These B vitamins power one-carbon metabolism, which regulates homocysteine--a metabolite linked to vascular and cognitive health. Supplementation reliably lowers homocysteine; cognitive outcomes vary by population and baseline status, so we frame them as supportive cofactors that keep the methylation pathways that underlie normal brain function running smoothly.

B6 (pyridoxine), B9 (folate), and B12 (cobalamin) all have some positive evidence for improving cognition, especially in cases of deficiency or elevated homocysteine. The effect sizes of each component on cognition are small (~0.1). Although they are water-soluble, megadoses can cause issues so aim for B6 < 20mg, folate < 1mg, B12 < 500mcg/day. These vitamins convert neurotoxic homocysteine to methionine/cystine–critical since elevated homocysteine is linked to brain atrophy and reduced cerebral blood flow.

Citations:
Porter K, et al. Causes, Consequences and Public Health Implications of Low B-Vitamin Status in Ageing. Nutrients. 2016 Nov 16;8(11):725. doi: 10.3390/nu8110725. PMID: 27854316; PMCID: PMC5133110.
Nelson Siu Kei Lam, et al. The potential use of folate and its derivatives in treating psychiatric disorders: A systematic review, Biomedicine & Pharmacotherapy, Volume 146, 2022