A Buyer's Guide to Seaweed Supplements

One of the seven products below is ours. That is the disclosure that matters, and the earlier version of this page buried it under a line about not using affiliate links. There are no affiliate links here, which is true and much less important.

We also name the other six without linking to them. You can find any of them in a search, and we would rather not run a checkout funnel to somebody else's store from our own blog. That is a commercial position rather than a principled one, and it seemed better to say so than to quietly drop them.

First, the kinds

"Seaweed supplement" covers four quite different things, and knowing which one you are buying is most of the work:

- Brown seaweed extracts, usually sold for fucoidan. This is where concentration and extraction method matter. See our guide to fucoidan. - Whole dried seaweed, milled into capsules or powder. Kelp products are usually this. Closest to eating the food, and the most variable in iodine. - Sea moss, which has become its own category. Mostly Chondrus crispus and Genus gracilaria, usually as a gel or capsule. - Algae that are not seaweed at all. Spirulina and chlorella are microalgae, and algal omega-3 comes from cultured microalgae rather than from anything harvested at sea. Filed here by the shops, not by biology. We went into that in Seaweed 101.

The seven

LIMU Original

This is ours, so weigh it accordingly. A liquid drink built on limu moui, a brown seaweed harvested off Tonga, blended with fruit juice because on its own it is not pleasant. Partner.Co's fact sheet puts a bottle at about 33 one-ounce servings, with 2 to 4 fl oz a day as the usual amount.

We think liquid preparations are generally less processed than powders and capsules, which is a preference rather than a finding. Full detail on the product page.

Athletic Greens

A comprehensive daily greens powder rather than a seaweed product, but spirulina is prominent in it. Worth knowing about because it is the best-marketed product in this category and the one most people have heard of. It is expensive, and what you are buying is a long ingredient list rather than a concentration of any one thing.

Iwi Omega-3

Algal omega-3, which is worth understanding: fish do not manufacture omega-3, they accumulate it from algae. Going to the algae directly skips the fish, and the product is vegan and free of the contamination questions that attach to fish oil.

Swanson Full Spectrum Brown Seaweed

Wakame in capsules, and not much else. On the list precisely because the label is short. If you want seaweed without a proprietary blend around it, this is the shape of product to look for.

Oregon's Wild Harvest Kelp

Atlantic kelp, milled and encapsulated. A straightforward whole-seaweed product from a company with a long track record.

Nature's Life Icelandic Kelp

North Atlantic kelp, kept whole and processed simply. Kelp is the seaweed most Westerners can picture, and it is also among the highest in iodine, which is the reason for the caution below.

Starwest Botanicals Kelp+

Kelp capsules with added iodine, aimed at people specifically after that. Which makes the next section the most important part of this page for anyone considering it.

The iodine caution, which no label on this list states well

Seaweed is the richest natural source of iodine, and kelp is at the high end of seaweed. Content varies enormously between species, between harvests and between products, and very few labels give a figure at all.

More is not better. If you have been told to watch your iodine intake, or you are under a doctor's care for your thyroid, raise this with them before adding a daily kelp capsule, not after.

This is the single most common gap in seaweed supplement marketing, this page's earlier version included.

Contamination, and what a certification actually covers

Ask an answer engine about seaweed supplements and contamination comes up in the first screen, usually as "look for lab-tested or third-party verified brands". That is good advice and it is incomplete, because it does not say what the testing covers, and the answer is often "not this".

Why seaweed in particular. The thing that makes seaweed worth eating is the thing that creates the risk. It concentrates minerals out of the water it grew in, which is why it is the richest natural source of iodine, and the same mechanism takes up whatever else is in that water.

The risk is species-specific rather than general, which is the part that gets flattened. The clearest evidence is a UK Food Standards Agency survey of 31 seaweed samples: inorganic arsenic was found only in hijiki, in all nine hijiki samples, and in none of the arame, kombu, nori or wakame tested. The FSA advises against eating hijiki and names the others as the ones to choose instead. "Seaweed is contaminated" is not a useful sentence; "which seaweed, grown where" is the question.

And a certification mark is not a blanket. Ours is a worked example rather than a boast. LIMU Original carries the Cologne List, which screens for substances on the World Anti-Doping Agency's prohibited list at a laboratory that specialises in doping analysis. That is a real, demanding test and it is not a contaminant screen. It tells a competing athlete what they need to know and it tells you nothing about arsenic or cadmium.

So the question to put to any seller on this page, ours included, is not "is it third-party tested" but "tested by whom, and for what". A seller who answers the second version knows their own supply chain. One who answers the first is quoting a badge.

The four questions worth asking

Since labels are not much help, these are:

1. Which species, and from where? "Brown seaweed" is not an answer. Vagueness usually means a broker rather than a relationship. 2. How was it processed or extracted? Most sellers cannot say. 3. Is there third-party testing, and for what? Seaweed concentrates whatever is in the water it grew in, so this matters more here than for most supplements. 4. What is the claim? If the marketing tells you what it does to a condition, that tells you about the seller. The evidence base does not support those claims for any product on this list, ours included.

What we are not going to tell you

Which one is best. It depends on what you want it for, and the honest answer for most people is that a varied diet with some seaweed in it is a better starting point than any capsule.

If you would rather gather your own, that is genuinely viable in a lot of places and we wrote about doing it on the Oregon coast.

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.