Vitamin B-12, or cobalamin, is a water-soluble vitamin found in foods of animal origin and fortified cereals. It's necessary for red blood cell production, DNA synthesis and normal nervous system function. The Food and Nutrition Board of the Institutes of Medicine says adults need 2.4 mcg per day from food or supplements. Specific acids, enzymes and proteins are necessary for absorption of vitamin B-12.
Hydrochloric Acid and Pepsin
Vitamin B-12 is attached to protein in food, and it must be separated from the protein before it can be absorbed. The stomach produces digestive juices that contain hydrochloric acid and pepsin, which work together to release the vitamin from the proteins. The acidic environment allows vitamin B-12-binding proteins to attach to B-12 before it moves into the small intestine, along with another protein called intrinsic factor.
Pancreatic Enzymes and Intrinsic Factor
The protein-bound vitamin B-12 enters the duodenum, which is the first portion of your small intestine. Your pancreas secretes enzymes into the duodenum that release the vitamin B-12 from the binding proteins. Next, the free vitamin B-12 attaches to the intrinsic factor, which helps transport it through the walls of the ileum by a process called receptor-mediated endocytosis.
Absorption Problems
Vitamin B-12 deficiency occurs when you don't make enough stomach acid to break the vitamin away from food proteins. According to the Office of Dietary Supplements of the National Institutes of Health, vitamin B-12 absorption problems are more common in older adults and people with gastrointestinal disorders. People with pernicious anemia do not make intrinsic factor and cannot transport vitamin B-12 across the intestinal wall.
Vitamin B-12 Supplements
The vitamin B-12 in oral supplements is not bound to protein, so it doesn't require hydrochloric acid for digestion, but you still need intrinsic factor to absorb a regular dose of B-12. Information published in 2003 in "American Family Physician" says people with pernicious anemia can absorb approximately 1 percent of a large oral dose of vitamin B-12, even without intrinsic factor. Some people require B-12 injections or nasal gel. If you believe you have a B-12 deficiency, speak to your doctor before taking vitamin B-12 supplements.
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