Vitamin B-12, or cobalamin, is a member of the B-complex family. Even though it is water soluble -- excess intake is easily eliminated in your urine -- your liver stores enough vitamin B-12 to last for up to six years, according to "The Merck Manual of Diagnosis and Therapy." Vitamin B-12 is the only vitamin that contains an essential mineral -- cobalt -- and it possesses the most complex molecular structure of all vitamins. Absorption of food-based vitamin B-12 involves a coordinated series of intestinal events.
Vitamin B-12 must first be liberated from foods through the actions of gastric acid, which is secreted by parietal cells in your stomach lining. Once freed, B-12 is quickly bound by R proteins -- another product of the cells lining your stomach -- which transport vitamin B-12 into your small intestine. Pancreatic enzymes in the upper part of your small intestine cleave R proteins from vitamin B-12, which is immediately recaptured by intrinsic factor, yet another protein secreted by your gastric parietal cells. The intrinsic factor-B12 complex, or IF-B12, is eventually absorbed when it binds to special receptors in the lower end of your small intestine.
A number of intestinal problems can interrupt vitamin B-12 processing and prevent its absorption into your bloodstream. If your stomach stops producing acid or intrinsic factor -- common occurrences in older people, in people taking certain ulcer medications and in a condition called pernicious anemia -- vitamin B-12 is not efficiently liberated from your foods or absorbed through your lower intestine. Other conditions, such as Crohn's disease and celiac disease, interfere with the uptake of IF-B12. The January 2011 issue of "The New England Journal of Medicine" describes genetic disorders that alter the IF-B12 receptors in your intestinal wall and prevent B-12 absorption.