Vitamin B12 Deficiency - Causes

Causes

• Inadequate dietary intake of vitamin B₁₂. Vitamin B₁₂ occurs in animal products (eggs, meat, milk) as well as in some algae, such as Chlorella and Susabi-nori (Porphyra yezoensis). Vegans and vegetarians to a lesser degree may be at risk for B₁₂ deficiency due to inadequate dietary intake of B₁₂, since they may not be aware of the few non-animal dietary sources of vitamin B₁₂. However a 2000 Tuft study found not eating meat was not correlated with lower levels. Children are at a higher risk for B₁₂ deficiency due to inadequate dietary intake, as they have fewer vitamin stores and a relatively larger vitamin need per calorie of food intake.

• Selective impaired absorption of vitamin B₁₂ due to intrinsic factor deficiency. This may be caused by the loss of gastric parietal cells in chronic atrophic gastritis (in which case, the resulting megaloblastic anemia takes the name of "pernicious anemia"), or may result from wide surgical resection of stomach (for any reason), or from rare hereditary causes of impaired synthesis of intrinsic factor.

• Impaired absorption of vitamin B₁₂ in the setting of a more generalized malabsorption or maldigestion syndrome. This includes any form of structural damage or wide surgical resection of the terminal ileum (the principal site of vitamin B₁₂ absorption).

• Forms of achlorhydria (including that artificially induced by drugs such as proton pump inhibitors) can cause B₁₂ malabsorption from foods, since acid is needed to split B₁₂ from food proteins and salivary binding proteins. This process is thought to be the most common cause of low B₁₂ in the elderly, who often have some degree of achlorhydria without being formally low in intrinsic factor. This process does not affect absorption of small amounts of B₁₂ in supplements such as multivitamins, since it is not bound to proteins, as is the B₁₂ in foods.

• Surgical removal of the small bowel (for example in Crohn's disease) such that the patient presents with short bowel syndrome and is unable to absorb vitamin B₁₂. This can be treated with regular injections of vitamin B₁₂.

• Long-term use of ranitidine hydrochloride may contribute to deficiency of vitamin B₁₂.

• Coeliac disease may also cause impaired absorption of this vitamin, though this is due not to loss of intrinsic factor, but rather damage to the small bowel.

• Some bariatric surgical procedures, especially those that involve removal of part of the stomach, such as Roux-en-Y gastric bypass surgery. (Procedures such as the adjustable gastric band type do not appear to affect B₁₂ metabolism significantly).

• Bacterial overgrowth in parts of the small bowel are thought to be able to absorb B₁₂. An example occurs in so-called blind loop syndrome.

• The diabetes medication metformin may interfere with B₁₂ dietary absorption.

• Hereditary causes such as severe MTHFR deficiency, homocystinuria, and transcobalamin deficiency.

• Some studies have shown that giardiasis, or similar parasitic infections may be a cause of vitamin B₁₂ deficiency.

• Malnutrition of alcoholism.

• Nitrous oxide abuse.