[bio] [mcg] Watanabe: "Vitamin B12 Sources and Bioavailability"

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Vegetables.Many studies have been performed to measure vitamin B₁₂ content in various vegetables. For decades, edible bamboo shoots have been believed to contain considerable amounts of vitamin B₁₂. However, it turned out that they do not contain appreciable amounts of vitamin B₁₂; however, certain compounds showing vitamin B₁₂–like activity (known as the alkali-resistant factor) were found in them (44). Similar results were found in cabbage, spinach, celery, garland chrysanthermum, lily bulb, and taro (44). Only trace amounts of vitamin B₁₂ (< 0.1 μg/100 g of wet weight edible portion), which was estimated by subtracting the alkali-resistant factor from total vitamin B₁₂, were found in broccoli, asparagus, Japanese butterbur, mung bean sprouts, tassa jute, and water shield (44). These vegetables may have the ability to take up vitamin B₁₂ found in certain organic fertilizer.Mozafar (45) demonstrated that the addition of an organic fertilizer, cow manure, significantly increases the vitamin B₁₂ content in barley kernels and spinach leaves. Mozafar and Oeftli (46) investigated uptake of vitamin B₁₂ by soybean roots under water culture conditions. Sato et al. (47) reported that a high level of vitamin B₁₂ is incorporated into a vegetable, kaiware daikon (radish sprout), by soaking its seeds in vitamin B₁₂ solutions before germination. The amount of vitamin B₁₂ incorporated into kaiware daikon increases up to about 170 μg/100 g of wet sprout with 3-hr soaking of seeds in 200 μg/ml vitamin B₁₂ solution. These vitamin B₁₂–enriched vegetables may be of special benefit to vegans or elderly persons with food-bound vitamin B₁₂ malabsorption.

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Tea Leaves and Tea Drinks.Considerable amounts of vitamin B₁₂ are found in various types of tea leaves: green (0.1–0.5 μg vitamin B₁₂ per 100 g dry weight), blue (about 0.5 μg), red (about 0.7 μg), and black (0.3–1.2 μg) tea leaves (48).When a corrinoid compound was isolated from Japanese fermented black tea (Batabatacha), the compound was identified as vitamin B₁₂ (49). When vitamin B₁₂–deficient rats were fed this tea drink (50 ml/day, equivalent to a daily dose of 1 ng vitamin B₁₂) for 6 weeks, urinary methylmalonic acid excretion (an index of vitamin B₁₂ deficiency) of the tea drink–supplemented rats decreased significantly compared with that of the deficient rats (49). These results indicate that the vitamin B₁₂ found in the fermented black tea is bioavailable in rats. However, only 1–2 liters of consumption of fermented tea drink (typical regular consumption in Japan), which is equivalent to 20–40 ng vitamin B₁₂, is not sufficient to meet the RDA of 2.4 μg/day for adult humans.

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Edible Algae.Various types of edible algae are used for human consumption the world over. Dried green (Enteromorpha sp.) and purple (Porphyra sp.) lavers (nori) are the most widely consumed among the edible algae and contain substantial amounts of vitamin B₁₂(32 to 78 μg/100 g dry weight; Ref. 39). In Japanese cooking, several sheets of nori (9 × 3 cm; about 0.3 g each) are often served for breakfast. A large amount of nori (< 6 g) can be consumed from certain sushi, vinegared rice rolled in nori. However, edible algae other than these two species contain none or only traces of vitamin B₁₂ (39). Dagnelie et al. (53) reported the effect of edible algae on the hematologic status of vitamin B₁₂–deficient children, suggesting that algal vitamin B₁₂ appears to be nonbioavailable. As bioavailability of the algal vitamin B₁₂ is not clear in humans, my colleagues and I characterized corrinoid compounds to determine whether the dried purple and green lavers and eukaryotic microalgae (Chlorella sp. and Pleurochrysis carterae) used for human food supplements contain vitamin B₁₂ or inactive corrinoids. My colleagues and I found that these edible algae contain a large amount of vitamin B₁₂ without the presence of inactive corrinoids (54–57).To measure the bioavailability of vitamin B₁₂ in the lyophilized purple laver (Porphyra yezoensis), the effects of feeding the laver on various parameters of vitamin B₁₂were investigated in vitamin B₁₂–deficient rats (58). Within 20 days after vitamin B₁₂–deficient rats were fed a diet supplemented with dried purple laver (10 μg vitamin B₁₂/kg diet), urinary methylmalonic acid excretion became undetectable and hepatic vitamin B₁₂ (especially coenzyme vitamin B₁₂) levels significantly increased. These results indicate that vitamin B₁₂ from the purple lavers is bioavailable in rats.A nutritional analysis for the dietary food intake and serum vitamin B₁₂ level of a group of six vegan children aged 7 to 14 who had been living on a vegan diet including brown rice for 4 to 10 years suggests that consumption of nori may keep vegans from suffering vitamin B₁₂ deficiency (59). Rauma et al. (60) also reported that vegans consuming nori and/or chlorella had a serum vitamin B₁₂ concentration twice as high as those not consuming these algae.

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Edible Cyanobacteria.Some species of the cyanobacteria, including Spirulina, Aphanizomenon, and Nostoc,are produced at annual rates of 500–3000 tons for food and pharmaceutical industries worldwide (61). Tablets containing Spirulina sp. are sold as a health food fad, since it is known to contain a large amount of vitamin B₁₂ (62). We found that commercially available spirulina tablets contained 127–244 μg vitamin B₁₂ per 100 g weight (63). When two corrinoid compounds were characterized from the spirulina tablets, the major (83%) and minor (17%) compounds were identified as pseudovitamin B₁₂(adeninly cobamide) and vitamin B₁₂, respectively (Fig. 2⇓). Several groups of investigators indicated that pseudovitamin B₁₂ is hardly absorbed in mammalian intestine with a low affinity to IF (64, 65). Furthermore, researchers showed that spirulina vitamin B₁₂ may not be bioavailable in mammals (63, 66). Herbert (67) reported that an extract of spirulina contains two vitamin B₁₂ compounds that can block the metabolism of vitamin B₁₂. And van den Berg et al. (68) demonstrated that a spirulina-supplemented diet does not induce severe vitamin B₁₂ deficiency in rats, implying that the feeding of spirulina may not interfere with the vitamin B₁₂metabolism. Further studies are needed to clarify bioavailability of spirulina vitamin B₁₂ in humans.

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Vitamin B₁₂–Fortified Cereals.Ready-to-eat cereals fortified with vitamin B₁₂ are known to constitute a great proportion of dietary vitamin B₁₂ intake in the United States (7). Several groups of investigators suggested that eating a breakfast cereal fortified with folic acid, vitamin B₁₂, and vitamin B₆ increases blood concentrations of these vitamins and decreases plasma total homocysteine concentrations in elderly populations (73). Fortified breakfast cereals have become a particularly valuable source of vitamin B₁₂ for vegetarians and elderly people.