Vitamin<unk>  cannot be produced by the human body, and must be obtained from the diet. When foods containing<unk>  are eaten, the vitamin is usually bound to protein and is released by proteases released by the pancreas in the small intestine. Following its release, most<unk>  is absorbed in the ileum, the last part of the small intestine, after binding to a protein known as intrinsic factor. Causes of deficiency are categorized as impaired absorption, inadequate dietary intake, or increased requirements. Inadequate dietary intake is not common, as the liver can normally store the vitamin for three to five years. Impaired absorption *Impaired absorption of vitamin<unk>  leads to pernicious anemia a type of megaloblastic anemia. Pernicious anemia comes about from the lack of gastric intrinsic factor produced by parietal cells in the stomach, and needed in the ileum for the absorption of vitamin<unk> . Pernicious anemia is the most common cause of<unk> . Any disruption that leads to the loss of parietal cells can lead to malabsorption, and includes atrophic gastritis, a condition that often affects the elderly, and gastric surgeries that involve the removal of all or part of the stomach, such as Roux-en-Y gastric bypass surgery. Surgical removal of the small bowel such as in Crohn's disease results in short bowel syndrome and the inability to absorb vitamin<unk> . Impaired absorption can also result from blind loop syndrome where an overpopulation of bacteria in the small intestine absorb the vitamin. Some infections such as giardiasis, and diphyllobothriasis caused by parasites can also cause malabsorption. * Forms of achlorhydria (including that artificially induced by drugs such as proton pump inhibitors and histamine 2 receptor antagonists) can cause<unk>  malabsorption from foods, since acid is needed to split<unk>  from food proteins and salivary binding proteins. This process is thought to be the most common cause of low<unk>  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<unk>  in supplements such as multivitamins, since it is not bound to proteins, as is the<unk>  in foods. * Long-term use of ranitidine hydrochloride may contribute to deficiency of vitamin<unk> . * Untreated celiac disease may also cause impaired absorption of this vitamin, probably due to damage to the small bowel mucosa. In some people,<unk>  may persist despite treatment with a gluten-free diet and require supplementation. * The diabetes medication metformin may interfere with<unk>  dietary absorption. * A genetic disorder, transcobalamin II deficiency can be a cause. * Nitrous oxide exposure, and recreational use. * Chronic exposure to toxigenic molds and mycotoxins found in water damaged buildings. *B12 deficiency caused by Helicobacter pylori was positively correlated with CagA positivity and gastric inflammatory activity, rather than gastric atrophy. Inadequate intake Inadequate dietary intake of animal products such as eggs, meat, milk, fish, fowl (and some type of edible algae) can result in a deficiency state. Vegans, and to a lesser degree vegetarians, are at risk for<unk>  deficiency if they do not consume either a dietary supplement or vitamin-fortified foods. Children are at a higher risk for<unk>  deficiency due to inadequate dietary intake, as they have fewer vitamin stores and a relatively larger vitamin need per calorie of food intake. Increased need Increased needs by the body can occur due to AIDS and hemolysis (the breakdown of red blood cells), which stimulates increased red cell production.
Vitamin B12 deficiency
Vitamin A deficiency
Copper deficiency
Biotin deficiency
Megavitamin-B6 syndrome
Vitamin E deficiency
Fluorine deficiency
Zinc deficiency