“Sea moss” is a popular term for coloured seaweeds sold as dried fronds (leaf structures), powders, capsules, or gels.
The 3 main types are ‘Chondrus crispus’ (also known as ‘Irish moss’ which tends to be reddish-purple, but can be other colours), harvested in the north Atlantic, and ‘Gracilaria’ species from warmer waters, as well as ‘Eucheuma cottoni’.
They are naturally high in soluble fibre (notably the ‘carrageenan’ component contained within it) – and they also contain many minerals (especially iodine), and various bioactive compounds.
-
Thyroid support:
Sea moss contains iodine which is essential for thyroid hormone production. However, seaweeds vary enormously in iodine levels depending on species and harvest location.
This means they can tip intake into deficiency correction or excess.
In the UK, the NHS recommends 140 micrograms per day of iodine for adults, while the Scientific Advisory Committee on Nutrition (SACN) and the European Food Standards Agency (EFSA) set the adult upper safe limit at around 600 micrograms per day.
Exceeding this regularly could disrupt thyroid function.
Sea moss can help top up iodine intake, but uncontrolled daily use risks over-supplementation, depending on the iodine content – into excess, with consequences.
-
Gut health and cholesterol:
The soluble fibre contained within sea moss (known as carrageenan and its related compounds) may act as a prebiotic and, like other fibres, may help lower LDL cholesterol / ‘bad cholesterol’.
But direct clinical trials in humans using sea moss are scarce; most favourable results come from general seaweed or animal studies. Evidence for sea moss itself remains limited.
-
Nutrient source:
Sea moss contains minerals (e.g. potassium, magnesium, calcium, iodine, iron, zinc and more), protein, omega-3 and omega-6, and polyphenols.
In some quarters, it is believed sea moss has strong anti-inflammatory properties and anti-viral properties.
Risks and safety concerns
-
Iodine excess:
Concerns generally surround the iodine content in sea moss – with sources citing wide variability.
Too much iodine can cause both hypothyroidism (an under active thyroid) and hyperthyroidism (an over active thyroid), in otherwise healthy people.
Groups at particular risk: those with thyroid nodules, autoimmune thyroid disease, pregnant women, and babies / young children.
The British Dietetic Association (BDA) warns that iodine content in dried seaweed can be “very high and highly variable”.
The UK government currently does not recommend taking an iodine supplement.
The BDA recommend getting our iodine intake from other food sources such as dairy, fish and eggs.
For vegans, they may be able to get their iodine from fortified plant-milks, nori, and kombu (a type of seaweed) as examples.
However, the BDA also stress the importance of sufficient and safe iodine levels in pre-pregnancy and pregnant women with respect to the baby’s brain development – including for breast-feeding.
Similarly, they also caution on too much iodine in pregnancy and breast-feeding. The BDA does not recommend supplementing with seaweed or kelp as an iodine source.
-
Heavy metals and contaminants
Seaweeds like Irish moss are generally lower risk than the Japanese ‘hijiki’ seaweed, but testing for contaminants is essential.
-
‘Carrageenan’:
Carrageenan is the soluble fibre component, and can be extracted from Irish moss, and other types.
Carrageenan is also used as a food additive in the UK and EU, typically as a thickener, stabiliser and gelling agent.
Safety concerns with respect to carrageenan relate mainly to ‘degraded carrageenan’ (known as poligeenan) from studies using animals. Poligeenan is not allowed in foods as a result.
Special considerations
Pregnancy and breastfeeding: Health authorities caution against seaweed supplements with unquantified iodine, as excess can affect the baby’s thyroid and impair neurodevelopment.
Possible medication interactions:
- Thyroid medicines (levothyroxine, carbimazole, propylthiouracil)
- Blood-pressure/heart drugs (ACE inhibitors, potassium-sparing diuretics)
- Warfarin (because of possible vitamin K content in some seaweeds)
- Kidney disease: some seaweeds are high in potassium, which may be dangerous in advanced chronic kidney disease (CKD).
(This is not an definitive list and you should always consult a medical professional with you have a health condition and taking medication and decide to supplement).
How much is safe?
There is no official UK dose for sea moss.
Guidance centres around iodine safety:
1. Adults need ~140 micrograms per day iodine (NHS).
2. Upper safe limit: about 600 micrograms per day (EFSA, SACN).
3. If a supplement doesn’t state iodine content, avoid daily use, or try and confirm.
Official positions in the UK and Europe
NHS (UK): Adults need 140 micrograms / day; most can get this from dairy and fish.
Food Standards Agency / Commitee On Toxicology (UK): Avoid hijiki; iodine levels in seaweeds can exceed safe limits; people should be cautious with supplements.
EFSA (EU): Sets adult iodine upper limit at 600 micrograms / day; warns that dried seaweeds often lead to excess iodine intake.
-
Who might consider sea moss?
Those with low iodine, provided the product states its iodine content and stays within safe limits.
People interested in extra soluble fibre from a natural source.
Some people who have an interested in sustainable food practices might also perceive sea moss as a climate-friendly way to supplement.
-
Who should avoid or get medical advice first:
Pregnant / breastfeeding women, anyone with thyroid disease, people with health conditions and / or on medications, or with kidney disease –
(this is not a definitive list and you should always consult a medical professional).
✔ Check iodine content per serving (micrograms (written as “µg”)) and compare with NHS guideline (140 µg/day) and EFSA Upper Limit (600 µg/day).
✔ Look for contaminant testing (heavy metals, including inorganic arsenic).
✔ Buy only from reputable UK or EU suppliers following food safety standards.
✔ Avoid products without species / origin labelling.
More research is needed to determine the risk profile of seaweeds in relation to contaminants.
Want to read more about supplements? Have a read of my blog articles on:
Disclaimer:
No responsibility can be accepted for this article which does not constitute medical advice, and is for educational purposes only. You should always consult with a medical professional when deciding to take supplements and use them as directed, should you choose voluntarily to take them.
REFERENCES:
ANSES (2018) Opinion on the risks of iodine excess from seaweed foods/supplements. French Agency for Food, Environmental and Occupational Health & Safety. Available at: https://www.anses.fr/en/system/files/NUT2017SA0086EN.pdf
BBC GoodFood (2023) Top 5 benefits of sea moss Available at: https://www.bbcgoodfood.com/health/nutrition/top-5-health-benefits-of-sea-moss
British Dietetic Association (2022) What is iodine and how does it help the body? Available at: https://www.bda.uk.com/resource/iodine.html
Circuncisão, A. R., Catarino, M. D., Cardoso, S. M., and Silva, A. M. S. (2018) ‘Minerals from macroalgae origin: Health benefits and risks for consumers’. Marine Drugs, 16(11), 400. doi: 10.3390/md16110400 Available at: https://www.mdpi.com/1660-3397/16/11/400
European Food Safety Authority (EFSA), Dujardin, B., Ferreira de Sousa, R. and Gómez Ruiz, J.Á., (2023) ‘Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European population’. EFSA Journal, 21(1), p.e07798. Available at: https://www.efsa.europa.eu/en/efsajournal/pub/7798
European Food Safety Authority Panel on Dietetic Products, Nutrition and Allergies (2014) ‘Scientific opinion on dietary reference values for iodine’. EFSA Journal, 12(5), p.3660. Available at: https://efsa.onlinelibrary.wiley.com/doi/abs/10.2903/j.efsa.2014.3660
Food Standards Agency (2022) Importing fruits and vegetables. Available at: https://www.food.gov.uk/business-guidance/importing-fruit-and-vegetables
Healthline (2025) What is carrageenan and should you remove it from your diet? Available at: https://www.healthline.com/health/food-nutrition/carrageenan
Katsuhiko Yokoi, Aki Konomi, (2012) ‘Toxicity of so-called edible hijiki seaweed (Sargassum fusiforme) containing inorganic arsenic’. Regulatory Toxicology and Pharmacology, Volume 63, Issue 2, pages 291-297, doi: 10.1016/j.yrtph.2012.04.006. Available at: https://www.sciencedirect.com/science/article/abs/pii/S0273230012000773
Men’s Health (2023) Irish sea moss is the popular supplement you definitely don’t need. Available at: https://www.menshealth.com/uk/nutrition/a44562805/irish-sea-moss-benefits/
NHS (2020): Vitamins and minerals – Iodine. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/iodine/
Rose, M et al,. (2007) ‘Arsenic in seaweed — Forms, concentration and dietary exposure’. Food and Chemical Toxicology, Volume 45, Issue 7, pages 1263-1267. doi:10.1016/j.fct.2007.01.007. Available at: https://www.sciencedirect.com/science/article/abs/pii/S0278691507000257
Scientific Advisory Committee on Nutrition (2014) SACN Statement on Iodine and Health. Available at: https://assets.publishing.service.gov.uk/media/5a7e469ced915d74e62253f3/SACN_Iodine_and_Health_2014.pdf
Recent Comments