Vitamin B12

What is Vitamin B12?

Vitamin B12 (cobalamin) is a water-soluble vitamin found exclusively in animal products — meat, fish, dairy, eggs — and the only vitamin that requires a digestive enzyme for absorption. That enzyme, intrinsic factor, is produced by cells lining the stomach wall. When those cells are damaged — by atrophic gastritis, stomach surgery, or long-term use of proton pump inhibitors (PPIs) — absorption falls even when the diet contains sufficient B12. In the blood, B12 circulates bound to two proteins: transcobalamin (the active transport protein that delivers B12 to cells) and haptocorrin (which carries a biologically less active fraction). The standard blood test measures total B12 — both fractions combined — in pmol/L or pg/mL (1 pg/mL ≈ 0.738 pmol/L). That total value is a useful starting point but can mislead: some people with a normal total B12 still have a cellular deficiency, while others with a low total B12 function perfectly well. The most sensitive and specific test for a true cellular deficiency is methylmalonic acid (MMA): it rises specifically when the mitochondrial B12 cofactor is absent, even when serum B12 looks formally normal. B12 operates through two central enzymatic functions: building myelin (the protective sheath around nerves) and converting homocysteine to methionine in the methylation cycle. For that second function, B12 and folate work together — a deficiency in either one raises homocysteine.

Why is Vitamin B12 relevant?

B12 is worth tracking for several reasons, but the most urgent is the slow, insidious nature of a deficiency. The body's reserve — stored mainly in the liver — is large enough that a diet without animal products, or impaired absorption, can continue for years before clinical signs appear. But once neurological damage begins, it can be partially irreversible before it is noticed. Measuring early, even without symptoms, makes a material difference in at-risk groups. The groups at elevated risk are well defined. Vegans and strict vegetarians get little to no B12 from diet. Long-term PPI users reduce the gastric acid needed to release B12 from food proteins. Metformin — the most widely prescribed drug for type-2 diabetes — actively blocks B12 absorption in the gut and causes a gradual deficiency in a proportion of long-term users. Older adults progressively lose intrinsic-factor production. And people with coeliac disease or inflammatory bowel disease have a reduced absorptive reserve in the small intestine. The third reason is the link with homocysteine. Elevated homocysteine — even without anaemia or neurological symptoms — has been associated with cardiovascular risk, cognitive decline, and kidney damage. B12, together with folate, is one of the most straightforwardly addressable causes of elevated homocysteine. Always read the two markers together.

Vitamin B12 high or low — what it means

Reference ranges for total B12 vary considerably between laboratories — as a broad orientation, values above 200–250 pmol/L (roughly 270–340 pg/mL) are generally considered adequate, but any borderline result in the 150–300 pmol/L range deserves additional assessment. Add MMA and homocysteine to the panel when in doubt: a raised MMA (> approximately 271 nmol/L, lab-dependent) is strong evidence of a functional cellular deficiency, even when total B12 appears formally normal. For someone in a risk group — vegan, PPI user, metformin user, over 60 — who reports tingling or numbness in the hands or feet, unexplained fatigue, memory complaints, or mood changes, it is worth looking beyond total B12 alone and adding MMA and homocysteine. Together they give the most reliable picture of actual B12 status. After starting supplementation or injections, serum B12 can rise rapidly and markedly, even before cellular recovery is complete. Re-test MMA or homocysteine after three to six months for an honest view of functional recovery — not the serum level. Oral B12 (cyanocobalamin or methylcobalamin) works well for most people, including those with reduced intrinsic-factor production, provided the dose is sufficient (500–1000 µg/day); for severe malabsorption or confirmed intrinsic-factor deficiency, intramuscular injections are the more reliable route.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.