80% of long term vegans are deficient in vitamin B12, which is needed for proper mental function.
50% of long term vegetarians are deficient in vitamin B12.
B12 deficiency causes dementia, cognitive impairment, depression, and degenerative mental disorders.
Kids who eat a vegan diet are deficient in B12 and have impaired brain function. This reverses when they start eating animal products.
Vegetarians and vegans have lower muscle creatine and carnosine levels.