Vitamin B-12 is a water soluble vitamin naturally present in animal proteins such as meat, poultry, fish, and, to a lesser extent, milk. It is required for the proper development and function of the brain, nerves, blood cells and other parts of the body. The recommended daily allowance, or RDA, of vitamin B12 for adults and youth 14 years or older is 2.4 mcg, according to the Food and Nutrition Board at the Institute of Medicine. This requirement is usually amply satisfied by a typical Western Diet.
Mechanism of Absorption
Vitamin B-12 that is naturally occurring in food requires the activity of hydrochloric acid and the enzyme gastric protease in the stomach to be released from the protein for absorption. B-12 that is added to fortified foods, such as cereals, and to dietary supplements is already in free form and does not require this separation process. The next step in the absorption process for all B-12 is its binding to intrinsic factor, which is a glycoprotein secreted by parietal cells in the stomach lining. The resulting B-12-intrinsic factor complex is then absorbed in the part of the intestine known as the distal ileum.
B12 Deficiency
The most common causes of B-12 deficiency in the general population are pernicious anemia and food-bound vitamin B-12 malabsorption, which commonly afffects the elderly. Pernicious anemia is an autoimmune disorder that results in destruction of the stomach cells by one's own antibodies and causes decreased secretion of the acid and enzymes needed to release food-bound B-12 and decreased production of intrinsic factor needed for B-12 absorption. Food-bound B-12 malabsorption is a result of atrophic gastritis, which is a chronic inflammation of the stomach lining that reduces stomach acid production. Because intrinsic factor is usually still available in these individuals, they can still absorb B-12 from fortified foods and dietary supplements, according to the Linus Pauling Institute.
Symptoms of B12 Deficiency
Symptoms of B-12 deficiency may take five to 10 years to appear because the body stores a large amount of B-12. One of the most serious complications of B-12 deficiency is megaloblastic anemia, or a lack of mature, normal-size red blood cells. Symptoms include fatigue, shortness of breath, weakness, and confusion. Neurologic complications of vitamin B-12 deficiency can result in numbness or persistent tingling in your hands and feet, irritability, forgetfulness, confusion, or other personality changes. Without treatment for B-12 deficiency, neurologic complications can become permanent.
Treating B12 Deficiency
The traditional treatment for B-12 deficiency has been intramuscular injection of B-12 in the form of cyanocobalaimin, according to a 2009 report by Drs. Dali-Youcef and Andres in the "Quarterly Journal of Medicine." But studies conducted by their research group and others, as well as an evidence-based review of the medical literature by the Cochrane Colalboration in 2005 demonstrate that replacement with oral vitamin B-12 is effective, even when the cause of deficiency is pernicious anemia, or lack of intrinsic factor. The reason this works is that approximately 1 to 5 percent of B-12 in free form is absorbed along the entire intestine by a process called passive diffusion, which does not require intrinsic factor. Therefore, large doses of between 1,000 and 2,000 micrograms of B-12 given daily at first, then weekly, have resulted in absorption of sufficient B-12 to normalize blood levels and improve symptoms of deficiency.
References
- MedlinePlus: Vitamin B12; November 19, 2010
- "Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin and choline"; Institute of Medicine; 1998
- "Quarterly Journal of Medicine"; An Update on Cobalamin Deficiency in Adults; N. Dali-Youcef, et al.; January 2009
- Linus Pauling Institute: Vitamin B12; Jane Higdon; March 2003
- NIH Office of Dietary Supplements: Vitamin B12
- MayoClinic.com: Vitamin Deficiency Anemia; March 2011
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