Medical Diagnosis Exams

Vitamin B12, also referred to as cobalamin, is a complex organometallic compound in which a cobalt atom is situated within a corrin ring. It is a water-soluble vitamin which is synthesised by microorganisms. It cannot be synthesised in the human body and is seldom found in products of plant origin.

Main sources of vitamin B12 are meat, fish, eggs and dairy products. The uptake in the gastrointestinal tract depends on intrinsic factor, which is synthesised by the gastric parietal cells, and on the cubam receptor in the distal ileum.

The most frequent cause of severe vitamin B12 deficiency is a lack of intrinsic factor due to autoimmune atrophic gastritis. The disease is historically called pernicious anaemia, even though many patients present with mainly neurologic manifestations.

Examples of other causes for vitamin B12 deficiency are malabsorption due to gastrectomy, inflammatory bowel disease or dietary deficiency, e.g. in strict vegetarians (vegans).

Vitamin B12 is the cofactor for two enzymes, methionine synthase and methylmalonyl CoA mutase. Methionine synthase, located in the cytoplasm, requires vitamin B12 in the form of methylcobalamin and catalyses the conversion of homocysteine to methionine, an essential amino acid.

In the form of 5deoxyadenosylcobalamin, vitamin B12 is also required for the mitochondrial enzyme methylmalonyl CoA mutase, which converts methylmalonyl CoA to succinyl CoA. This is a step in the oxidation of oddchain fatty acids and catabolism of ketogenic amino acids.

Thus, vitamin B12 is important for DNA synthesis, regenerating methionine for protein synthesis and methylation, as well as for the development and initial myelination of the central nervous system (CNS) and for the maintenance of normal CNS function.

Vitamin B12 deficiencies are common in wealthier countries principally among the elderly and are most prevalent in poorer populations. In general the prevalence increases with age. Vitamin B12 deficiency impacts red blood cell synthesis, resulting in megaloblastic anaemia due to abnormal DNA synthesis.

In addition it impairs neurological function, in particular demyelination of nerves in part due to abnormal methylation, leading to peripheral neuropathy, dementia, poor cognitive performance, and depression.

Other effects of vitamin B12 deficiency or depletion are increased risk of neural tube defects, osteoporosis, cerebrovascular and cardiovascular diseases. Early diagnosis is essential, because of the latent nature of this disorder and the risk of permanent neurological damage.