What is bladderwrack? A plain guide

Bladderwrack (Fucus vesiculosus) is a brown seaweed that grows on rocky shorelines in the North Atlantic, and the dried whole plant is sold as powder, flakes, capsules and tea. The name comes from the air-filled bladders on its fronds, not from any link to the human bladder. It is also a source of iodine, in amounts that vary widely from one lot to the next.

What bladderwrack is

Bladderwrack is a seaweed, not a land plant, so it has no root, stem or leaves in the botanical sense. It anchors to rock with a holdfast and has flat, branching fronds with a midrib; pairs of small air bladders along the fronds keep it afloat in the water. That is where the name comes from, and why searches such as "bladderwrack for bladder" or "bladderwrack root" are built on a misreading of the name.

It grows on rocky shores in the intertidal zone of the North Atlantic, the North Sea and the Baltic, and on parts of the Pacific coast. Where it is harvested matters, because seaweed takes up minerals, including metals, from the water around it.

What is bladderwrack powder?

Bladderwrack powder is the dried whole plant, milled. The National Institutes of Health's LactMed monograph on seaweed describes bladderwrack as the dried whole plant and says it contains soluble fibre (alginic acid), vitamin B12, iron and iodine, and can also contain heavy metals such as arsenic, cadmium and lead. The same page notes that supplements often differ from what their labels say.

Whole-plant powder is a different thing from a bladderwrack extract. Extracts are concentrated for particular compounds, such as fucoidan or polyphenols, and the human trials that used bladderwrack mostly used extracts. What those trials found is covered in bladderwrack benefits: what the research does and doesn't show.

Bladderwrack is not kelp, but labels blur the two

"Kelp" properly means large brown seaweeds such as Laminaria. Supplement labels and some medical papers use "kelp" loosely, and LactMed lists Fucus species and Ascophyllum nodosum (also called egg wrack or rockweed) together under bladderwrack. The difference matters because iodine content differs enormously between seaweeds. A survey of 12 seaweed species found iodine from 16 mcg per gram in nori to over 8,165 mcg per gram in processed Laminaria kelp granules, and an Italian analysis of 14 seaweeds found Laminaria digitata highest in iodine, arsenic and cadmium.

Often confused with	What it is
Kelp	Large brown seaweeds (Laminaria and relatives), generally much richer in iodine
Rockweed / egg wrack	Ascophyllum nodosum, a related brown seaweed that is sometimes sold under the bladderwrack name
Dulse	A red seaweed (Palmaria palmata)
Sargassum	Another brown seaweed genus, used in some trials alongside bladderwrack
Spirulina	A blue-green algae (a cyanobacterium), not a seaweed
Burdock root	The root of a land plant; see what is burdock root

The iodine catch

Iodine is needed to make thyroid hormone. A review of excess iodine found that high intakes are tolerated by most healthy people, but in some, too much may precipitate thyroid over- or under-activity, goitre or thyroid autoimmunity, and that people with existing thyroid disease may be more susceptible. Seaweeds and iodine-containing supplements are both named as sources.

A Norwegian analysis of 96 seaweed products found iodine in supplements ranging from 5 to 5,600 mcg per daily dose, and species and iodine labelling that was inadequate or inaccurate for several products. For a plant like bladderwrack, that means the milligrams on a bottle say little about the iodine in it. See bladderwrack side effects for what has been reported.

Can you eat bladderwrack, and what does it taste like?

Bladderwrack is edible and has been eaten, usually dried and rehydrated or powdered into other foods. Expect a strong, briny, ocean flavour. Eating it whole is not the low-risk option it might seem: in the Norwegian analysis, one portion of a whole-food seaweed product (across all species) ranged from 128 to 62,400 mcg of iodine.

Questions people ask

What does bladderwrack do for the body?

Nutritionally, it supplies iodine, fibre and some minerals. Beyond that, human trials of bladderwrack are few, mostly used concentrated extracts, and mostly found little or nothing; the details are in the benefits review.

What is bladderwrack and burdock root?

They are two unrelated plants that appear together in some combination supplements: a brown seaweed and the root of a land plant in the daisy family. Each has its own guide on this site.

Where does bladderwrack come from?

It is harvested from rocky coastlines, mainly in cold North Atlantic waters. The harvest site affects what the seaweed has absorbed, which is why metal testing matters; see how to choose a good bladderwrack.

Is bladderwrack the same as the bladderwrack used in soaps and tonics?

The plant is the same; the products are different. This guide is about the dried plant as a supplement ingredient. We don't give advice on pet use, so ask a vet about dogs.