Overview
Vitamin B12 (cobalamin) is an essential water-soluble vitamin critical for nerve function, DNA synthesis, red blood cell formation, and energy metabolism. Unlike most vitamins, B12 is stored in the body (primarily liver), found only in animal foods, and requires specific absorption mechanisms in the gut (intrinsic factor).
Primary applications focus on preventing/treating B12 deficiency (vegans, elderly, absorption issues), preventing pernicious anemia, supporting nerve function and preventing neurological damage, energy metabolism (in deficient individuals only), and methylation support.
Evidence quality is strong for treating deficiency and preventing deficiency-related complications, with no evidence for benefits in replete (non-deficient) individuals.
Safety is excellent even at high doses (B12 is water-soluble with minimal toxicity), though benefits exist only for those with deficiency or absorption issues - B12 doesn't enhance energy or cognition in replete individuals.
What it means
If you're vegan, over 50, or have gut issues: you probably need B12 (1000mcg daily). If you eat meat and you're young: you probably don't. B12 "energy shots" for non-deficient people = expensive placebo. Real deficiency causes irreversible nerve damage - get tested if at risk. Methylcobalamin vs cyanocobalamin debate is mostly marketing; both work fine.
Functions and Deficiency
B12 functions: cofactor for methionine synthase (conversion of homocysteine to methionine, critical for methylation and DNA synthesis), cofactor for methylmalonyl-CoA mutase (fatty acid metabolism), supporting myelin synthesis (nerve insulation), and red blood cell maturation.
Deficiency symptoms develop gradually (body stores last years) and include fatigue and weakness, megaloblastic anemia (large, immature red blood cells), neurological symptoms (tingling, numbness, balance problems, memory issues, cognitive decline), glossitis (inflamed tongue), depression and mood changes, and in severe cases, irreversible neurological damage.
What it means
Deficiency sneaks up slowly (body stores last 3-5 years). By the time you feel it, nerve damage might be permanent. At-risk groups MUST supplement preventively, not wait for symptoms.
At-Risk Populations
Vegans and vegetarians (B12 only in animal products - supplementation essential), adults over 50 (reduced stomach acid impairs absorption), people with GI disorders (Crohn's, celiac, gastric bypass), those on metformin or PPIs long-term (interfere with B12 absorption), and pernicious anemia patients (lack intrinsic factor).
Forms and Dosing
Forms: Cyanocobalamin (synthetic, most studied, requires conversion to active forms, very affordable), methylcobalamin (active form, preferred by some for neurological support and methylation, more expensive), hydroxocobalamin (injectable form, long-acting, used medically), and adenosylcobalamin (mitochondrial form, less common in supplements).
For most people, cyanocobalamin is effective and affordable. Methylcobalamin might be preferable for neurological conditions or those wanting pre-converted active form.
What it means
Methylcobalamin vs cyanocobalamin war is 90% marketing. Both work. Cyanocobalamin is cheap, well-studied, converts to active forms fine. Methylcobalamin is pre-activated, costs more. Either works - choose based on budget.
Typical doses: 25-100 mcg daily for maintenance in at-risk populations. 500-1000 mcg daily for mild deficiency or malabsorption. 1000-2000 mcg daily for significant deficiency or strict vegans. Very high doses (sublingual or spray) can bypass intrinsic factor absorption issues.
Injection (hydroxocobalamin or cyanocobalamin) for severe deficiency or pernicious anemia under medical supervision.
Safety and Testing
B12 is extremely safe. Water-soluble,excess excreted. No established upper limit. High-dose supplementation (even 1000+ mcg) safe with no documented toxicity.
Testing: Serum B12 levels (basic but can miss functional deficiency). Methylmalonic acid (MMA) and homocysteine (more sensitive for functional B12 status). If at risk, consult healthcare provider about testing and appropriate supplementation.
B12 supplementation is essential for vegetarians/vegans, highly valuable for older adults and those with absorption issues, and critical for correcting deficiency to prevent irreversible neurological damage.
References
Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040.
Pawlak R, Lester SE, Babatunde T. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. Eur J Clin Nutr. 2014;68(5):541-548.