Cobalamin is a family of complex molecules, consisting of a
cobalt-containing tetrapyrrole ring and side nucleotide chains
attached to the cobalt atom.4 It is synthesized by anaerobic
bacteria and is found in foods of animal origin (e.g., fish,
meat, dairy products, and eggs), as well as fortified
cereals.5–7 The Recommended Daily Allowance (RDA) of vitamin B12
is 2.4 μg per day (mcg/day) for persons over the age of 14
years. In the United States, the average daily adult dietary
intake of vitamin B12 is about 5 mcg–30 mcg, of which only 1
mcg–5 mcg are effectively absorbed, given its complex absorption
process. It is estimated that only 50% of dietary vitamin B12 is
absorbed by healthy adults.7 Defects at any step of the
absorption process can cause cobalamin deficiencies of varying
degrees; 50%–90% of cobalamin stores in the body (3 mg–5 mg) are
located in the liver. These stores help delay, often for up to 5
years, the onset of clinical symptoms due to insufficient
cobalamin absorption. Dietary cobalamin is