Vitamin B12 requirements
Most estimates of vitamin B12 requirements are based on the amounts given parenterally to maintain normal health in patients with pernicious anemia due to a failure of vitamin B12 absorption. This overestimates normal requirements, because of the enterohepatic circulation of vitamin B12; in people lacking intrinsic factor, or secreting anti-intrinsic factor antibodies, the vitamin that is excreted in the bile will be lost in the feces, whereas normally it is almost completely reabsorbed.
The total body pool of vitamin B12 is of the order of 2.5 mg (1.8 μmol), with a minimum desirable body pool of about 1 mg (0.3 μmol). The daily loss is about 0.1% of the body pool in subjects with normal entero-hepatic circulation of the vitamin; on this basis requirements are about 1–2.5 μg/day and reference intakes for adults range between 1.4 μg and 2.0 μg.