"Few studies have looked carefully at the risk of dementia in vegetarians versus other people, and the data is contradictory. One small study of California residents found that meat eaters were more likely to become demented than their vegetarian counterparts. Another study in Alzheimer’s patients, however, found that adhering to a strict vegetarian diet resulted in lower cognition compared to a pescatarian diet (i.e., a diet that includes fish)."
https://www.alzdiscovery.org/cognitive-vitality/blog/vegetar...