, flies, and yeast, which tend to live longer and exhibit healthier aging when consuming lower-protein diets. However, laboratory animals are typically raised in tightly controlled environments, making it difficult to translate these results directly to humans.

Human biology is more complex, and laboratory findings often do not apply to people. Additionally, the review suggests that focusing on total protein may not be the only factor; restricting specific amino acids, particularly methionine, isoleucine, and valine, can produce similar benefits in animal studies as reducing total protein.

However, studying humans is more challenging, as most intervention studies only lasted a few weeks, making it impossible to determine if restricting protein affects lifespan or healthy aging. Furthermore, higher-protein diets can help preserve muscle mass, especially in older adults, and when combined with resistance exercise, can reduce the risk of frailty and sarcopenia (muscle loss and weakness).

While eating more protein can make people feel fuller for longer and assist in managing weight, particularly for those dealing with obesity, it is essential to consider the overall diet. Consuming protein mainly from red and processed meat may also involve higher intakes of saturated fats and salt, which have been linked to worse health and early death.

Large studies have found associations between high animal protein intake, particularly red and processed meat, and adverse health outcomes. Still, these studies cannot conclusively prove that protein itself is responsible. This complexity makes it difficult to separate the effects of protein from the foods providing it or an individual's overall dietary habits.

Moreover, longevity is influenced by more than just protein intake. The Mediterranean diet, which includes moderate protein from legumes, fish, and nuts, along with plenty of vegetables, fruits, whole grains, and olive oil, has strong evidence for healthy aging. Conversely, the traditional Okinawan diet from Japan, characterized by low protein and an emphasis on plant foods like sweet potato and tofu, is also associated with healthy aging.

Both diets demonstrate that isolating protein alone might overlook a more significant picture. Longevity is shaped by various factors beyond diet, such as regular physical activity, good sleep, strong social connections, stress management, and avoiding smoking. While protein is undoubtedly an essential piece of the puzzle, focusing solely on protein may not provide the full picture.

The current review indicates that the relationship between protein and longevity is more nuanced than a simple "less is better" approach. At present, the strongest evidence supports adhering to current protein recommendations, particularly as we age. Rather than being swayed by the latest nutrition headlines, it is advisable to choose primarily minimally processed protein foods as part of an overall healthy diet.