The parietal cells in your stomach produce a protein called intrinsic factor. In order for you to properly absorb vitamin B-12, intrinsic factor must bind to the vitamin in the stomach and carry it to the small intestine. The villi in the small intestine absorb the vitamin and deposit it into the bloodstream, where it becomes available to your cells. Any disruptions in this process can lead to problems with proper vitamin B-12 absorption.
Lack of Intrinsic Factor
One of the most common reasons for a vitamin B-12 absorption problem is a lack of intrinsic factor. In some cases, a lack of intrinsic factor develops as a result of an abnormal autoimmune response. During an autoimmune response, your immune system releases antibodies that mistakenly interpret normal structures, in this case the parietal cells in the stomach, as potentially harmful organisms. The antibodies attach to the parietal cells and attempt to destroy them. As a result, the parietal cells can no longer produce intrinsic factor. Without this important protein, your body cannot absorb vitamin B-12 regardless of the amount of the vitamin you consume.
A lack of intrinsic factor can also occur as a result of surgical removal of all or part of the stomach or prior surgery that resulted in damage to the parietal cells. In rare cases, a child may be born without the ability to produce intrinsic factor, a condition referred to as congenital pernicious anemia.
Digestive Disorders
You may also have a problem absorbing vitamin B-12 if you have a digestive disorder that interferes with proper nutrient absorption in the small intestine, which is the area of the digestive system that absorbs vitamin B-12. Celiac disease causes damage to the intestine's villi. Damaged villi cannot absorb nutrients. Other possible disorders that can lead to vitamin B-12 absorption problems include Crohn's disease and ulcerative colitis.
Malabsorption in the Small Intestine
Other factors may also decrease the small intestine's ability to properly absorb vitamin B-12. Your small intestine normally contains beneficial bacteria that aid in the digestion process. If the wrong types of bacteria grow and multiply in your small intestine, it can lead to a decreased ability to absorb vitamin B-12. The National Heart, Lung and Blood Institute notes that this is most common in older adults. Tapeworm infections, certain medications and surgical removal of all or part of the small intestine may also lead to an inability to absorb vitamin B-12.
Considerations
If you cannot properly absorb vitamin B-12, no amount of vitamin B-12 in the diet will prevent a deficiency. The only way for you to get the proper amounts of vitamin B-12 is through intravenous injections, which allow vitamin B-12 to bypass the stomach and small intestine and enter the bloodstream directly.
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