Vitamin B-12, or cobalamin, is an important component for several metabolic reactions in the body. Although Vitamin B-12 deficiency is rare, it can result in anemia and serious cases of neuropathy and dementia. Daily recommendations for adults are 2.4 micrograms a day. B-12 is typically found in animal sources.
Vitamin B-12 Use in the Body
Vitamin B-12 acts in different enzymatic pathways. These pathways are involved in DNA synthesis, amino acid metabolism, and energy metabolism. While these pathways occur in all cells, they are especially prevalent in cells in the gastrointestinal tract, bone marrow and nervous tissue.
The recommended dietary allowance of vitamin B-12 is 2.4 micrograms in men and women over the age of 14. A 3-ounce hamburger contains the daily recommended amount of vitamin B-12. Vitamin B-12 is only found in animal products. Good sources include oysters, crab, fish, pork, poultry, eggs, milk, yogurt, and cottage cheese. It is often added to cereals and nondairy milk as well. Many of these items are common in diets, which is why deficiency is rare although it can be seen in strict vegetarians.
Vitamin B-12 digestion requires several steps. B-12 is bound to proteins when food is eaten. Once it enters the stomach, enzymes and stomach acid liberate it from these proteins, and it binds to R proteins released from gastric juices. R proteins help guide B-12 to the small intestine, and also prevent vitamin B-12 from being used by bacteria in the gut. In the small intestine, pancreatic enzymes break down R proteins. Vitamin B-12 then binds to intrinsic factor, which guides B12 to be absorbed by the small intestine cells, or enterocytes. From the enterocytes, B-12 then enters the blood stream to be used in metabolic pathways.
Deficiency is often caused not by a lack of B-12 in the diet, but rather impaired absorption. Too much or too little gastric juices, a lack of pancreatic enzymes, or a reduction in size of the small intestine, either through surgery or autoimmune diseases like celiac disease, can cause vitamin B-12 deficiency. When deficiency occurs, several events take place. Serum B-12 levels drop, followed by a decrease in stores, followed by a decrease in DNA synthesis. In the early stages, large, immature red blood cells appear in the blood stream. This is known as macrocytic megaloblastic anemia and can be ameliorated with vitamin B-12 supplementation. If deficiency continues, peripheral neuropathy or loss of sensation in the limbs can occur. This cannot be corrected by additional B-12 supplementation. Deficiency can also result in loss of concentration, memory loss, disorientation, fatigue and even dementia.
- The University of Maryland Medical Center: Vitamin B12 (cobalamin)
- Medscape: Megaloblastic Anemia
- Advanced Nutrition and Human Metabolism, 5th Edition; Gropper, Sareen, Smith, Jack, Groff, James