Kelp (Saccharina japonica): Uses, Evidence and Safety

Safety first: iodine, the thyroid and arsenic

Kelp is a brown seaweed, and the main problem is iodine. Iodine is needed to make thyroid hormones, but kelp products can hold far too much of it, and the amount is often not reliable on the label. In a laboratory study of 96 seaweed products from Norway, the iodine in one portion of whole seaweed ranged from 128 to 62,400 micrograms, and in a daily supplement dose from 5 to 5,600 micrograms. For 54 products a single portion or dose was above the upper limit the authors used (600 micrograms a day). [1] Regular excess iodine is linked to an underactive thyroid, an overactive thyroid, goitre and autoimmune thyroid disease. [1] A case report describes a 70-year-old woman with no earlier thyroid disease who developed a temporary overactive thyroid about 3 months after starting kelp tablets; it cleared after she stopped them and took anti-thyroid treatment. [2] The American Thyroid Association (ATA) advises people of all ages, including pregnant and breastfeeding women, not to take iodine or kelp supplements providing more than 500 micrograms of iodine a day, and names infants, older adults, pregnant or breastfeeding women and people with thyroid disease as more vulnerable. [3]

The second problem is contamination. Seaweed takes up arsenic from sea water; a UK survey of 31 dried retail seaweed samples found arsenic in every one. [4] A case report of long-term arsenic poisoning has been linked to an Icelandic kelp supplement, and trade-group authors dispute that interpretation, so this is debated, not settled. [5] Ask a doctor or pharmacist before using kelp if you have a thyroid condition, take thyroid medicine or lithium, are pregnant or breastfeeding, or have kidney disease. See thyroid conditions and iodine or herbal products and seaweed, iodine and the thyroid. This page is information, not a recommendation to use kelp.

Overview

"Kelp" is a loose name for large brown seaweeds, so products differ. This page covers two that are sold under the name. Saccharina japonica (formerly Laminaria japonica; sold as kombu or kunbu) is in the family Laminariaceae. [6] Ascophyllum nodosum (knotted wrack, rockweed) is a different seaweed in the family Fucaceae, which GBIF lists as an accepted name. [6] Native range: not found in the sources we read. The part used is the whole seaweed body (thallus). It is eaten as a food and sold as powder, tablets, capsules and extracts, often in slimming or "thyroid" products. Product labels do not always name the species: in the Norwegian study, 12 products gave no information on the seaweed type. [1] Related but different: bladderwrack, which is Fucus vesiculosus and is sometimes sold as "brown kelp". Seaweed as a food and sea moss are covered in seaweed, sea moss and kelp.

Traditional use (not proof of benefit)

Brown seaweeds are a long-standing part of the diet in many coastal and Asian food cultures, and narrative reviews describe their medicinal use in several traditions. [7] The sources we read did not give a clear list of traditional medical uses of kelp, so we do not list them. A long history of eating or using a seaweed is not proof that it treats anything.

What research shows

Because the first row is about iodine content and not a treatment, none of these rows shows that kelp supplements help a health condition.

Forms and why the form matters

Whole dried kelp, kelp tablets, capsules and extracts are not interchangeable. In the Norwegian study, whole-food seaweeds averaged much more iodine per portion than supplements, and iodine in 18 of 24 products with a declared value differed from the label by more than 50%. [1] Preparation changes iodine too: typical cooking or processing cut iodine by about 11 to 89% in dry weight, though a few products rose. [1] A result or limit for one product cannot be assumed for another. How much to use is a decision for a clinician or pharmacist, not this page.

Safety in more detail

• Thyroid: excess iodine can cause both an underactive and an overactive thyroid. [1] Case report: temporary overactive thyroid after kelp tablets, in a person without earlier thyroid disease. [2] The authors of that report advise supervision and thyroid monitoring for people using kelp supplements. [2]
• Iodine limits quoted by the sources: the ATA says the upper limit for most people is 1,100 micrograms a day, with lower limits of 500 to 1,100 in pregnancy and breastfeeding, and advises against kelp or iodine supplements above 500 micrograms a day. [3] The Norwegian authors used 600 micrograms a day and cite a lowest-adverse-effect level of 1,800. [1] These are reference figures, not advice for any person.
• Arsenic and metals: arsenic was found in all 31 seaweed samples in the UK survey (18 to 124 mg/kg total). The more harmful inorganic form was found only in hijiki, a different seaweed, and was below the detection limit in the other types tested. [4] A letter from a herbal-industry trade group argues that most arsenic in food seaweed is the less toxic organic form and that the iodine in the supplement in the poisoning case could explain the symptoms. [5] The 2019 review lists arsenic among the risks of eating seaweed. [8] Lead, cadmium and mercury in kelp supplements: Proper evidence not found in the sources we read.
• Medicines: NIH Office of Dietary Supplements statements on iodine and anti-thyroid medicines were not read directly for this page. For thyroid medicines and lithium, see supplements and thyroid medicines and how herb and medicine interactions work.
• Pregnancy and breastfeeding: the ATA advises against kelp or iodine supplements above 500 micrograms of iodine a day in pregnancy and breastfeeding. [3] See pregnancy and breastfeeding safety.
• Children and older adults: the ATA names infants and older adults as more vulnerable to excess iodine. [3]
• Liver and kidney: the sources we read do not describe a kelp-specific liver or kidney warning. We could not read NIH's LiverTox entry, so we make no claim either way.
• Allergy and surgery: not covered by the sources we read. See supplements before surgery.
• Quality: labels are often incomplete or inaccurate on species and iodine. [1] See heavy metals in supplements and herbal products.

Official statements

• American Thyroid Association (US), first published June 2013, last updated March 2024: advises against iodine or kelp supplements above 500 micrograms of iodine a day. [3]
• UK Food Standards Agency: advised consumers to avoid hijiki seaweed because of inorganic arsenic (as reported in a 2007 survey). This is about hijiki, not kelp. [4]
• Not found in the sources we read: a current NCCIH, Memorial Sloan Kettering, EMA or FDA statement specific to kelp.

Related pages

• Bladderwrack
• Seaweed, sea moss and kelp
• Seaweed, iodine and the thyroid
• Kelp and seaweed supplements: iodine content and risks
• Iodine
• Herbs for weight loss: claims checked