1.6.6 Vitamin B12 (cobalamin)
Vitamin B12 is yet another group of compounds, this time with an atom of the metal called cobalt (present in only trace quantities in the body) in their structure, hence the alternative name ‘cobalamin’. Vitamin B12 works alongside folate and if levels of it are low, folate deficiency symptoms occur too. It is stored in the liver and in general the body does not appear to need a regular intake. Many people have enough B12 stored in their liver to last for up to 30 years. Unlike most vitamins, vitamin B12 is found only in foods obtained from animals. In ruminant animals such as cattle and sheep, the bacteria in their stomachs synthesise vitamin B12, hence its presence in their meat, milk and dairy products. They too store it in their liver and hence eating liver is a rich source of the vitamin. Vegetarians are likely to take in sufficient vitamin B12 due to contamination of their food by yeasts and bacteria, but strict vegans may need to supplement their diet to ensure sufficient intake. If the diet contains excess amounts of vitamin C, this can bind to vitamin B12 and limit its availability. Vitamin B12 is essential for the formation of the protective coating of myelin, which is found around some nerve fibres (neurons) and so its deficiency can lead to malfunction of the nerves and eventual paralysis and dementia. Like folate, vitamin B12 is also vital for cell division, especially in the bone marrow, since it also plays a role in DNA synthesis.
Using your learning about folate and cell division, what condition might you expect to see in a person with vitamin B12 deficiency?
Since red blood cells are produced in the bone marrow, you would expect some type of anaemia to develop.
Deficiency of vitamin B12 due to dietary insufficiency is uncommon, but this vitamin can be deficient due to a condition that prevents its normal absorption. Because the B12 molecule is particularly large, in order to be absorbed by the body it has to be linked to a protein known as ‘intrinsic factor’, which is produced by the lining of the stomach. The combined ‘complex’ is then absorbed into the blood when the food reaches the small intestine. If the cells that produce intrinsic factor are destroyed, or the intrinsic factor is inactivated, vitamin B12 cannot be absorbed. This situation may occur due to an autoimmune disease in which the body produces antibodies against the cells that produce the intrinsic factor or against the intrinsic factor itself. Women are more commonly affected than men and this type of autoimmunity tends to run in families. The intrinsic factor-producing cells can also fail to function efficiently in a patient with ulcers, stomach cancer or other conditions such as Crohn's disease that affect the digestive system. The condition that results in all these cases is called pernicious anaemia, for which the treatment is regular injections of vitamin B12.
Before the availability of injectable vitamin B12, the treatment for pernicious anaemia was to feed the patient large quantities of raw liver every day. Why would this treatment be only partly effective?
The cause of pernicious anaemia is a failure in the absorption of vitamin B12 from the food, due to the lack of intrinsic factor to which it binds. Taking in large quantities of liver would increase the amount of B12 ingested, since liver is a rich source, but would not help the underlying lack of intrinsic factor. Presumably enough would be absorbed directly, without the binding factor, to have some effect in alleviating the anaemia.