Essential for nerves and red-cell production; deficiency causes anaemia and neurological symptoms.
What it is
Serum vitamin B12 (cobalamin), often ng/L or pmol/L depending on the lab. Active B12 or MMA may be used when results are borderline.
Vitamin B12 (cobalamin) from animal foods or fortification is absorbed with intrinsic factor in the terminal ileum. Serum B12 assays vary; borderline results may lead to active B12, MMA, or antibody tests.
Why it matters
Low B12 may relate to diet (vegan/vegetarian without fortification), absorption problems including pernicious anaemia, or medicines such as metformin and PPIs. Neurological symptoms (pins and needles, balance, memory change) need prompt clinical care, do not wait on DIY supplements alone if severe.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
B12 is a cofactor for DNA synthesis and myelin maintenance. Deficiency produces megaloblastic anaemia and can cause irreversible nerve injury if prolonged.
B12 cofactors support methionine synthase and methylmalonyl-CoA mutase: DNA synthesis and odd-chain fatty acid metabolism. Deficiency injures myelin and produces megaloblastic anaemia; nerve injury can become permanent if delayed.
For athletes
Plant-based endurance athletes should plan fortified foods or supervised supplementation and periodic checks. Nitrous oxide recreational use depletes B12 functionally: relevant in some sports social scenes.
Vegan/vegetarian athletes need a planned B12 source. Metformin users should mention it to clinicians. Neurological symptoms (paraesthesia, ataxia, cognitive change) need prompt care, not watchful waiting on supplements alone.
