I am writing this as a quick article that I hope at some point to come back and fix up, but the bottom line is this: Vitamin B-12 is essential for the functioning of brain and nervous system and for the formation of blood. It is involved in the metabolism of every cell in the body, but is not found in any non-animal sources as it comes from bacteria.
This means that vegetarians are deficient in B-12. The research below shows that vegetarians generally are deficient in B-12, and then proof that a deficiency in vitamin B-12 is related to cognitive decline.
The easiest thing you can do is to supplement it. You’ll want to take around 5000 micrograms (not grams) and you want to look it in the form called methylcobalamin. Most multivitamins use cyanocobalamin and that is a pretty good way to know that your vitamin is not using the best sources for its inputs. Even though I eat meat daily (grass-fed beef mostly), I still take one of these B-12 losengers every day:
Here is some research showing that vegetarians are deficient and that a deficiency is linked to cognitive impairment:
“It recognized that vegetarians have consistently lower vitamin B-12 concentrations than do nonvegetarians and that vegetarians are at greater risk of vitamin B-12 deficiency than are nonvegetarians. Because vitamin B-12 is produced in nature only by vitamin B-12–producing microorganisms, humans must receive vitamin B-12 solely from the diet…The data are compelling, and they indicate that vegetarians should routinely take cobalamin or vitamin B-12 supplements, which in their generic form are relatively inexpensive…The lack of a comprehensive initiative to protect vegetarians from vitamin B-12 deficiency can lead to a whole generation of cobalamin-deficient children (and adults) who are incapable of making good decisions because of the additional burden of neurologic deficits induced by cobalamin deficiency.
This study discusses cognitive impairment in those with B12 decencies and how supplementation can fix it:
Vitamin B12 levels in the subclinical low-normal range are associated with Alzheimer’s disease, vascular dementia, and Parkinson’s disease. Vegetarianism and metformin use contribute to depressed vitamin B12 levels and may independently increase the risk for cognitive impairment. Vitamin B12 deficiency is associated with cognitive impairment. Vitamin B12 supplements administered orally or parenterally at high dose (1 mg daily) were effective in correcting biochemical deficiency…Low serum vitamin B12 levels are associated with neurodegenerative disease and cognitive impairment.