Vitamin B12

Vitamin B₁₂

Structure of vitamin B₁₂

Structurally, vitamin B₁₂ consists of a corrin nucleus attached with 5,6 dimethyl benzimidazole moitey, an aminopropanol unit, a ribose unit and a phosphate group. A cobalt atom (Co) is present at the centre of the corrin ring structure. One of the valencies of cobalt is filled by either CN- (cyano cobalamin) or H₂O (aqua cobalamin) or OH- (hydroxo cobalamin) or CH₃ (methyl cyano cobalamin).

Functions

Of all vitamins, vitamin B₁₂ is the most complex. The trace mineral cobalt is an essential part of the molecule. Vitamin B₁₂ is required for the maturation of red blood cells in the bone marrow and for the synthesis of proteins.

Sources

Milk, eggs, cheese, meet, fish and poultry supply ample amounts of vitamin B₁₂. Plant foods supply no vitamin B₁₂ and use of an exclusively vegetarian diet for a long period of time will lead to symptoms of deficiency.

Requirements

Early estimates of vitamin B₁₂ requirements were based on the amounts required to maintain normal RBC maturation in patients with pernicious anemia due to lack of intrinsic factor secretion. There is a considerable enterohepatic circulation of vitamin B₁₂. It is secreted in the bile and re-absorbed in the small intestine. However, in patients with defective secretion of intrinsic factor, the vitamin cannot be re-absorbed, but is excreted in the feces. This means that patients with impaired secretion of intrinsic factor have much higher requirement for vitamin B₁₂ than normal.

The average requirement of vitamin B12 is 3 μg/day.

 Infants         -   0.3 μg / day

 Children         -   1 - 2 μg / day

 Adults         -   3 μg/day

 Pregnant and lactating women   -   4 μg / day

Absorption and Storage

For the absorption of vitamin B₁₂ from the intestines, a factor called “Intrinsic Factor” (IF) secreted by the stomach is essential. Vitamin B₁₂ is stored in fair amounts in the liver.

Deficiency

Pernicious anemia is the disease resulting from vitamin B₁₂ deficiency. It is a genetic defect with the absence of intrinsic factor, hence the vitamin B₁₂ in the diet cannot be absorbed. Since vitamin B₁₂ is important for the maturation of red blood cells the deficiency of this vitamin leads to the formation of macrocytic red blood cells.