The two main causes of vitamin B12 deficiency are decreased intake and decreased absorption.
Vitamin B12 deficiency occurs most commonly in older adults. This is likely related to low gastric acid levels, which can decrease the absorption of dietary B12. Additionally, a lack of intrinsic factor (IF) — a protein that transports vitamin B12 through the stomach to the intestine — will result in poor absorption of vitamin B12. Over time, a vitamin B12 deficiency will develop, and this will lead to decreased red blood cell production. This condition is called pernicious anemia.
Other reasons for decreased absorption are atrophic gastritis and inflammatory bowel diseases such as Crohn’s disease. Some medications can cause a decrease in the absorption of B12, such as metformin, colchicine, and antacids.[1][2][3]
Decreased intake of B12 can also happen in the elderly, who tend to have lower appetites and less varied diets. Vegetarians and vegans often need to supplement vitamin B12, since the dietary sources are mostly animal products. Any situation that decreases intake, such as severe nausea or a psychiatric cause of poor intake, could result in a B12 deficiency.[4][5]