What Causes Low B12 Levels?

Vitamin B12 belongs to the group of eight vitamins referred to as the B-complex vitamins. It is water-soluble, which means that it dissolves in water in the body. Vitamin B12 plays important roles in DNA synthesis, nerve signal transmission and formation of red blood cells, according to the Office of Dietary Supplements. If left untreated, low levels of B12 in the body can eventually lead to a lack of red blood cells, called pernicious anemia, and permanent nerve damage 1.

Lack of Intrinsic Factor

Intrinsic factor is a protein produced by the cells of the stomach, called the parietal cells. When vitamin B12 is consumed, it must bind to intrinsic factor in order to be absorbed by the small intestine. Without the presence of intrinsic factor, the body cannot physically absorb the vitamin. One of the most common causes of a lack of intrinsic factor is an autoimmune disorder that causes the destruction of the parietal cells in the stomach. When the parietal cells are destroyed, they are unable to produce and release intrinsic factor. A lack of intrinsic factor can also develop as a side effect of prior surgeries performed to remove all or a portion of the stomach. In rare cases, a child may be born without the ability to produce intrinsic factor, according to the National Heart Lung and Blood Institute 1.


After vitamin B12 is ingested, it moves from the stomach into the small intestine where it is absorbed into the bloodstream. Any condition or factor that disrupts the proper absorption process in the small intestine can also cause low B12 levels in the body. An overgrowth of bacteria in the small intestine is a common cause of low B12 levels in older adults, according to the National Heart Lung and Blood Institute 1. The excess bacteria consume the B12 before the body is able to absorb it. Digestive diseases, such as celiac disease and Crohn’s disease, may also result in malabsorption of B12 in the small intestine. Certain medications, surgical removal of part of the small intestine and a tapeworm infection can also cause low B12 levels.

Inadequate Dietary Intake

Inadequate dietary intake of vitamin B12 is the least common reason for low B12 levels; however, it is possible. Strict vegetarians and vegans are considered the highest risk group because meat, poultry, eggs, fish and dairy are the best dietary sources of vitamin B12. Alcoholics also make up a high-risk group for low levels of vitamin B12 because they often do not consume enough food to sustain levels of B12 in the body.