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.

The usual dietary sources of Vitamin B₁₂ are animal-derived foods, although a few plant-based foods contain substantial amounts of Vitamin B₁₂. To prevent Vitamin B₁₂ deficiency in high-risk populations such as vegetarians, it is necessary to identify plant-derived foods that contain high levels of Vitamin B₁₂. A survey of naturally occurring plant-derived food sources with high Vitamin B₁₂ contents suggested that dried purple laver (nori) is the most suitable Vitamin B₁₂ source presently available for vegetarians. Furthermore, dried purple laver also contains high levels of other nutrients that are lacking in vegetarian diets, such as iron and n-3 polyunsaturated fatty acids. Dried purple laver is a natural plant product and it is suitable for most people in various vegetarian groups.

Vitamin B12 is synthesized only by certain bacteria and archaeon, but not by plants. The synthesized vitamin B12 is transferred and accumulates in animal tissues, which can occur in certain plant and mushroom species through microbial interaction. In particular, the meat and milk of herbivorous ruminant animals (e.g. cattle and sheep) are good sources of vitamin B12 for humans. Ruminants acquire vitamin B12, which is considered an essential nutrient, through a symbiotic relationship with the bacteria present in their stomachs. In aquatic environments, most phytoplankton acquire vitamin B12 through a symbiotic relationship with bacteria, and they become food for larval fish and bivalves. Edible plants and mushrooms rarely contain a considerable amount of vitamin B12, mainly due to concomitant bacteria in soil and/or their aerial surfaces. Thus, humans acquire vitamin B12 formed by microbial interaction via mainly ruminants and fish (or shellfish) as food sources. In this review, up-to-date information on vitamin B12 sources and bioavailability are also discussed.

Oxidative stress is implicated in various human diseases and conditions, such as a neurodegeneration, which is the major symptom of vitamin B12 deficiency, although the underlying disease mechanisms associated with vitamin B12 deficiency are poorly understood. Vitamin B12 deficiency was found to significantly increase cellular H2O2 and NO content in Caenorhabditis elegans and significantly decrease low molecular antioxidant [reduced glutathione (GSH) and L-ascorbic acid] levels and antioxidant enzyme (superoxide dismutase and catalase) activities, indicating that vitamin B12 deficiency induces severe oxidative stress leading to oxidative damage of various cellular components in worms. An NaCl chemotaxis associative learning assay indicated that vitamin B12 deficiency did not affect learning ability but impaired memory retention ability, which decreased to approximately 58% of the control value. When worms were treated with 1mmol/L GSH, L-ascorbic acid, or vitamin E for three generations during vitamin B12 deficiency, cellular malondialdehyde content as an index of oxidative stress decreased to the control level, but the impairment of memory retention ability was not completely reversed (up to approximately 50%). These results suggest that memory retention impairment formed during vitamin B12 deficiency is partially attributable to oxidative stress.