An 85 year old man had a preoperative assessment for a knee replacement. His full blood count was normal apart from haemoglobin 95 g/l and mean corpuscular volume 105 fl. He drank no alcohol. Further testing showed that his vitamin B-12 was low: 90 pmol/l (reference range 160-800 pmol/l). Folate, ferritin, thyroid stimulating hormone, and liver function tests were normal. He had no other medical or surgical history and ate a balanced diet that includes meat.

Vitamin B-12 is found only in foods of animal origin. Dietary B-12 is freed from food protein by pepsin in the acid gastric environment and binds to haptocorrin, a protein secreted in saliva. In the small intestine, haptocorrin is degraded by pancreatic enzymes; vitamin B-12 is released and binds with intrinsic factor, which is secreted by gastric parietal cells. This complex binds to receptors at the terminal ileum and is actively absorbed. A small fraction (1-2%) of the intake of B-12 is passively absorbed …