How does one get deficient in B12? The classic cause, most frequent in older adults, particularly those of northern European ancestry, is pernicious anemia, in which the body is unable to absorb B12 due to lack of the protein needed to allow it to pass through the intestine into the body. It was thought that dietary lack was uncommon because B12 is found in so many foods, but these are all of animal origin, and it is easy for strict vegans to become deficient if they do not take supplements. There may be decreased absorption of B12 after gastric bypass surgery or in the setting of gastrointestinal diseases such as celiac or Crohn’s diseases. Some medications, particularly the diabetes treatment metformin or long-term use of acid-suppression, may cause decreased absorption.

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