Vitamin B12 is found in several forms known as "cobalamins" due to the presence of the element cobalt. This water-soluble vitamin plays many essential and important roles in the body, including in red blood cell formation, energy metabolism, DNA synthesis and neurological function.
Sources
Vitamin B12 is naturally produced by microorganisms within the bodies of animals. The best and most reliable sources of dietary cobalamins are meat and meat products, poultry, fish, shellfish--especially clams and oysters--eggs--especially the yolk--and dairy products. Food products that are fortified with non-protein-bound Vitamin B12, industrially synthesized by fermenting bacteria, are also good sources.
Vitamin B12 Digestion and Absorption
Dietary vitamin B12 is linked to proteins found in food. The link must be cleaved by hydrochloric acid and a gastric enzyme called pepsin once the food enters the stomach. Vitamin B12 that is found in supplements or fortified food does not require this step. In the stomach, the cobalamins are bound to a non-enzymatic protein called R protein. The bound vitamin travels to the small intestine. At this time, a glycoprotein called intrinsic factor, or IF, is secreted by the gastric parietal cells. Both the R protein-B12 complex and IF enter the small intestine, where enzymes break down the R protein and leave B12 free to bind with IF. The cobalamin-IF complex then travels to the ileum, the furthest part of the small intestine, where it is absorbed. B12 can be absorbed anywhere along the ileal surface, but absorption occurs mainly in the last third.
Risk for Deficiency
Given that stomach acid and intrinsic factor are both needed for the successful digestion and absorption of vitamin B12, the elderly--since stomach acid secretion naturally declines with age--in addition to individuals with pernicious anemia, an autoimmune disorder that results in the decline of gastric tissue and parietal cells and thus intrinsic factor, can be at risk for deficiency. Individuals with pancreatic insufficiency can also be at risk for deficiency, given the reduction of pancreatic enzymatic action on the cleavage of the R-protein-B12 complex, leaving the B12 unavailable to bind with IF and be absorbed. In addition, anyone who has a gastrointestinal disorder, such as Crohns, or has had a surgery that either removes or bypasses the stomach or distal ileum is at risk for deficiency. The absorption of vitamin B12 found naturally in food requires an intact and functioning stomach, exocrine pancreas, intrinsic factor and small bowel. Those who do not regularly eat animal foods may also be at risk if they do not include fortified food products or nutritional supplements in their diet.
Signs and Symptoms of Deficiency
Some signs of deficiency include megaloblastic anemia, characterized by large immature red blood cells--though this can be masked by high folic acid intake--weakness, fatigue, shortness of breath, insomnia, loss of concentration, tingling and numbness in extremities, soreness of the mouth and tongue, confusion, dementia, loss of concentration, memory loss and disorientation. It is important to note that neurological symptoms can occur without anemia and that early detection and treatment is needed to avoid irreversible damage.
References
- "Advanced Nutrition and Human Metabolism," 4th Edition; S. Gropper, J. Smith and J, Groff; 2005
- National Institutes of Health: Dietary Supplement Fact Sheet, Vitamin B12
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