Vitamin B12Cobalamin

Vitamin B12 (cobalamin) is an essential vitamin the body cannot make, obtained almost entirely from animal foods or supplements. It is needed to build healthy red blood cells and to keep the nervous system working.

It matters because a shortfall causes fatigue, a particular type of anaemia and, if prolonged, nerve problems. Deficiency is more common in people eating little or no animal produce, in older adults and where absorption is impaired.

In practice, B12 is read alongside folate and the full blood count — a shortfall of either can enlarge red cells (raising MCV) — and is corrected with diet, supplements or, where absorption is the problem, injections.