Causes of B12 deficiency

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The immune system normally makes antibodies to attack bacteria and viruses. Pernicious anaemia is caused by an autoimmune disease, which causes the immune system to make antibodies against other parts of your body. In pernicious anaemia, antibodies are formed that attack the stomach lining and damage the cells that produce intrinsic factor. This stops intrinsic factor from attaching to B12, and so the vitamin cannot be absorbed into your body.

Another cause is where the bowel cannot absorb the vitamin B12 because it has been damaged by disease (e.g. Crohn's disease) or shortened by surgery (usually to treat bowel disease). If the bowel has been shortened by surgery, these problems can contribute to a condition known as short bowel syndrome. Short bowel syndrome is a group of problems affecting people who have had half or more of their small intestine removed. Common symptoms are diarrhoea, cramping and heartburn. Some people become malnourished because their remaining small intestine is unable to absorb enough water, vitamins, and other nutrients from food.

Occasionally, some people who follow a Vegan diet may become deficient in B12. This is because is not found in vegetable foods (such as fruit, vegetables and grains).