159 11.2 Vitamin B12
Vitamin B12 is unique among vitamins in that it contains an element (cobalt) and is found almost exclusively in animal products. Neither plants nor animals can synthesize vitamin B12. Instead, vitamin B12 in animal products is produced by microorganisms within the animal that the products came from. Animals consume the microorganisms in soil or bacteria in ruminant animals that produce vitamin B121. Some plant products, such as fermented soy products (tempeh, miso) and the sea algae supplement, spirulina, are advertised as being good sources of B12. However, fermented soy products are not a reliable vitamin B12 source2 and spirulina contains a pseudovitamin B12 compound that is not bioavailable3. For vegans, supplements, nutritional yeast, and fortified products like fortified soy milk can help them meet their vitamin B12 needs4.
Vitamin B12’s scientific name is cobalamin, which makes sense when you consider it contains cobalt and many amine groups, as shown in the figure below.
The other feature that is important in cobalamin is the circled R group. This is what differs between the different cobalamins, whose names and R groups are shown in the following table.
Table 11.21 Different cobalamin forms
| R Group | Name |
| CN | Cyanocobalamin |
| OH | Hydroxocobalamin |
| H2O | Aquocobalamin |
| NO2 | Nitritocobalamin |
| 5′-deoxyadenosyl | Adenosylcobalamin* |
| CH3 | Methylcobalamin* |
*Cofactor Forms
The 2 cofactor forms are adenosylcobalamin and methylcobalamin. We can convert most cobalamins into these 2 cofactor forms. Most foods contain adenosylcobalamin, hydroxocobalamin, or methylcobalamin6. The most common form found in supplements is cyanocobalamin, with some also using methylcobalamin7. Cyanocobalamin is a synthetic form of the vitamin B12.
The uptake, absorption, and transport of vitamin B12 is a complex process. The following descriptions explain, and figures illustrate, this process.
Vitamin B12 is normally bound to protein in food. Salivary glands in the mouth produce haptocorrin (formerly known as R protein), which travels with the food into the stomach. In the stomach, acid converts pepsinogen into pepsin, and the protein intrinsic factor is released from the parietal cells1,8.
As pepsin frees B12 from protein, haptocorrin binds to the newly freed vitamin B12 (haptocorrin + B12). Intrinsic factor escapes digestion and, along with haptocorrin + B12, exits the stomach and enters the duodenum1,8.
In the duodenum, pancreatic proteases break down haptocorrin, and again vitamin B12 is freed. Intrinsic factor then binds vitamin B12 (intrinsic factor + B12); intrinsic factor + B12 continues into the ileum to prepare for absorption1,8.
In the ileum, intrinsic factor + B12 is believed to be endocytosed by cubulin (aka intrinsic factor receptor), forming an endosome inside the enterocyte. Intrinsic factor is broken down in the enterocyte, freeing vitamin B12. The free vitamin B12 is then bound to transcobalamin II (TC II + B12); TC II + B12 moves into circulation8.
The liver is the primary storage site for vitamin B12. Unlike most other water-soluble vitamins, the liver is able to maintain significant stores of vitamin B12. Uptake into the liver occurs through the binding of TC II + B12 to the TC II Receptor and the endocytosis of both the compound and the receptor8. Vitamin B12 is once again freed after degradation of TC II. Vitamin B12 is primarily stored in the liver as adenosylcobalamin6,8.
The overall bioavailability of vitamin B12 is believed to be approximately 50%3, with the different cobalamin forms having similar bioavailabilities7. Sublingual supplements of vitamin B12 have been found to be equally efficacious as oral supplements7. Excretion occurs mostly through bile, with little loss in urine6.
Subsections:
11.22 Vitamin B12 Deficiency & Toxicity
References & Links
1. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.
2. Craig W, Mangels A. (2009) Position of the american dietetic association: Vegetarian diets. J Am Diet Assoc 109(7): 1266-1282.
3. Watanabe F. (2007) Vitamin B12 sources and bioavailability. Exp Biol Med 232(10): 1266-1274.
4. http://www.vrg.org/nutrition/b12.php
5. http://en.wikipedia.org/wiki/File:Cobalamin.png
6. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
7. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
8. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
9. http://commons.wikimedia.org/wiki/File:Illu_small_intestine_catal%C3%A0.png