It takes a few weeks of a folate-free diet to become deficient. Absorption is largely through the jejunum. Everything that walks swims or flies contains vitamin B 12 . Nothing that grows out of the ground contains vitamin B 12 . Requirements are about 1-3 mcg per day.
Absorption is highly specialised.
Vitamin B 12 is protein bound in food, and must be digested free in the stomach by acid and pepsin. Absorption takes place only through the terminal ileum, and IF is an absolute requirement. Absence of R binder does not seem to matter. Pallor, mild jaundice, fever, fundal haemorrhages in severe cases, glossitis. Specific for vitamin B 12 deficiency are paraesthesiae, and dementia.