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What Is Sea Moss, and Why Does Its Iodine Vary So Much?
In this article
Sea moss is a red seaweed. The original is Chondrus crispus, the Irish moss of the North Atlantic shore; most of what is sold as sea moss today is a tropical farmed relative from the Eucheuma and Kappaphycus group, grown for its gelling carrageenan. No human trial of either has been published, and the "92 minerals" claim has no analytical source. The one nutrient in sea moss with a known effect and a known risk is iodine, and it varies more than almost anything else we eat: edible seaweeds span 16 µg per gram in nori to over 8,000 in processed kelp granules.1 Of 224 seaweed products on sale in the UK, only 10% stated their iodine content; where it could be estimated, the median serving carried 585 µg, and 26 products could exceed the 600 µg adult upper level.2 The adult reference intake is 150 µg a day, and the NHS says 0.5 mg or less a day from supplements is unlikely to cause harm.3, 4 So the figure to buy sea moss by is the declared iodine per serving. Who should not take sea moss at all, including the thyroid and levothyroxine questions, is on the sea moss side effects page.
What is sea moss, and where does it come from?
Chondrus crispus grows on rocks in the cold Atlantic from Ireland and Scotland to Nova Scotia, which is the Irish moss of the name. It is one of the better-mineralised red seaweeds: in a 2024 analysis against dulse and knotted wrack it was the richest in calcium, iron, manganese and zinc, and in a comparison of 14 seaweeds it carried the most sulphur.5, 6 Its iodine, measured in 30 shop samples of dulse and Irish moss in 2020, averaged 3.86 µg per gram dry weight, a low figure for a seaweed; four grams a day supplied about a quarter of a child's recommended intake, and an adult would need to eat 286 g of dried Irish moss a day to pass the upper level the authors used.7
The sea moss in most gels and powders is not that plant. The boom since 2020 is supplied by Eucheuma and Kappaphycus species farmed on ropes in the Caribbean, East Africa and South-East Asia, the same seaweeds the food industry grows for carrageenan, and sold as "sea moss", "Irish moss", "wildcrafted" or "gold sea moss" interchangeably. They are different species with different mineral and iodine profiles, and "wildcrafted" is a marketing word with no regulated meaning. A label that names the species tells you which plant you are buying.
Does sea moss really contain 92 minerals?
No analysis has ever shown it. The figure comes from a self-styled healer's teaching, "92 of the 102 minerals the body needs", repeated across social media as fact; the body does not need 102 minerals, and the published element analyses of seaweeds measure a couple of dozen, with differences between species of up to 10,000-fold.6 What a seaweed does carry is whatever was dissolved in the water it grew in, essential and otherwise, in proportions set by species, season and site. In Great Britain a label may not make the "92 minerals" claim, and no mineral in sea moss carries an authorised claim unless the product declares it at 15% of the reference intake or more, which is why a gummy that declares its iodine, vitamin D and vitamin C can say what those nutrients do and a bag of dried seaweed cannot.
How much iodine is in sea moss?
Anywhere from almost none to a dangerous amount. Three things set the figure. Species first: the Irish moss samples above averaged under 4 µg per gram, nori about 16, and kelps run into the thousands, so a "sea moss" blend that includes kelp, or that is a tropical species with a different profile, can carry a hundred times the iodine of another jar with the same word on the front.1, 7 Harvest second: the same kelp varied 13-fold between sun-bleached and fresh blades.1 Preparation third: iodine is water-soluble, and soaking and boiling leach it out, with Chondrus crispus losing a measurable share on rehydration and boiling; a gel made by soaking for a day will carry less iodine than the dried weed it came from, by an unmeasured amount.8
The arithmetic for a typical serving shows the spread. Five grams of dried seaweed carries about 19 µg of iodine at the Irish moss figure, about 550 µg at the UK products survey's median of 110 µg per gram (its median estimated serving carried 585 µg), and several thousand at a kelp figure.2, 7 Against that, the adult reference intake is 150 µg, pregnancy raises it to 200 µg, the NHS puts the supplement level unlikely to cause harm at 0.5 mg or less a day, and the upper level above which thyroid problems become likely in susceptible people is 600 µg a day.3, 4, 9 This is why the British Dietetic Association advises against seaweed and kelp supplements as an iodine source: the dose is unknown. A declared figure solves it.
| Source | Iodine | Against 150 µg a day |
|---|---|---|
| 5 g dried Irish moss (Chondrus crispus, Spanish retail samples) | about 19 µg | 13%7 |
| Median UK seaweed product serving (224 products) | 585 µg | 390%; near the 600 µg upper level2 |
| 5 g of a high-iodine kelp | over 5,000 µg | more than 30 times the reference intake1 |
What does sea moss do in your body?
No trial in people has measured an outcome of eating sea moss: not skin, digestion, weight, energy, the liver or kidneys, or men's or women's health. What is written about what it does "to a woman's body" or "to your face" is extrapolated from laboratory work on seaweed extracts or from the minerals it contains. Its carrageenan behaves as a gelling, soluble fibre in the gut; the minerals it carries do what those minerals do; and the iodine feeds the thyroid's two hormones, which is the one role with authorised wording in Great Britain. For anyone eating seaweed regularly, the useful check is a thyroid function test if symptoms appear.
In a sea moss gummy the authorised wording attaches to the declared nutrients, not to the seaweed. Iodine contributes to normal production of thyroid hormones and normal thyroid function, vitamin D to the maintenance of normal bones and teeth, and vitamin C contributes to normal function of the immune system. Do not stack iodine sources: a seaweed-containing greens or reds powder declares its own iodine figure, and a multivitamin may add 150 µg more.
Which sea moss should you buy, and what about heavy metals?
Buy by what is declared. A species name, an iodine figure per serving, and a heavy-metal test tell you more than "wildcrafted", "gold" or "raw". Seaweed concentrates what is in the sea, so arsenic, cadmium and lead turn up in analyses of every species; in 31 UK retail samples total arsenic ranged from 18 to 124 mg/kg, but the inorganic, carcinogenic form appeared only in the nine hijiki samples, at 67 to 96 mg/kg, which is why the Food Standards Agency advises against eating hijiki.6, 10 Red seaweeds carry arsenic mostly in organic forms, and the Italian analysis judged the cadmium and lead in 14 species low enough not to threaten consumers at normal intakes while noting they need controlling.6 A product that publishes its heavy-metal certificate has answered the question. For any blend, check the label for the iodine figure; one that lists kelp powder without a declared iodine figure counts as an unknown when adding up iodine sources.
Frequently asked questions
Does sea moss interact with any medications?
Through its iodine, yes, with thyroid medicines: anyone on levothyroxine, or with Hashimoto's or Graves' disease, should not add any iodine source without the prescriber. The claim that its carrageenan thins the blood comes from laboratory work on purified extracts, not from eating the seaweed; tell the anticoagulant clinic anyway. The detail is on the side effects page.
Is sea moss hard on your kidneys or liver?
No trial has looked. The kidney question is about load: a high-iodine seaweed and whatever metals it has accumulated are cleared by the kidneys, so anyone with kidney disease should treat seaweed as a food to ask about, not a supplement to add.
What time of day should you take sea moss?
Any consistent time, with food. Nothing has been measured that depends on timing. The one rule is not to take it in the same day as another iodine supplement, kelp or a seaweed-containing greens powder.
Is sea moss better than soursop, or spirulina?
They are unrelated foods taken for different reasons, so there is no basis for ranking them: spirulina has many human trials, sea moss and soursop have none. Soursop is a tropical fruit; spirulina and chlorella are freshwater algae with no iodine to speak of.
Does sea moss gel flatten your stomach?
No trial has measured weight or waist on sea moss. Its carrageenan is a gelling soluble fibre, and that is as far as the evidence goes.
Sources
- Teas J, et al. Variability of iodine content in common commercially available edible seaweeds. Thyroid, 2004. PubMed 15588380.
- Bouga M, Combet E. Emergence of seaweed and seaweed-containing foods in the UK: focus on labeling, iodine content, toxicity and nutrition. Foods, 2015. PubMed 28231201.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for iodine. EFSA Journal, 2014. PubMed 42125360.
- NHS. Vitamins and minerals: iodine. nhs.uk.
- Čmiková N, et al. Seaweed nutritional value and bioactive properties: insights from Ascophyllum nodosum, Palmaria palmata, and Chondrus crispus. Life, 2024. PubMed 39598320.
- Desideri D, et al. Essential and toxic elements in seaweeds for human consumption. Journal of Toxicology and Environmental Health A, 2016. PubMed 26817952.
- Darias-Rosales J, et al. Risk assessment of iodine intake from the consumption of red seaweeds (Palmaria palmata and Chondrus crispus). Environmental Science and Pollution Research, 2020. PubMed 32803579.
- Correia H, et al. Seaweeds rehydration and boiling: impact on iodine, sodium, potassium, selenium, and total arsenic contents and health benefits for consumption. Food and Chemical Toxicology, 2021. PubMed 34237393.
- Farebrother J, Zimmermann MB, Andersson M. Excess iodine intake: sources, assessment, and effects on thyroid function. Annals of the New York Academy of Sciences, 2019. PubMed 30891786.
- Rose M, et al. Arsenic in seaweed: forms, concentration and dietary exposure. Food and Chemical Toxicology, 2007. PubMed 17336439.
