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Vitamin B12 bioavailability significantly decreases with increasing intake of this vitamin per meal. Vitamin B12 is partially degraded and loses its biological activity during cooking and storage of foods.

The intrinsic factor-mediated gastrointestinal absorption system in humans has evolved to selectively absorb active vitamin B12 from naturally occurring vitamin B12 compounds, including its degradation products and inactive corrinoids. This absorption system is estimated to be saturated at about 1.5 - 2.0 mcg of cobalamin (B12) per meal, 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 it. Most of the edible blue-green algae (cyanobacteria) used for human supplements predominantly contain pseudovitamin B12, which is inactive in humans.

The bioavailability of vitamin B12 in healthy humans from fish meat, sheep meat, and chicken meat averaged 42%, 56%-89%, and 61%-66%, respectively, in eggs it seems to be poorly absorbed (< 9%). 

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