MINIREVIEW Vitamin B12 Sources and Bioavailability

The usual dietary sources of vitamin B12 are animal foods, meat, milk, egg, fish, and shellfish. As the intrinsic factor-mediated intestinal absorption system is estimated to be saturated at about 1.5–2.0 lg per meal under physiologic conditions, vitamin B12 bioavailability significantly decreases with increasing intake of vitamin B12 per meal. The bioavailability of vitamin B12 in healthy humans from fish meat, sheep meat, and chicken meat averaged 42%, 56%–89%, and 61%–66%, respectively. Vitamin B12 in eggs seems to be poorly absorbed (<9%) relative to other animal food products. In the Dietary Reference Intakes in the United States and Japan, it is assumed that 50% of dietary vitamin B12 is absorbed by healthy adults with normal gastrointestinal function. Some plant foods, dried green and purple lavers (nori) contain substantial amounts of vitamin B12, although other edible algae contained none or only traces of vitamin B12. Most of the edible blue-green algae (cyanobacteria) used for human supplements predominately contain pseudovitamin B12, which is inactive in humans. The edible cyanobacteria are not suitable for use as vitamin B12 sources, especially in vegans. Fortified breakfast cereals are a particularly valuable source of vitamin B12 for vegans and elderly people. Production of some vitamin B12-enriched vegetables is also being devised. Exp Biol Med 232:1266–1274, 2007