Vitamin B12

Vitamin B12 is a water soluble vitamin that also contains a mineral in the form of cobalt. Thus, the cobalt compound cyanocobalamin is often used in proprietary vitamin B12 supplements. Unique amongst the water-soluble vitamin class, B12 cannot be synthesised by plants (animal-derived foods are best dietary sources) and appreciable amounts can be stored in the body in the liver. Via interaction with the folate group of coenzymes, vitamin B12 is crucial to the maintenance of healthy red blood cells and is also vital for the process of nerve myelination.   


Functions

Vitamin B12 has profound implications for human health, with important roles in ;

  • the formation of red blood cells
  • the metabolism of fatty acids and amino acids
  • the synthesis and regulation of DNA 
  • cell growth and development 
  • neurological function via nerve myelination 


Vitamin B12 is also integral to the prevention of the haematological disorder megaloblastic anaemia, the mechanism for which is a loss of B12-dependent folate recycling. Folate deficiency-mediated inhibition of DNA synthesis during red blood cell production results in the prevention of normal cell division and the formation of large, immature and dysfunctional red blood cells in the bone marrow (megaloblasts) and the presence of abnormal neutrophils in the circulation.   


Reference Intake (RI)

    AGE

   CHILDREN

    MALES

FEMALES

MALES

FEMALES

PREGNANCY

  YEARS

1-3

4-6

7-10

11-14

15-17

11-14

15-17

18-64

65+

18-64

65+

 

VITAMIN B12 (µg / d)

0.5

0.8

1.0

1.2

1.5

1.2

1.5

1.5

1.5

1.5

1.5

(No increment)  

 

    MONTHS

0-3

4-6

7-9

10-12

LACTATION

VITAMIN B12 (µg / d)

0.3

0.3

0.4

0.4  

  (+0.5)

 

Food Sources

PRODUCTS

   Clams (cooked)

   Liver 

  Mackerel

  Crab

(µg / 100 g)

   98.9 

   83.1

  19.0

  11.5

PRODUCTS

   Silken Tofu

   All Bran

  Milk (skimmed)

 Cheese (Swiss)

   (µg / 100 g)

   2.4 

   20

          0.5

  3.3  

 

Deficiency / Toxicity

Vitamin B12 deficiency is rare as almost all animal-derived foodstuffs are substantial sources and appreciable bodily stores are maintained in the liver, the turnover of which is extremely slow. However, deficiency has been recorded in individuals adhering to a strict vegetarian or vegan regimen, where virtually no vitamin B12 is obtained through dietary intake. 

In order to reach the circulation, vitamin B12 must bind to a protein cofactor secreted by the parietal cells of the stomach known as "instrinsic factor". Disruption of instrinsic factor secretion in the intestine results in malabsorption of the vitamin and manifests clinically as pernicious anaemia. 

Vitamin B12 exhibits low toxicity in humans with administration of therapeutic injections up to 3 mg / d displaying no adverse effects.