Seaweed Supplements: Kelp, Bladderwrack and Sea Moss, and What the Label Does Not Tell You

By Jayson Byles, forager & chef · September 2026

Seaweed supplements are capsules, tablets and powders made from dried seaweed or an extract of it. Most are kelp or bladderwrack sold for their iodine; a smaller number are purified fucoidan or powdered carrageen sold as sea moss. We do not make or sell any of them. We cut whole seaweed on the Fife shore and sell it as food, and this is the page for the people on our workshops who ask whether the kelp tablets in the cupboard are safe, and what the number on the label means.

The short version: iodine is real, the amounts in seaweed are large and variable, the labels are often wrong, and the only lawful claims are a short list of sentences about normal body function.

What is actually in a seaweed supplement

Read the ingredients, not the front of the pack. Four things turn up.

• Kelp. On a UK label this usually means dried Ascophyllum nodosum (egg wrack) or a Laminaria species, milled and capsuled, and heavy with iodine. The capsules sold as Hebridean seaweed are Ascophyllum from Lewis, declared at 350 micrograms of iodine per capsule, 233 per cent of the daily reference value.
• Bladderwrack. Fucus vesiculosus, the plant we cut as the wracks. Sold dried or as a tincture. Lower in iodine than the kelps, higher in polyphenols.
• Fucoidan. A sulphated sugar extracted from brown seaweed and sold as a purified powder. In Great Britain it is an authorised novel food only as extracts of Undaria pinnatifida and Fucus vesiculosus, at no more than 250 mg a day.
• Sea moss. Powdered or gelled red seaweed, sold on the strength of social media rather than trials; it has its own section below.

None of them contains a standardised dose, which is the problem this page is about. For the chemistry of the compounds, see seaweed nutraceuticals; for what a plate of seaweed gives you, seaweed benefits and nutrition.

Bladderwrack versus kelp

People use the two words as if they were one plant. Kelp is the big subtidal browns, oarweed, sugar kelp and their relatives, and on supplement labels often the egg wrack Ascophyllum. Bladderwrack is Fucus vesiculosus, the mid-shore wrack with paired air bladders on the rocks at Kingsbarns and Elie. It holds far less iodine per gram than a kelp and far more of the polyphenols called phlorotannins, which is why it is bitter and why skincare makers like it (see our seaweed skincare page).

Its reputation as a slimming aid is old and traceable. The 1918 United States Dispensatory records that in 1862 the French physician Duchesne-Duparc reported “very good results in the treatment of morbid obesity” with Fucus, and that kelp charcoal had earlier been sold as Aethiops vegetabilis against goitre. Both worked, when they worked, through iodine on an underactive thyroid. There is no authorised weight-loss claim for any seaweed in Great Britain.

Sea moss: what it is and what it is not

Sea moss is the fastest-growing seaweed supplement in Britain, and the one with the least behind it. The name covers two different plants. The original is carrageen, Chondrus crispus, the red seaweed of Irish and Scottish shores that has set puddings for centuries and grows on our own rocks. Most of what is sold as sea moss gel or capsules today is a warm-water relative, Gracilaria or Eucheuma, farmed on ropes in the Caribbean and South East Asia. Both are mostly carrageenan, the gelling sugar, with modest minerals; neither carries anything like the iodine of a kelp, which is the one safety point in its favour. The claims made for it, 92 minerals, thyroid, immunity, skin, have no authorised wording behind them, and a January 2026 trade report quoted dietitians warning that its iodine and heavy-metal content vary with where it grew and that the gel is a food, not a medicine. If you want carrageen, we cut the real thing on the Fife shore; our carrageen page shows how to set a pudding with it, which is what it is good at.

How much iodine is in kelp tablets?

The reference points: the NHS says adults need 140 micrograms of iodine a day, and that 0.5 mg (500 micrograms) or less a day from supplements is unlikely to cause any harm. The European tolerable upper level for adults is 600 micrograms a day, set where thyroid hormone signalling starts to shift (Nordic scoping review, 2023).

Now the tablets. In 2021 researchers at Norway’s Institute of Marine Research bought 14 seaweed supplements and measured them: 5 to 5,600 micrograms of iodine per daily dose, and for most the label was wrong by more than the measurement uncertainty. In Glasgow, Martha Redway and Emilie Combet surveyed UK retailers three years running and found 2.3 times as many seaweed products as in 2015, a median of over 400 micrograms of iodine per serving in every year, and some products, supplements especially, above the tolerable upper level in a single dose. Most did not state their iodine content at all.

Two things follow. A capsule of milled kelp is a large dose, and the label may be out by a wide margin. And whole seaweed is not safer by default: a gram of dried kelp can hold more iodine than a week of capsules, which is why we treat the kelps as a seasoning. The portion-by-portion numbers for every species we cut, and what soaking and boiling do to them, are on the benefits and nutrition page.

Are kelp supplements safe?

The dietitians’ answer is plain. The British Dietetic Association’s iodine factsheet says seaweed or kelp supplements should not be used as an iodine source, because the amount can vary considerably from the label and can be excessive; it also advises against eating brown seaweed or kelp more than once a week, especially in pregnancy. The Committee on Toxicity reviewed excess iodine around pregnancy in 2022, recorded kelp at 333 to 4,782 micrograms of iodine per gram freeze-dried, and concluded that high consumers of seaweed may be exposed to levels that could pose a toxicological risk to maternal health.

The newest human data point the same way: when 49 habitual seaweed eaters in Norway stopped for six weeks, their thyroid-stimulating hormone fell, most in those who had eaten the most. If you have a thyroid condition, are pregnant or breastfeeding, or are giving supplements to a child, talk to a GP or pharmacist before taking kelp in any form.

Arsenic is the other question. Brown seaweeds take up arsenic from seawater, mostly in organic forms the body handles well. A 2007 case report found detectable arsenic in eight of nine kelp supplements bought in California, at 1.6 to 65.5 parts per million; it drew published criticism for not separating organic from inorganic arsenic or stating the capsule weight. Read it as a reason to ask for a certificate of analysis, not as proof of harm.

What the label is allowed to say

Only wording on the Great Britain Nutrition and Health Claims Register may be attached to a food or supplement. For iodine the authorised sentences are that iodine contributes to normal thyroid function, to the normal production of thyroid hormones, to normal cognitive function, to normal energy-yielding metabolism, to normal skin and to the normal functioning of the nervous system. They may be used only where a portion supplies at least 15 per cent of the reference value, and section 15 of the CAP Code requires the claim to name the nutrient, keep the authorised meaning and say how much you need to take.

What is not allowed is anything that sounds like medicine: treating an underactive thyroid, shrinking a goitre, detoxing, boosting immunity, burning fat. Under the medicines regulator’s guidance on what is a medicinal product, a food presented as treating or preventing disease becomes a medicine by presentation. There are no authorised health claims at all for fucoidan, fucoxanthin, phlorotannins or sea moss.

A short history of seaweed, iodine and the thyroid

The kelp trade that made Scottish shores an industry, told on our history page, reached Orkney when James Fea of Stronsay introduced kelp burning around 1722, and the islands were making about 3,000 tons of ash a year for glass and soap between 1780 and 1830. Iodine was found in it by accident: in 1811 the Paris saltpetre maker Bernard Courtois added sulphuric acid to seaweed ash and watched a violet vapour condense into dark crystals. In 1820 the Geneva physician Jean-François Coindet gave iodine tincture to patients with goitre, with striking results, then a year of self-dosing and poisonings that led Geneva to restrict its sale in 1821. When the soda market collapsed, Scottish kelp found a second life feeding iodine works through the nineteenth century.

What the research says now

Studies from 2023 onwards that bear on supplements, with the level of evidence said plainly.

• Market survey, Glasgow, 2023. Redway and Combet’s three-year survey of UK retailers, described above. Observational; the most useful single paper on the UK market.
• Sugar kelp fucoidan and the gut, 2025. A randomised, double-blind, placebo-controlled trial in 90 healthy adults gave 125 mg or 500 mg of a Saccharina latissima fucoidan extract twice a day for 90 days. The higher dose shifted stool microbiome markers and was well tolerated. Made and funded by Oceanium in Oban; microbiome measures, not health outcomes. Small human trial.
• Fucoidan and wellbeing, Aberystwyth, 2024 to 2025. A registered, remote, placebo-controlled trial gave young adults spray-dried fucoidan or placebo for 28 days and measured gut symptoms and wellbeing by questionnaire. Innovate UK funded; not yet published. Small human trial, self-report.
• Fucoidan and chemotherapy fatigue, Mayo Clinic, due 2027. A phase 2 crossover trial in 34 patients on platinum chemotherapy, eight weeks of oligo-fucoidan against eight of placebo. Recruiting; estimated completion February 2027.
• Habitual seaweed eaters, Norway, 2026. The 49-adult cessation study above. Non-randomised human study.

Whole seaweed instead

We are not against iodine. We are against not knowing the dose. A pinch of dried dulse or wrack on a baked potato is a food with a long history of use, and it is easy to keep small. A food that is a source of iodine may say that iodine contributes to normal thyroid function, and that is the whole of what may be said; we say no more on our own labels. Cook the kelps: soak, boil, discard the water. Our page on drying and storing seaweed explains how heat changes what is in the bag; anyone gathering their own should read the complete guide to seaweed foraging in Scotland and our plain guide to the law first.

If you are a small brand rather than a cook: know the species, the shore and the batch. We supply identified, hand-cut Scottish seaweed as food and raw material, fresh or dried; testing and a certificate of analysis are available on request. We do not make supplements, and we will not help write a label that says more than the register allows. See commercial supply, and for the wider picture, the other uses of seaweed.

Sources

The figures and findings above come from these sources; the research papers are open access where a PMC link is given.

• Iodine: vitamins and minerals (NHS)
• Iodine: a scoping review for Nordic Nutrition Recommendations 2023 (Food & Nutrition Research, 2023)
• Commercially available kelp and seaweed products: valuable iodine source or risk of excess intake? Aakre et al. (Food & Nutrition Research, 2021)
• Seaweed as food: survey of the UK market and appraisal of opportunities and risks in the context of iodine nutrition. Redway & Combet (British Food Journal, 2023)
• Statement on the potential effects that excess iodine intake may have during preconception, pregnancy and lactation (Committee on Toxicity, 2022)
• Iodine food fact sheet (British Dietetic Association)
• Impact of habitual seaweed consumption on iodine nutrition and thyroid function: a non-randomised pre-post clinical study (European Journal of Nutrition, 2026)
• Risks and benefits of consuming edible seaweeds (Nutrition Reviews, 2019)
• Case report: potential arsenic toxicosis secondary to herbal kelp supplement (Environmental Health Perspectives, 2007)
• Pure Organic Seaweed capsules, product page (Doctor Seaweed)
• Fucoidan extract from the seaweed Undaria pinnatifida, authorised novel food (Food Standards Agency register)
• Fucoidan extract from the seaweed Fucus vesiculosus, authorised novel food (Food Standards Agency register)
• Great Britain nutrition and health claims (NHC) register (Department of Health and Social Care)
• Food: health claims, CAP Code section 15 (Advertising Standards Authority)
• A guide to what is a medicinal product, Guidance Note 8 (MHRA)
• Sea moss: is the trend worth its salt? (NutraIngredients, January 2026)
• Gut microbiome modulation and health benefits of a novel fucoidan extract from Saccharina latissima: a double-blind, placebo-controlled trial (Microorganisms, 2025)
• Exploring the effects of a natural seaweed supplement on health and wellbeing, ISRCTN25101555 (ISRCTN registry, 2024)
• Fucoidan for preventing chemotherapy-related fatigue, NCT06855524 (ClinicalTrials.gov, Mayo Clinic)
• Fucus, in The Dispensatory of the United States of America, 20th edition (1918)
• North Ronaldsay’s kelp industry in the 19th century (Archaeology Orkney)
• Bernard Courtois, Scientist of the Day (Linda Hall Library)
• Jean-François Coindet and the treatment of goitre with iodine (European Thyroid Association)
• History of Scottish seaweed (Scottish Seaweed Industry Association)

Questions we get asked

Are kelp supplements safe?

Not reliably. Kelp is very high in iodine and the amount in a capsule can differ widely from the label; one study measured 5 to 5,600 micrograms per daily dose across 14 products. The British Dietetic Association advises against using kelp supplements as an iodine source. If you have a thyroid condition, are pregnant or breastfeeding, ask a GP or pharmacist first.

How much iodine is in a kelp tablet?

It depends on the species and the batch. A retail Hebridean seaweed capsule declares 350 micrograms, two and a half times the adult daily need of 140 micrograms. Measured products have ranged from 5 to 5,600 micrograms per dose. The NHS says 500 micrograms a day or less from supplements is unlikely to cause harm; the European upper level is 600.

What is the difference between bladderwrack and kelp?

Kelp means the large brown seaweeds such as oarweed and sugar kelp, and on supplement labels often egg wrack. Bladderwrack is Fucus vesiculosus, a smaller mid-shore wrack with paired air bladders. Kelp carries far more iodine per gram; bladderwrack carries more phlorotannins, the polyphenols that make it bitter and interesting to skincare makers.

Can I take seaweed supplements for my thyroid?

The only lawful claim is that iodine contributes to normal thyroid function and normal thyroid hormone production. No seaweed product may claim to treat an underactive or overactive thyroid, and excess iodine can itself disturb thyroid function. If you have a diagnosed thyroid condition, take advice from your GP before adding kelp or seaweed in any form.

Is sea moss good for you?

Sea moss is carrageen or a warm-water relative, mostly carrageenan gel with modest minerals and far less iodine than kelp. No health claim for it is authorised in Great Britain, there are no clinical trials behind the claims made online, and its mineral and metal content varies with where it grew. Eat it as a food, not as a remedy.

This page is general information about a food and the rules around it, not medical advice. If you have a thyroid condition, are pregnant or breastfeeding, or take medicines, speak to a GP or pharmacist before using any seaweed supplement.