Vitamin B12 (Cobalamin)

What it does

Vitamin B12, or cobalamin, has two headline roles: making healthy red blood cells and keeping the nervous system working. It is needed for the same DNA-building steps as folate, so a shortage causes the same kind of large-cell (megaloblastic) anaemia, leaving people tired and breathless as oxygen delivery falls. Because B12 and folate are so closely linked, a folate supplement can mask the blood signs of B12 deficiency while nerve damage continues underneath.

Its second role is protecting nerves. Vitamin B12 is needed to maintain myelin, the insulating sheath around nerve fibres that lets signals travel quickly and cleanly. A prolonged deficiency can damage this sheath, causing numbness, tingling, balance problems and, if untreated, lasting neurological harm. This is why unexplained pins-and-needles should never be ignored.

Vitamin B12 is found almost exclusively in animal foods — meat, fish, eggs and dairy — so people following a vegan diet need a reliable fortified food or supplement. Absorbing it is a two-step process needing stomach acid and a protein called intrinsic factor, so deficiency is common in older adults and in people with certain gut conditions even when their diet contains plenty. The liver stores several years' worth, which is why a deficiency can take a long time to appear.