← DatabaseEssential Vitamin
Vitamin B12 (Cobalamin)
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A water-soluble essential vitamin required as a cofactor for DNA synthesis, red blood cell formation, and nerve myelination; deficiency causes well-defined hematologic and neurological disease.
How it works
Vitamin B12 is a nutrient your body cannot make on its own, so it must come from food (mainly animal products) or supplements. It plays a behind-the-scenes role in keeping your blood, nerves, and DNA-building machinery working properly. Because the body stores it in the liver, true deficiency can take years to appear, but when it does it can cause anemia, fatigue, and nerve problems.
Absorbing B12 is unusually complicated: it depends on stomach acid and a special protein called intrinsic factor, then a receptor in the lower small intestine. This is why people with autoimmune stomach conditions, gut surgery, certain medications, or strict plant-based diets can become deficient even when their diet seems adequate. In these cases supplementation or injections are used to bypass or overwhelm the faulty absorption step.
Correcting a genuine deficiency reliably reverses many of the symptoms and is a clear, established use. What is far less certain is whether extra B12 helps people who already have normal levels, for things like energy, mood, or cognition; the evidence there is weak. Because it is water-soluble and excess is excreted, high doses are generally considered low-risk, but that does not mean they provide benefit.
Mechanism · Detailed Analysis
Molecular targetCobalamin is the cofactor for two human enzymes. As methylcobalamin it supports cytosolic methionine synthase, which remethylates homocysteine to methionine and regenerates tetrahydrofolate. As adenosylcobalamin it supports mitochondrial methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA in the catabolism of certain fatty acids and amino acids.
Signaling & downstream effectsMethionine synthase activity feeds the folate and one-carbon cycles that produce S-adenosylmethionine and the nucleotide precursors for DNA synthesis; impairment causes a functional folate trap and megaloblastic, ineffective erythropoiesis. Mutase impairment leads to accumulation of methylmalonic acid and may contribute to the demyelinating neuropathy seen in deficiency. Serum homocysteine and methylmalonic acid therefore serve as functional markers of cobalamin status.
PharmacokineticsDietary B12 is released by gastric acid and pepsin, bound to haptocorrin, then transferred to intrinsic factor and absorbed via cubam receptors in the terminal ileum; this receptor-mediated route is saturable. A small fraction of any large oral dose is absorbed by passive diffusion independent of intrinsic factor, the basis for high-dose oral and parenteral repletion. Cobalamin is stored substantially in the liver, undergoes enterohepatic recirculation, and excess is excreted renally, giving body stores that buffer intake over long periods.
CaveatsRobust benefit is established for correcting true deficiency; routine supplementation in replete individuals has not shown convincing benefit for energy, mood, or cognition. Normal serum B12 does not exclude functional deficiency, and high intake can mask the hematologic signs of folate deficiency while neurological damage progresses. Regulatory status is as a recognized essential nutrient and dietary supplement; this entry is educational and not medical guidance.
Published EvidenceLoading cited studies from PubMed…
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Educational aggregation of public literature. Not medical advice and not a recommendation to use any compound. Many compounds here are not approved for human use. Consult a licensed clinician.