Because this vitamin is only found in animal byproducts, a B12 deficiency can be caused by a strict vegan diet. It is important for vegan patients to get regular labs and supplement this vitamin.
One of the most well known causes of a vitamin B12 deficiency is pernicious anemia, which is an autoimmune disorder where parietal cells are destroyed. Parietal cells are necessary for secreting intrinsic factor, which is crucial to properly absorb vitamin B12 in the distal ileum.
In healthy adults, vitamin B12 is broken down in the stomach and binds to intrinsic factor, which is released by the parietal cells in the stomach. The vitamin B12 and intrinsic factor complex is then absorbed in the ileum. In patients who have had gastric bypass surgery, most of the stomach and duodenum are bypassed, limiting the breakdown of vitamin B12 and subsequent binding to intrinsic factor. Without intrinsic factor, vitamin B12 absorption is significantly impaired.
Because vitamin B12 is absorbed in the distal half of the ileum, any inflammation or damage to the distal ileum can lead to malabsorption of vitamin B12.
Sprue, also called celiac disease, is an autoimmune disorder of the small intestine caused by the immune system attacking gliadin, a byproduct of gluten. In this disease, the inflammatory reaction of the small bowel leads to intestinal villous atrophy, and subsequent malabsorption of essential nutrients, including vitamin B12.
Vitamin B12 is absorbed in the distal half of the ileum and any inflammation or damage to this part of the small intestine can cause malabsorption of B12.
Crohn's disease commonly affects the distal ileum and can cause poor absorption of vitamin B12.
Diphyllobothrium latum is an intestinal tapeworm, which is contracted from consuming raw or undercooked fish. This tapeworm can cause a vitamin B12 deficiency because it can specifically absorb more than 80% of the host's B12.
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