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