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.[1]

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.[2,3]

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%,[2] prompting some experts to suggest that food fortification with vitamin B12 might be considered, similar to fortification of flour with folic acid.[4,5]

Vitamin B12 has been touted as an energy enhancer and metabolism booster. These claims are based on the fact that correcting vitamin B12 deficiency should improve the associated symptoms of fatigue and weakness; however, in the absence of a nutritional deficit, vitamin B12 supplementation does not affect physical performance.[6]

No form of vitamin B12 (intramuscular, oral, or other routes) has been credibly tested as an aid to weight loss. A search of MEDLINE using the search terms "cyanocobalamin OR vitamin B12" and "weight loss OR diet" yielded no clinical studies using vitamin B12 supplements. Although no adverse effects have been associated with excess vitamin B12 intake from food and supplements in healthy people,[2] weight loss programs that promote vitamin B12, particularly in injectable form, suggest treatment that is not based on sound evidence. Since vitamin B12 deficiency can cause weight loss, supplementation theoretically would be more likely to cause weight gain with repletion.

For patients who are concerned about vitamin B12 intake, suggest dietary sources such as meat, milk, eggs, fish, and shellfish, or, in the case of strict vegetarians, oral cyanocobalamin supplements.[2] Screening for vitamin B12 deficiency with a serum vitamin B12 level is an inexpensive approach in patients worried about deficiency or those in whom deficiency is suspected.[3

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