Vitamin B12 (Cobalamin)

Vitamin B₁₂ contains cobalt, making it the only vitamin that contains a metal ion. Vitamin B₁₂ is an essential part of coenzymes. It is necessary for fat and protein catabolism, for folate coenzyme function, and for hemoglobin synthesis. An enzyme requiring vitamin B₁₂ is needed by a folate-dependent enzyme to synthesize DNA. Thus, a deficiency in vitamin B₁₂ has similar consequences to health as folate deficiency. In children and adults vitamin B₁₂ deficiency causes macrocytic anemia, and in babies born to cobalamin-deficient mothers there is an increased risk for neural-tube defects. In order for the human body to absorb vitamin B₁₂, the stomach, pancreas, and small intestine must be functioning properly. Cells in the stomach secrete a protein called intrinsic factor that is necessary for vitamin B₁₂ absorption, which occurs in the small intestine. Impairment of secretion of this protein either caused by an autoimmune disease or by chronic inflammation of the stomach (such as that occurring in some people with H. pylori infection), can lead to the disease pernicious anemia, a type of macrocytic anemia. Vitamin B₁₂ malabsorption is most common in the elderly, who may have impaired functioning of digestive organs, a normal consequence of aging. Pernicious anemia is treated by large oral doses of vitamin B₁₂ or by putting the vitamin under the tongue, where it is absorbed into the bloodstream without passing through the intestine. In patients that do not respond to oral or sublingual treatment vitamin B₁₂ is given by injection.

Vitamin B₁₂ Relationship with Folate and Vitamin B₆

Vitamin B₁₂ and folate play key roles in converting homocysteine to amino acid methionine.  As mentioned in Section 11.5, high levels of homocysteine in the blood increases the risk for heart disease. Low levels of vitamin B₁₂, folate or vitamin B₆ will increase homocysteine levels, therefore increasing the risk of heart disease.

When there is a deficiency in vitamin B₁₂ , inactive folate (from food) is unable to be converted to active folate and used in the body for the synthesis of DNA. Folic Acid however (that comes from supplements or fortified foods) is available to be used as active folate in the body without vitamin B₁₂ .Therefore, if there is a deficiency in vitamin B₁₂ macrocytic anemia may occur. With the fortification of foods incorporated into people’s diets, the risk of an individual developing macrocytic anemia is decreased.

Dietary Reference Intakes

The RDAs and ULs for different age groups for  Vitamin B₁₂ are listed in Table 11.81 “Dietary Reference Intakes for Vitamin B₁₂“.

Table 11.81 Dietary Reference Intakes for Vitamin B₁₂^[1]

Dietary Sources

Vitamin B₁₂ is found naturally in animal products such as fish, meat, poultry, eggs, and milk products. Although vitamin B₁₂ is not generally present in plant foods, fortified breakfast cereals are also a good source of vitamin B₁₂. For the vitamin B₁₂ content of various foods, see Table 11.82 “Vitamin B₁₂ Content of Various Foods”.

Table 11.82 Vitamin B₁₂ Content of Various Foods^[2]