Seaweed attracts a particular kind of confident nonsense. It is unfamiliar enough in Western diets that almost anything said about it sounds plausible, and it is sold as a supplement, which supplies the motive.
An earlier version of this page contributed to that, so we have rewritten it and flagged what it got wrong. We sell a seaweed drink, which is the reason to read the corrections below closely rather than take our word for the rest.
Iodine: true, and it cuts both ways
The fact. Seaweed is the richest natural source of iodine in the food supply, and iodine is genuinely essential. The World Health Organization still records insufficient intake in parts of Europe and much of Africa, decades after iodised salt became common.
The part usually left out. Iodine content varies wildly by species and by where the seaweed grew. Nori is modest. Kombu can be extraordinarily high, high enough that regularly eating a lot of it can push intake well past what anyone needs. Too much iodine causes problems of its own, and "it is natural" is not a dosing strategy.
Most seaweed snacks, meanwhile, contain very little. They are a snack, not a supplement, whatever the packaging suggests.
If you are under a doctor's care for your thyroid, or have been told to watch your iodine, seaweed is a conversation to have with them before it is a food to add enthusiastically.
What this page used to say, and should not have. It listed "protection from nuclear fallout" as a benefit of iodine, with a link to a CDC page. That page is about potassium iodide tablets, taken at doses far beyond anything in food, during a radiological emergency. It has nothing to do with eating seaweed. It also listed "treating infections". Neither belongs here.
What is in it, minus the superlatives
Seaweed is nutritionally dense, and unusually so for a vegetable. It carries calcium, iron, magnesium, a range of B vitamins, and it takes up trace minerals directly from seawater, which is a more complete mineral solution than most soil.
What we are not going to repeat is that it is "the most nutritious food on the planet". That was on this page. It is not a measurable statement and nobody has ever measured it.
The honest framing is narrower and more useful: seaweed supplies several things that are hard to get elsewhere in a Western diet, and it does so in a food rather than a tablet. That is a good reason to eat it. It is not a reason to expect anything specific from it.
Seaweed is not one thing, and mostly not a plant
This is where the old version was garbled, and the real answer is better.
"Seaweed" is a everyday word covering three unrelated groups of algae: green, red and brown. They are not closely related to each other. Green algae are the group that land plants descended from, so in the strict sense they sit inside the plant lineage; red and brown algae do not, and brown algae in particular are about as distantly related to a plant as a mushroom is to you.
That matters practically, because fucoidan is found in brown algae specifically. Nori is a red alga and contains none of it. Spirulina is not a seaweed at all; it is a cyanobacterium.
The oxygen claim, corrected
You will see "seaweed produces 70% of the world's oxygen" repeated everywhere, including on the previous version of this page.
What is roughly true is that marine photosynthesis produces something like half of the oxygen entering the atmosphere, and the overwhelming majority of that comes from phytoplankton, which are microscopic and free-floating, not from seaweed. Seaweed's contribution is real and small.
The distinction is worth keeping because it is the more interesting fact.
Cooking with it
The claim that seaweed is difficult to prepare is the one piece of common scepticism that really is unfounded. Dried seaweed rehydrates in minutes, kombu goes into stock, wakame goes into soup and salad, and crushed dried seaweed works as a seasoning on more or less anything savoury.
The genuine barrier is taste, not technique, and it is why most people outside coastal Asia meet seaweed as a supplement instead. If you want to try harvesting your own, we wrote a practical guide to doing it on the Oregon coast, rules and daily limits included.
Where we come into it
We sell LIMU Original, a liquid seaweed drink, and the rest of the LIMU range. If you want the detail on the compound brown seaweed is known for, and on how thin the human evidence for it still is, that is our guide to fucoidan.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.
More on this subject: Fucoidan & Seaweed