Summary A review paper by the Watanabe group suggests that vegans can rely on nori for vitamin B12. I strongly advise against this.
There is a group of researchers in Japan who regularly publish papers in scientific journals about plant sources of vitamin B12. Fumio Watanabe is often the lead researcher, so I refer to them as the “Watanabe group.”
Some of their papers analyze the B12 in foods such as mushrooms, algae, and black tea, while other papers are just review articles of previous research. The latter is the case with their latest paper, Vitamin B12-containing Plant Food Sources for Vegetarians, published in the May 5, 2014 issue of Nutrients (1). A free version can be obtained at the link.
When the Watanabe group analyzes a food for B12, they often find molecules that they believe to be the vitamin. But a complication with simply finding B12 in food is that the food might also contain inactive B12 analogues that interfere with active B12. The Watanabe group is well aware of this and often analyzes the food for some of the typical inactive B12 analogues. Sometimes they feed the food to rats to see if it lowers the rats’ methylmalonic acid (MMA) levels, the prime indicator of B12 activity. Based on how much active B12 and inactive analogues they find, and any results with rats, they make recommendations as to whether a food can provide B12 for vegans.
In their latest review, based on the results of their various experiments combined with a study in which six vegan children stayed healthy eating large amounts of seaweed (my analysis here), they suggest that nori is a “suitable” source of B12 for vegans.
The biggest flaw in this theory is that there is a study that tested raw and dried nori using the gold standard of lowering MMA levels in humans (2), and although the authors of this study were optimistic about raw nori, the fact was that both dried and raw nori reduced B12 status in their subjects.
I often hear from people who say they have been vegan for some time, have not supplemented with B12, and are not B12 deficient. They take this to mean that vegans don’t need a supplemental source of B12. In most cases they do not know whether, in fact, they are B12 deficient or not, because they haven’t been appropriately tested for deficiency. And once you go vegan without a source of B12, you never know when deficiency symptoms might kick in – someone can be fine for years and then one day they start to feel tingling in their fingers or toes or they become severely fatigued. You don’t want to end up like any of the vegans listed in the Individual Cases of Deficiency.
There is also the problem of subclinical B12 deficiency where someone doesn’t feel any symptoms but has mild deficiency for years that can possibly develop into dementia or a stroke. As the Watanabe group says in their latest paper:
“However, Vitamin B12 deficiency may go undetected in vegetarians because their diets are rich in folic acid, which may mask vitamin B12 deficiency until severe health problems occur. Vitamin B12 deficiency contributes to the development of hyperhomocysteinemia, which is recognized as a risk factor for atherothrombotic and neuropsychiatric disorders, thereby negating the beneficial health effects of a vegetarian lifestyle.”
This does not mean that vegans need to get tested for B12 deficiency. On the contrary, I don’t see any need for that unless you decide not to supplement with B12 according to the recommendations here or you are supplementing but experiencing symptoms of B12 deficiency.
Whenever I post about B12, it is inevitable that someone, I suspect an internet troll in many cases, will pipe in to say that cyanocobalamin is not a good source of B12 but that methylcobalamin is. Except in very rare cases, this is not true (see Methylcobalamin & Adenosylcobalamin for more information). If you decide to rely on methylcobalamin, I recommend at least 1,000 µg per day.
If this post interested you, you might also want to read my August 2013 post about the Watanabe group, B12 in Plants and Algae Update.
In conclusion, there is no new evidence to suggest that nori is a reliable source of B12 for vegans and I advise against relying on it.
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1. Watanabe F, Yabuta Y, Bito T, Teng F. Vitamin B12-containing plant food sources for vegetarians. Nutrients. 2014 May 5;6(5):1861-73. | link
2. Yamada K, Yamada Y, Fukuda M, Yamada S. Bioavailability of dried asakusanori (porphyra tenera) as a source of Cobalamin (Vitamin B12). Int J Vitam Nutr Res. 1999 Nov;69(6):412-8. | link