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.
Various types of edible algae are consumed worldwide as food sources. Dried green laver (Enteromorphasp.) and purple laver (Porphyra sp.) are the most widely consumed edible algae, and they contain substantial amounts of Vitamin B12 (approximately 63.6 μg/100 g dry weight and 32.3 μg/100 g dry weight, respectively)  (Figure 2). However, excluding these two genera, other edible algae contain zero or only traces of Vitamin B12 . To determine whether dried purple and green lavers contain Vitamin B12 or inactive corrinoids, the algal corrinoid compounds were purified and confirmed as Vitamin B12[55,56]. A substantial amount (133.8 μg/100 g dry weight) of Vitamin B12 was found in dried Korean purple laver (Porphyra sp.), but seasoned and toasted laver products contain lower amounts of Vitamin B12(approximately 51.7 μg/100 g dry weight) . In particular, when the dried purple laver was treated by toasting until the laver’s color changed from purple to green, the decreases in the Vitamin B12 contents of the seasoned and toasted laver products were not due to the loss or destruction of Vitamin B12 during the toasting process . In vitro gastrointestinal digestion experiments indicated that the estimated digestion rate of Vitamin B12 from dried purple laver was approximately 50% at pH 2.0 (as a model of normal gastric function). The release of free Vitamin B12 from the purple laver significantly decreased to approximately 2.5% at pH 7.0 (as a model of severe atrophic gastritis) . Edible purple laver predominantly contains coenzyme forms (5′-deoxyadenosylcoblamin and methylcobalamin) of Vitamin B12 or hydroxocobalamin (or both) [57,58,59].
To measure the biological activity of Vitamin B12 in lyophilized purple laver (Porphyra yezoensis), the effects of laver feeding were investigated in Vitamin B12-deficient rats . Urinary methylmalonic acid excretion was undetectable within 20 days of initiating a diet supplemented with dried purple laver (10 μg of Vitamin B12/kg diet), and the hepatic Vitamin B12 (especially coenzyme Vitamin B12) levels significantly increased. These results indicate that Vitamin B12 obtained from purple laver is bioavailable in rats. A nutritional analysis of six vegan children who had consumed vegan diets including brown rice and dried purple laver (nori) for 4–10 years suggested that the consumption of nori may prevent Vitamin B12 deficiency in vegans . Our preliminary study indicated that similar dried purple laver products that are available in local markets in Taiwan (Hong-mao-tai, Bangia atropurpurea) and New Zealand (Karengo, a mixture of P. cinnamonea and P. virididentata) contained 28.5 ± 3.9 and 12.3 ± 1.9 μg of Vitamin B12 per 100 g weight, respectively (Figure 2).
For a long time, it was unclear whether algae have an absolute requirement for Vitamin B12 for growth, and why algae that lack a requirement of Vitamin B12 for growth contain substantial amounts of Vitamin B12. However, recent biochemical and bioinformatics studies have accurately defined the Vitamin B12requirements of various algae (half of all algal species absolutely require Vitamin B12 for their growth), and they have suggested possible physiological functions for Vitamin B12 in algae [61,62].
Furthermore, the standard tables of food composition in Japan  indicate that dried purple laver (per 100 g) contains various other nutrients that are lacking in vegetarian diets, such as Vitamin A (3600 μg of Vitamin A equivalent as provitamin A), iron (10.7 mg), and n-3 polyunsaturated fatty acids (1.19 g), as well as Vitamin B12 (77.6 μg). Purple laver also contains a large amount of a pigment protein, phycoerythrin, which is digested in the intestine to release the covalently linked chromophore moiety, a phycoerthrobilin compound (a potent antioxidant) .