The effect of dietary protein on neurodegeneration

Question

I have recently come across the following two studies:

• The first paper was performed on an Alzheimer's disease mouse model, and found that protein restriction cycles reduce IGF-1 and phosphorylated Tau, the latter of which is a commonly observed biomarker related to neurodegenerative diseases. It was also found that mice that had protein restriction cycles had improved cognition, among other findings.
• The second one was performed on cognitively normal adults, and found a decreased burden of brain beta-amyloid, which is also a biomarker that is usually correlated with cognitive decline.

While I understand that none of these studies attempt to suggest that either dietary protein consumption habit is better for humans, they seem to contradict themselves a bit. Though different biomarkers were observed, they both came to a different conclusion regarding cognitive function, albeit with obviously different experimental conditions as well.

These results got me asking the following question: what does the current literature suggest is better for humans regarding retaining or improving cognitive function, as it pertains to dietary protein?

Answer 1

There really isn't enough research to say more or less dietary protein is "better" for Alzheimer's in the long-term (Review Article - Protein and Alzheimer's).

In the short-term, Alzheimer's occurs mostly in older adults, who are at risk of muscle degradation with insufficient protein intake. Muscle degradation dramatically increases the risks of fall and generally decreases quality of life (Review Article - Sarcopenia); for the elderly, then, the benefits of consuming more protein would probably outweigh any slightly increased risk of Alzheimer's. Again, it isn't clear whether protein consumption plays a significant role either way (Review Article - Protein and Alzheimer's).

Most evidence regarding diet and long-term Alzheimer's risk says that eating a healthy diet (lots of vegetables, not so much red meat, etc.) is beneficial (National Institute on Aging). As with most diseases, the cumulative effect of a healthy diet is probably more important than any single aspect.