Observational studies have also demonstrated an association between vitamin B12 levels and cognitive function. In fact, the link is so strong that part of the evaluation process for anyone presenting with a dementing illness includes checking vitamin B12 levels. While we have no gold-standard, randomized, double-blind, placebo-controlled trials demonstrating any benefit from vitamin B12 replacement, it's still considered standard-of-care for those patients with low levels. As an aside, over the past two decades, I have yet to optimize someone's vitamin B12 and note a significant clinical and/or cognitive improvement.
Nevertheless, in a study published last month in Neurology , the authors concluded that vitamin B12 markers are associated with cognitive function and even brain size. Of course, the study was short at less than 5 years in duration; it also followed just 121 community-dwellers in the Chicagoland area. More importantly, the strongest association was not directed to serum vitamin B12 level but rather to a specific metabolite, methylmalonate, and a non-specific metabolite, homocysteine. Total brain volume appeared to be related to the former while the latter was associated with white matter hyperintensity, eg cerebral infarcts.
In the end, this study provides more support linking vitamin B12 to cognitive function. Just remember that conversely, we still have no proof that raising vitamin B12 will return someone's level of function back to baseline. Good luck!