Normally, B12 is absorbed through the ileum with the help of intinsic factor which is secreted by the parietal cells.
Various things can stop this happening - antibiodies to parietal cells, like in atrophic gastritis, antibodies to intrinsic factor like in pernicious anaemia, lack of a terminal ileum, or disease of the ileum - like Crohn's disease, and reduced b12 intake.
It takes a while for the disease to become apparent as there's 3-5 years worth of reserves in the liver.
Lack of B12 means that DNA replication is slow, so cells divide less but grow bigger. This gives you your macrocytic or megaloblastic anaemia.
Lack of B12 means the conversion of homocysteine to methionine is reduced, so there are high levels of homocysteine - causing things like atherosclerosis, thromboembolism and osteoporosis.
More importantly, B12 is a cofactor in the conversion of methylmalonic acid into succinyl CoA. Without this, the dorsal and lateral spinal columns are demyelinated, through an unknown mechanism, so you get neurological symptoms.
FBC will show macrocytic anaemia, and maybe neutropaenia and thrombocytopenia.
A blood film will show anisocytosis and poikilocytosis.
Bilirubin may be increased because of haemolysis.
Then you can look for autoantibiodies, and maybe an absorption test like the Schilling test.
Symmetrical, legs > arms
Loss of position sense
Loss of vibration sense
Low grade pyrexia, weight loss, diarrhoea, jaundice due to haemolysis, pallor due to anaemia, premature greying of the hair.
If B12 levels are low, then patients need intramuscular B12 and a haematology referal. If there is neurological involvement, 5 - 6 loading doses of 1000mcg, followed by maintainence of 1000mcg every three months.
If B12 levels are borderline, then oral B12 may have a reponse and be diagnostic.
Patients often feel better within 24hours of starting treatment.
If patients are asymptomatic, there is debate about their treatment- monitoring seems preferred.
B12 is naturally found in animal products - fish, meet, eggs, milk. It's not generally present in plant foods, but is in fortified cereals. It's worth mentioning that pabrinex doesn't contain B12!