Vitamin B12 deficiency causes

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Causes

• Inadequate dietary intake of vitamin B₁₂. As the vitamin B₁₂ occurs naturally only in animal products (eggs, meat, milk) a vegan diet can produce a deficiency unless one uses supplements or eats enriched food.

• Selective impaired absorption of vitamin B₁₂ due to intrinsic factor deficiency. This may depend on loss of gastric parietal cells in chronic atrophic gastritis (in which case, the resulting megaloblastic anaemia takes the name of "pernicious anaemia"), or on wide surgical resection of stomach (such as in bariatric surgery), or on rare hereditary causes of impaired synthesis of intrinsic factor. It takes years to develop deficiency after dietary absorption stops.

• Impaired absorption of vitamin B₁₂ in the setting of a more generalised 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), as well as bacterial overgrowth (such as in blind loop syndrome).

• Chronic intestinal infestation by the fish tapeworm Diphyllobothrium, that competes for vitamin B₁₂, seizing it for its own use and therefore leaving insufficient amount for the host organism. This is mostly confined to Scandinavia and parts of Eastern Europe (for example, in preparers of gefilte fish, who would acquire the tapeworm by sneaking bits of uncooked fish while making the Eastern European delicacy, now eaten by Jews at Pesach).

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

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