Vitamin B12, the heaviest and most complex of all vitamins, is a water-soluble vitamin naturally present in animal foods. Cyanocobalamin is the form of vitamin B12 used medically. Vitamin B12 is essential for the formation of red blood cells, DNA synthesis, and proper neurologic function. Additionally, it acts as a cofactor in multiple physiologic pathways in humans.
Clinical deficiency of vitamin B12 usually takes years to develop because the daily requirement is small and can be adequately met in most diets (except strict vegetarians) and because the body stores vitamin B12. Clinical symptoms of deficiency are subtle and include lethargy and weakness with anorexia, glossitis, weight loss, and malabsorption in more severe deficiency. Serious complications include hematologic manifestations, such as leukopenia and thrombocytopenia. Neurologic complications include numbness and tingling in the hands and feet and can progress to peripheral neuropathy, dementia, and psychosis.
Vitamin B12 deficiency resulting from inadequate absorption is not uncommon in older adults and patients with pernicious anemia or achlorhydria. Estimates of vitamin B12 deficiency in the general population range from 1.5% to 15%, prompting some experts to suggest that food fortification with vitamin B12 might be considered, similar to fortification of flour with folic acid.
Vitamin B12 has been touted as an energy enhancer and metabolism booster.