Vitamin B12

By Hasina Akhter, Ph.D.


Vitamin B12 is one of the nine water-soluble B vitamins. It is also known as cobalamin or cyanocobalamin as it contains cobalt, the essential trace element to all animals. Methylcobalamin and 5-deoxyadenosylcobalamin are other forms of this vitamin that are active in human metabolism (12).


Monot and Murphy first reported Vitamin B12’s effectiveness for the treatment of pernicious anemia, a fatal disease where body cannot produce red blood cells due to deficiency of this vitamin. Thereafter it was isolated in crystalline form from the liver by Folkers and Smith in 1948. Its structure was then determined by the crystallographic work of Noble laureate Dorothy Hodgkin in 1956. (3) Another Noble laureate Robert Woodward synthesized this vitamin in the laboratory in 1971.


The structure of vitamin B12 is based on a corrin ring having cobalt in the center. Four of the six coordination sites are corrin ring, the fifth one is dimethylbenzimidazole group, the sixth one is the center of reactivity which is variable, can be cyano or hydroxyl or a methyl or a 5’-deoxyadenosyl group to yield four different forms of this vitamin i.e.

cyanocobalamine, hydroxolcobalamine, methylcobalamine and adenosylcobalamine respectively (4).


Vitamin B12 is naturally found in animal sources such as meat, beef liver, fish, poultry, eggs, milk and dairy products. However, fortified breakfast cereals are good sources of Vitamin B12 and thus ideal foods for vegetarians. Some nutritional yeast products also have this vitamin.


0-6 months 0.4 µg 0.4 µg
7-12 months 0.5 µg 0.5 µg
1-3 years 0.9 µg 0.9 µg
4-8 years 1.2 µg 1.2 µg
9-13 years 1.8 µg 1.8 µg
14+ years and older 2.4 µg 2.4 µg 2.6 µg 2.8 µg

Most importantly, as 30% of older individuals (>50 years) may poorly absorb vitamin B12 from their diet, they should receive either foods fortified with this vitamin or supplements to meet their daily requirements.


  • Patients with pernicious anemia or gastrointestinal (GI) disorders cannot properly absorb this vitamin in the GI tract (812).

  • Individuals who have had gastrointestinal surgery may loss part of stomach or distal ileum and become unable to absorb or release of food-bound vitamin B12 (1315).

  • Older people with atrophic gastritis, a condition that decreases secretion of hydrochloric acid in the stomach, have poor absorption of this vitamin (2, 1618).

  • Pure vegetarians including pregnant and breastfeeding mothers who fully restrict animal foods are at a greater risk as vitamin B12 is naturally of animal origin (2, 1920). Infants of these vegan mothers may develop this vitamin deficiency and should be treated immediately to prevent serious neurological problems.

FUNCTIONS (12, 8, 21)

  • Vitamin B12 is essential for keeping nerve cells healthy.
  • Together with vitamin B9 or folic acid, this vitamin helps in the production of red blood cells, the vital component of blood.
  • It functions as a co-factor of enzymes involved in methionine synthesis. This amino acid is required to produce s-adenosylmethionine (SAM), a universal methyl donor for the production of our genetic material, DNA and RNA. SAM is also needed for the synthesis of other hormones, proteins and lipids.


  • In infants, vitamin B12 deficiency may cause developmental problems as well as megaloblastic anemia, a blood disorder characterized by large and abnormally shaped red blood cells (22).
  • Adults suffer from megaloblastic anemia and its associated symptoms such as fatigue, poor appetite, weight loss, paleness (8, 23).
  • Vitamin B12 deficiency may also cause neurological problems such as numbness and tingling in the hands and feet, muscle weakness, as well as memory loss . Patients with such problems may not have anemia.


Since it is a water soluble vitamin the excess amounts could be readily excreted in the urine and no toxicity reports have been detected.


Pernicious anemia

Taking supplements of vitamin B12 is effective for the treatment of this red blood cells disorder.

Cognitive function and dementia

Vitamin B12 deficiency leads to accumulation of homocysteine level in the blood and strong association between elevated levels of homocysteine and incidence of Alzheimer’s disease and dementia has also been reported . However, further research is needed to show that this vitamin has independent effect on cognitive function.

Cardiovascular diseases

Elevated levels of homocysteine, the sulfur containing amino acid derived from methionine, has been identified as a potential risk factor for cardiovascular

disease . Vitamin B12 together with folate and vitamin B6 are involved in homocysteine metabolism. Moreover, vitamin B12 deficiency causes a rise in homocysteine levels. Several studies show that vitamin B12 together with folate and vitamin B6 can decrease homocysteine levels in young, adult women suffering from diabetes or vascular disease . Another study on older men and women show that a multivitamin supplement for 8 weeks causes a decline in homocysteine levels . However, based on several other large prospective studies, the American Heart Association has concluded that the available evidence is inadequate to support the role of vitamin B12 or other B vitamins in reduction of cardiovascular risk.

Cyanide poisoning

For the treatment of cyanide poisoning, US Food and Drug Administration (FDA) approved the use of hydroxocobalamine.

Eye disease

Report shows that vitamin B12 with folate and vitamin B6 might prevent the age related macular degeneration.

Sports and training

Vitamin B12 rich drinks are also given to athletes to boost up performances by correcting fatigue and weakness.


Metformin Diabetes Reduce absorption of vitamin B12
Renitidine, Famotidine,Cimetidine Peptic ulcer disease Reduce absorption of vitamin B12
Omeprazole, Lansoprazole Peptic ulcer disease Reduce absorption of vitamin B12


This vitamin functions as a cofactor of two enzymes, methionine synthase and L-methylmalonyl-CoA mutase. The first enzyme catalyzes the formation of methionine from homocysteine and methionine is essential in the formation of S-adenosylmethionine, which in turn is involved in the synthesis of DNA, RNA, hormones, proteins and lipids. The second enzyme, L-methylmalonyl-CoA mutase catalyzes an essential biochemical reaction in fat and protein metabolism as well as hemoglobin synthesis . Hemoglobin of blood carries oxygen to different parts of the body. In this way, vitamin B12 is supplying energy as well as oxygen to the athletes and is a good choice for them to replenish the depleted energy after strenuous workout or sports.


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