Is there vitamin B12 in nori seaweed?

A new article by the "Watanabe group" (as Jack Norris calls them), has recently published a new article (Koseki et al. 2023) about nori seaweed containing vitamin B12, real vitamin B12.

The Watanabe group is a research group in Tottori, Japan, under the leadership of Fumio Watanabe, and over the years they have published many scientific articles reporting analyses of B12 levels in seaweeds and other plant foods.

Here are a few (my) highlights from the new paper by Koseki et al.:

The article says ...

  • "Information on the contents of [.] vitamin B12 [..] in edible seaweeds is limited [...]"
  • The analyses were conducted by means of "high-performance liquid chromatography" ... I don't see mass spectrometry listed, which would be more reliable, as far as I understand.
  • "Dried purple laver (Neopyropia yezoensis) [= nori] products contain higher levels of vitamin B12 (approximately 30-60 μg/100 g dry weight) [...] than other seaweed products [...]".
    • The sentence above contains the most important info: the amount of B12. As far as I understand, the authors make the "mistake" (?) of assuming that dry nori sheets (normal nori that the consumer is familiar with - as opposed to freshly harvested nori) is 100% "dry", i.e., 100% dry mass (maybe it's not a mistake but the authors rather think it's obvious and doesn't make much of a difference and can therefore be ignored). I'm not sure what the dry mass percentage or water content of dry nori are exactly. But I would assume that the water content is between 6 and 14%. That my guess because the water content is ~7% in "Seaweed, dried" and ~14% in "Seaweed, Canadian Cultivated EMI-TSUNOMATA, dry". That would mean that the dry mass is 100% minus that amount (water), i.e., ~86 - 94%. The B12 amount of "30-60 μg/100 g dry weight" can then be converted to 30-60 μg/116.279 g "fresh" weight ("fresh weight" as in normal dry nori sheets), which would be 0.26 - 0.52 µg/g "fresh" weight. One "typical" dried nori sheet weighs ~2.5 grams - but better double-check the packaging that you might have because nori sheets are available in many different sizes. So, the B12 content would be 0.64 - 1.29 µg/one sheet (2.5 g) of nori. The most recent dietary intake recommendation for optimizing the functional biomarkers of vitamin B12 is the one from Europe (by EFSA [2015] and the one from Germany [Ströhle et al. 2019]), which is 4 µg/day. So, this intake could - THEORETICALLY - be achieved by eating about 8 - 15 g of nori per day, i.e., about 3.5 - 6 sheets of nori per day.
    • By "seaweed products", they just mean seaweed and seasoned seaweed.
    • The term "nori" is used for a few different "similar" species of seaweed, as far as I understand. I think, all nori belongs to the genus Pyropia (also known as Neopyropia), and typically nori comes from ones of these two species: Neopyropia yezoensis (formerly Pyropia yezoensis) and Neopyropia tenera (formerly Pyropia tenera) [Niwa et al. 2020]. 
  • The authors (Koseki et al.) conclude: "These findings suggest that dried purple laver (nori) products are suitable sources of vitamin B12 [...] for humans, especially vegetarians." I don't agree. Rather, the findings indicate that nori, if eaten in large amounts, might hypothetically be a B12 source for vegetarians and vegans. However, nori should be tested in further analytical studies, with mass spectrometry, testing a wide variety of dry nori sheets (the way the consumer buys and eats them) from a wide variety of sources (different brands, different regions, different countries of origin). In addition, nori should be tested in humans (I know, a bit tricky) to see if, over the course of many months, nori can be a sufficient vitamin B12 source and can keep the two functional biomarkers of B12 in blood (methylmalonic acid [MMA] and homocysteine - the two bad guys so to speak) low. MMA and homocysteine increase in the case of suboptimal B12 status - and this isn't good. So, an adequate intake of B12 should keep MMA and homocysteine down.
  • "The red alga Neopyropia spp. [spp = species] (formerly Porphyra spp.) is one of the most commercially available edible seaweeds because it is farmed in Japan, Korea, and China."
  • "Commercially available dried Neopyropia products are called purple laver (Europe and the United States), nori (Japan), zicai (China) [紫菜, zǐcài; I was told: "海苔", hǎitái], and kim (Korea) [Wikipedia says "노리", i.e., "nori"; "김", i.e. "chim", seems to be "seaweed" in general (?)]. ... I think, everyone in Europe says "nori" too."
  • "[...] B12, which is the sole vitamin not found in plant-based food sources." Technically, this is incorrect because vitamin D and (preformed) vitamin A are not found in (!) plants either as far as I know. But that is another topic.
  • "This finding suggests that nori is the B12 source suitable for vegetarians." I don't agree because this information is not reliable enough.
  • The authors (Koseki et al.) write that different nori products assessed were: "Purple laver (Neopyropia yezoensis, previously Porphyra yezoensis) products (dried, toasted, and seasoned and toasted) [...]. [...] products were obtained from local markets in Tottori City, Japan, [in] 2022." I'm not too familiar with food in Japan but I had this idea: Is there a possibility that the nori was seasoned with fish sauce? But my silly idea is proven wrong by the table below: The higher B12 amounts were found in unseasoned (not seasoned) nori.

Figure 1 from Koseki et al. (2023): "Fig. 1. Structural formula of vitamin B and partial structures of pseudovitamin B. (1) Vitamin B [real B12] and (2) pseudovitamin B [pseudo B12]." ... This means: If the B12 molecule contains the little molecule within the dashed line (or a small number of similar one) it is real B12. But if instead of the one inside of the dashed line is replaced with the one next to the arrow (or other similar ones) then it's "fake B12", i.e., an inactive vitamin B12 analogue that does not work in humans.

The authors (Koseki et al.) also provide a table with the B12 content of different seaweeds and the different types of nori they tested in this study.

They write ...

  • "Dried purple laver (nori) products contained substantial amounts (approximately 30–60 μg B12/100 g dry weight) of B12. These values were much higher than those in other edible seaweeds (<<0.5 μg/100 g dry weight) (Table 1). Moreover, nori products did not contain pseudovitamin B12 (Fig. 1) mostly found in edible cyanobacteria [like spirulina] because only a single peak of B12 was detected during HPLC [...]." However, this too should be tested and confirmed (if true) in other studies (with mass spectrometry), i.e., that nori does NOT contain pseudo B12. Also, the statement that nori did not contain pseudo B12 seems to be contradicted (?) by their Figure 4, under which they write: "Fig. 4. HPLC chromatograms for authentic B12 and corrinoid compounds [pseudo B12] present in dried purple laver products. (A) Authentic B12. (B) Corrinoid compounds present in dried purple laver products. These are typical HPLC chromatograms of authentic B12 and corrinoid compounds present in dried purple laver products for three independent experiments."

Table 1.Total B12 and folate contents of commercially available edible seaweed products.

Total folate compounds

Total B12

(µg/100 g)

(µg/100 g)

Dried purple laver (N. yezoensis)

1309.0 ± 53.4

59.7 ± 18.2

Toasted purple laver (N. yezoensis)

1259.6 ± 46.2

58.4 ± 18.0

Seasoned and toasted purple laver (N. yezoensis)

876.8 ± 136.4

28.9 ± 11.6

Dried kombu (S. japonica)

230.3 ± 23.4

0.1 ± 0.1

Boiled and dried hijiki (S. fusiformis)

149.0 ± 30.0


Dried wakame (U. pinnatifida)

66.5 ± 27.0

0.5 ± 0.1

After each of the commercially available edible seaweed products described in the table was extracted and treated with tri-enzymes, total folate compounds were determined using the microbiological method. B12 was extracted from seaweed products, purified with a B12-immunoaffinity column, and determined using HPLC. Data are represented as mean ±± SD (n = 3). ND, not detected.


Furthermore, the authors (Koseki et al.) write ...

  • "Miyamoto et al. [Miyamoto et al. 2009][also the "Watanabe group"] reported that the B12 contents of the seasoned and toasted purple laver products were reportedly about half of the values of the dried purple laver products. Similar results were obtained in this study (Table 1). No B12 content was reduced in dried purple laver products during the toasting process [.], suggesting that the decreased B12 contents in the seasoned and toasted laver products may be due to B12 destruction caused by the interaction of various seasonings rather than the toasting process." ... Another conceivable reason could be that the seasoned nori is not 100% nori but rather part nori and part (plant) oil and spices, which are B12-free.
  • Actually the authors (Koseki et al.) write that one nori sheet weighs 3 g rather than 2.5 g. This may be typical in Japan (?): "Because the weight of one sheet (20 × 20 cm) of dried purple laver products is approximately 3 g [Watanabe et al. 1999] [...]."
  • And then they conclude: "[...] two sheets (approximately 6 g) of dried purple laver products would be sufficient to meet the recommended dietary allowance [...] of adults for B12 (2.4 μg/day)." They come to this conclusion (2 nori sheets per day) because (1) they use bigger nori sheets (3 g rather than 2.5g), (2) they assume a slightly higher B12 content (0.30-0.60 μg/g rather than 0.26-0.52 µg/g), and they assume a lower recommended daily intake (of 2.4 µg/day rather than 4 µg/day). However, even based on their assumptions, 2 sheets of nori per day, weighing 3 g each (i.e., 6 g/day), assuming the lower B12 content they found (0.30 µg/g, ignoring the "dry mass" aspect), would still be only 1.8 µg/day. That is, this would not achieve their lower recommended vitamin B12 intake of 2.4 µg/day.
  • They also comment on the iodine content of nori (we shouldn't consume too little iodine but not too much either). They suggest that "[...] dried purple laver [nori] products reportedly contained less iodine (approximately 4–8 mg [4000-8000 µg]/100 g dry weight) [they cite Watanabe et al. 1999]. Therefore, consuming two sheets (approximately 6 g/day) of dried purple laver products would not lead to an excessive intake of iodine (approximately 0.2–0.5 mg [200-500 µg]/day)." This is correct, The tolerable upper intake level for iodine, for adults, is considered 600 µg/day in Europe [EFSA 2018] and 1100 µg/day in the United States [Institute of Medicine 2010]. It's also possible that the iodine content of nori is lower, around 16 µg/g [Teas et al. 2004] (rather than the above-mentioned ~40-80 µg/g dry weight, or 36-72 µg/g normal dried nori). In another publication I found this statement that might be accurate (?): "Consumption of 4 to 6 g/d seaweed, typical for most people in Japan [...]" [Teas et al. 2009]. This would be similar to the 6 g of nori per day recommended by Koseki et al. But lower than the 8 - 15 g of nori per day, i.e., about 3.5 - 6 sheets of nori per day, that I estimated above.


Kyohei Koseki, Ryusei Yoshimura, Koki Ido, Kiho Katsuura [who analysed the B12 content of nori in this study], Tomohiro Bito, Fumio Watanabe: Determination of Vitamin B12 and Folate Compounds in Commercially Available Edible Seaweed Products, Front Biosci (Elite Ed), 2023 May 5;15(2):10. doi: 10.31083/j.fbe1502010;