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MINIREVIEW

Vitamin B₁₂ Sources and Bioavailability

School of Agricultural, Biological and Environmental Sciences, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan

To whom requests for reprints should be addressed at ¹ School of Agricultural, Biological and Environmental Sciences, Faculty of Agriculture, Tottori University, 4-101 Koyama-Minami, Tottori 680-8553, Japan. E-mail: watanabe{at}muses.tottori-u.ac.jp

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 µg 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 gastro-intestinal 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.

Key Words: vitamin B₁₂ • cobalamin • food source • bioavailability • deficiency • human