B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system.

The daily cobalamin (vitamin B12) requirement is 1-2 mcg. An intake of 5-20 mcg/day is usual in a Western diet. The total body can store 2-5 mg. 60% of vitamin B12 is efficiently absorbed via the ileum when it is bound to intrinsic factor (IF).

The first deficiency symptom that was discovered was anemia characterized by enlarged blood corpuscles, so-called megaloblastic anemia.

The anemia is thought to be due to problems in DNA synthesis, specifically in the synthesis of thymine, which is dependent on products of the MTR reaction. Other cell lines such as white blood cells and platelets are often also low. Bone marrow examination may show megaloblastic hemopoiesis. The anemia is easy to cure with vitamin B12.

Far more serious is the damage to the nervous system that may occur due to deficiency.

For vegans, the risk is very high because none of their natural food sources contain B₁₂. One American study found blood levels below normal in 92 % of vegans, 64 % of lacto-vegetarians, and 47% of lacto-ovo vegetarians.^[1] The study applied the old normal values, so in reality a considerably greater proportion may have been deficient.