Does Vitamin B12 Promote Weight Loss?

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. 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.

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, 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. 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.