Vitamin B12 (cobalamin) is an essential water-soluble vitamin required for neurological function, red blood cell formation and DNA synthesis. It is found almost exclusively in animal products. Deficiency is common in strict vegetarians and vegans, and in older adults who lose the ability to absorb it from food. When deficient, supplementation is critical — irreversible neurological damage can result from prolonged deficiency.
B12 supplementation is essential for vegans and vegetarians. Older adults often need it regardless of diet due to absorption issues. In people with normal status, B12 does nothing — the 'energy vitamin' marketing claim applies only to people who are deficient.
Each row grades the claimed effect by strength of human evidence, not mechanism or marketing.
B12 supplementation is essential for vegans and for older adults with absorption impairment. Widely recommended by nutritional authorities. A medical priority, not optional.
Sometimes marketed as an energy enhancer, cognitive booster or metabolism optimizer for the general population — none of these claims apply in replete people.
The deficiency prevention case is unambiguous. The gap is that people with no deficiency buy B12 energy shots expecting a performance effect — and get none. The other gap is that deficiency symptoms are non-specific (fatigue, brain fog, tingling) and take years to develop, during which neurological damage can accumulate.
Vitamin B12 is found almost exclusively in animal products (meat, fish, dairy, eggs) — strict vegans will develop deficiency within years without supplementation.
Older adults lose intrinsic factor (required for food-bound B12 absorption) — absorption of supplemental B12 (crystalline, not food-bound) is less affected.
Metformin (diabetes medication) and proton pump inhibitors (PPIs) reduce B12 absorption — people on these should monitor levels.
Methylcobalamin and hydroxocobalamin are biologically active forms; cyanocobalamin must be converted but is stable, cheap and effective.
High-dose oral B12 can work even when absorption is impaired because a small amount is absorbed by passive diffusion; intramuscular injection bypasses gut absorption entirely.
Neurological damage from prolonged deficiency may be irreversible — this makes early detection and supplementation in at-risk groups critical.
Mechanism is not outcome. Each mechanism is labelled by how far it has been validated in humans.
B12 is required for methionine synthase activity — converting homocysteine to methionine, a key step in DNA methylation and neurotransmitter synthesis.
B12 is required for methylmalonyl-CoA mutase activity, which is necessary for fatty acid metabolism and myelin sheath synthesis in the nervous system. Deficiency causes demyelination.
B12 is required for DNA synthesis in rapidly dividing cells. Deficiency causes large, immature red blood cells (megaloblastic anaemia) with impaired oxygen transport.
Exceptionally safe. No tolerable upper limit established. Water-soluble and excreted in excess. Even doses of thousands of mcg/day in clinical settings show no toxicity.
This page is educational and not medical advice. If you are vegan, older, or on metformin/PPIs, get your B12 tested. Deficiency is serious and sneaks up slowly.
A small, curated set — not a literature dump. Each reference comes with a single-line takeaway.
Lower B12 status associated with greater brain volume loss over time in older adults — reinforcing the importance of maintaining adequate levels.
Up to 30% of long-term metformin users develop B12 deficiency; peripheral neuropathy risk is significantly elevated.
High-dose oral B12 is as effective as intramuscular injection for correcting deficiency in most individuals — including those with absorption impairment.