Fumio Watanabe,* Yukinori Yabuta, Tomohiro Bito, and Fei Teng
Abstract
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.
Discussion
Vitamin B12 (molecular weight = 1355.4) belongs to the “corrinoids” group, which comprises compounds that contain a corrin macrocycle. The term “Vitamin B12” is usually restricted to cyanocobalamin, which is the most chemically stable and unnatural form of cobalamin [1], but Vitamin B12 refers to all potentially biologically active cobalamins in the present review. Cyanocobalamin is included in most human dietary supplements, and it is readily converted into the coenzyme forms of cobalamin, i.e., methylcobalamin functions as a coenzyme for methionine synthase (EC 2.1.1.13; involved in methionine biosynthesis), and 5′-deoxyadenosylcobalamin functions as a coenzyme for methylmalonyl-CoA mutase (EC 5.4.99.2; involved in amino acid and odd-chain fatty acid metabolism in mammalian cells) [2,3] (Figure 1). Corrinoids with a base other than 5,6-dimethylbenzimidazole as the lower ligand (cobalt-coordinated nucleotide) were recently found in certain foods and they are inactive in humans [4].
Vitamin B12 is synthesized only by certain bacteria, and it is primarily concentrated in the bodies of predators located higher in the food chain [5]. Vitamin B12 is well-known to be the sole vitamin that is absent from plant-derived food sources. Foods (meat, milk, eggs, fish, and shellfish) derived from animals are the major dietary sources of Vitamin B12 [4]. The recommended dietary allowance (RDA) of Vitamin B12 for adults is set at 2.4 μg/day in the United States (and Japan) [6,7]. The major signs of Vitamin B12 deficiency are megaloblastic anemia and neuropathy [6]. Vegetarians are at a higher risk of Vitamin B12 deficiency than non-vegetarians [8]. The frequencies of the deficiency among vegetarians were estimated as 62%, 25%–86%, 21%–41%, and 11%–90% in pregnant women, children, adolescents, and elderly subjects, respectively, by review of the 18 reports evaluating Vitamin B12 status of vegetarians [9]. The objective of this review is to present up-to-date information on Vitamin B12-containing plant-derived food sources to prevent vegetarians from developing Vitamin B12 deficiency.