Hope of deliverance: is Mankai duckweed a good source of vitamin B12 for vegans?

Updated 2 February 2021

Short answer: we don't know yet. The B12 content may be low and the cadmium content may be too high.

Long answer:

A tiny plant called "Mankai duckweed" has made some headlines towards the end of 2020 as being the next big superfood, the superfood of 2021, an amazing source of protein, etc. All this doesn't seem all that interesting to me because vegans don't exactly lack good protein sources. What a vegan diet does unfortunately lack is reliable plant-source origin of vitamin B12.

Of course, we (vegans) are also fine using vitamin B12 supplements or foods fortified with B12 but a lot of vegans around the world still resist the idea of using "unnatural" supplements. Many would be much more open to using a "natural" "superfood" such as Mankai duckweed.

Mankai duckweed's full name is Wolffia globosa (strain Mankai; i.e. not all Wolffia globosa is "Mankai duckweed"), and it's traditionally eaten in Thailand - although many modern city-dwelling Thai people may tell you they have never eaten it in their lives. Duckweed is not an alga. It's a plant - like chlorella.

There are three things that vegans - and venture capitalists, start-up people, etc. - may want to know:

First, there already is a company in the US that sells Mankai duckweed as a superfood (eatmankai.com - I have nothing to do with them and know nothing about them). I came across them just googling after reading this article. If this (Mankai duckweed) ends up being an actual, reliable and healthy (see below) source of B12, then I'd like to see vegan-owned start-ups make a lot of money with this.

Second, before vegans can rely on Mankai duckweed as a source of vitamin B12 we would need to know (1) how much B12 does it contain, (2) what type of B12 (methylcobalamin, adenosylcobalamin, hydroxycobalamin) it contains, (3) what other, inactive corrinoids (fake B12, inactive B12 analogues, inactive in humans) it contains, and (4) how consistent these amounts of these molecules are present (i.e. how large the variability of B12 and fake B12 content is from one batch to the next).

Partly these questions have already been answered. Sela et al. (2020) have tested Mankai duckweed (see table below).

You can find some more info on Jack Norris' website veganhealth.org.

Third, and this is quite important, Mankai duckweed - and Wolffia globosa in general and other Wolffia species - is known as a cadmium accumulator, i.e. a plant that accumulates cadmium from the water it grows in. Cadmium has no function in the human body, and we really want as little as possible cadmium in our foods - especially vegans who (in our polluted world) maybe be exposed to more cadmium from food than non-vegans. This seems to be a very good reason to not consume Mankai duckweed, but I imagine that at least theoretically it should be possible to grow Mankai duckweed in cadmium-free water or at least in very low-cadmium media that would keep the cadmium content extremely low.

"The results indicated that Wolffia is a strong Cd accumulator [...]." (Xie et al. 2013)

The company eatmankai.com has given me some feedback regarding cadmium (see table below).

Table: vitamin B12 and cadmium content in Mankai duckweed (what is known so far)

	Sela et al. (2020)*	Info by the company eatmankai.com (personal communication 30 January 2021)
B12 content	~0.5 μg per 100 g of frozen Mankai duckweed (as used in the study as an ingredient in a shake) # This would not be a lot of B12 for vegans. A daily amount of ~2 x 2 µg/day from foods may be more suitable for vegans.	-
B12 types	OH-B12 Adenosyl-B12 CN-B12	-
Consistency of B12 content	consistent in different seasons (spring, summer, autumn, winter); variability see “B12 content”	-
Fake B12 content	“no pseudo B12 was detected”	-
Consistency of fake B12 content	-	-
Cadmium content	not tested	<1 µg/g; typical levels 0.01 µg/g dry matter (according to the company eatmankai.com); this would mean a typical content of 0.1 µg/100 g frozen Mankai duckweed – but up to 99 µg of cadmium!) §
Consistency of cadmium content	-	no information
Cobalamin: vitamin B12 = active in humans Adenosyl-B12: 5-deoxyadenosylcobalamin CN-B12: cyanocobalamin OH-B12: hydroxocobalamin # Vitamin B12 content found in Mankai duckweed: 1.6 – 3.8 µg B12 per 100 g dry weight. 100 g dry weight are equivalent to 500 g frozen Mankai duckweed as used in the study by Sela et al. (2020). § An amount of 0.1 µg of additional cadmium per day (i.e. per 100 g of frozen Mankai duckweed) seems to be an acceptable amount. But if the specifications allow amounts of up to 99 µg of cadmium in the daily serving size (i.e. 100 g of frozen Mankai duckweed), this - if this would really occur - would be very high. Vegans may already have a higher intake of cadmium, and the tolerable upper intake of cadmium – an amount we should not regularly exceed – is 25 µg per day (EFSA) or 58 µg per day (WHO/FAO) (from all food sources combined) for a human being with a body weight of 70 kg (EFSA 2012, Satarug et al. 2017). * Note that this Mankai duckweed was grown in an “axenic” medium, i.e. in a medium free from any other living organisms (including bacteria) apart from the Mankai duckweed itself. However, it is assumed that the B12 in Mankai duckweed is produced by bacteria that live inside the Mankai duckweed cells. So, these would also be present.