By the Nutriveda editorial team · Updated 5 November 2026
Kelp is the name for large brown seaweeds such as Laminaria and Saccharina; kombu, the dried kelp used for Japanese stock, is Saccharina japonica, and in Europe also Laminaria digitata, sold as Atlantic kombu. Fresh kelp has 43 kcal per 100 g and gives magnesium, calcium, folate and vitamin K, but people eat it by the gram, not by the 100 g. What really sets kelp apart is iodine: dried kelp can carry 1,000 to 10,000 µg of iodine per gram, so a single gram can be more than the 600 µg a day that EFSA sets as the safe upper limit for adults. Treat kelp as an occasional seasoning, not as a daily food or a thyroid supplement.
Kelp nutrition per 100 g and per portion
| Nutrient | Kelp, raw, 100 g | Portion: 10 g (2 tablespoons) |
|---|---|---|
| Energy | 43 kcal | 4 kcal |
| Protein | 1.7 g | 0.2 g |
| Fat | 0.6 g | 0.1 g |
| Carbohydrate | 9.6 g | 1.0 g |
| of which sugars | 0.6 g | 0.1 g |
| Fibre | 1.3 g | 0.1 g |
| Calcium | 168 mg | 17 mg |
| Magnesium | 121 mg | 12 mg |
| Iron | 2.9 mg | 0.3 mg |
| Folate | 180 µg | 18 µg |
| Vitamin K | 66 µg | 7 µg |
| Sodium | 233 mg | 23 mg |
| Iodine | no data in this record | no data |
Source: USDA FoodData Central, SR Legacy: “Seaweed, kelp, raw” (FDC 168457), checked 5 November 2026. The record does not name the species. The 10 g measure (2 tablespoons) is a USDA portion; the portion column is calculated. Values rounded. The record has no iodine value, so the iodine figures in the next section come from the German Federal Institute for Risk Assessment (BfR).
This is fresh kelp, with 82% water. Dried kelp, the form most often sold in Europe, has far less water and is more concentrated per gram; we had no USDA record for it, so we give no numbers. Per 100 g, fresh kelp covers about 90% of the EU reference intake for folate (200 µg), 88% for vitamin K (75 µg), 32% for magnesium (375 mg) and about a fifth for calcium (800 mg) and iron (14 mg). A realistic 10 g portion gives a tenth of that.
Iodine in kelp: how much is too much
Adults need 150 µg of iodine a day; that is the NIH recommended intake and also the EU reference intake. EFSA’s tolerable upper intake level for adults is 600 µg a day (the US limit is 1,100 µg). Seaweed is one of the best food sources of iodine, but NIH stresses that the amount varies greatly between species: commercial whole or sheet seaweeds tested between 16 and 2,984 µg per gram.
| Dried seaweed | Iodine, µg per gram |
|---|---|
| Laminaria digitata (finger kelp, Atlantic kombu) | 2,000-10,000 |
| Saccharina japonica (kombu, kelp) | 1,500-8,000 |
| Saccharina latissima | 1,000-5,000 |
| Wakame (Undaria pinnatifida), for comparison | 50-1,500 |
| Nori (Pyropia), for comparison | 10-400 |
Source: BfR Opinion No. 044/2026, Table 1 (given there in mg/kg dry weight, which equals µg per gram). The BfR notes that iodine varies with species, region, season and processing.
By these numbers, 1 g of dried kombu carries 1,500-8,000 µg of iodine, 2.5 to 13 times the EFSA limit for a whole day. Soaking, rehydrating, blanching or cooking lowers the iodine in the seaweed, by up to 80-90% in studies on Saccharina latissima, but the BfR says the effect is hard to predict. An EU draft proposes a maximum of 1 g of iodine per kg of dried seaweed (1,000 µg per gram), and the BfR recommends that ready-to-eat seaweed products state their iodine content.
Glycemic index
Kelp is not listed in the international GI tables (Atkinson 2008). Fresh kelp has 9.6 g of carbohydrate per 100 g, and at a few grams per meal its effect on blood sugar is negligible. More in What is the glycemic index? (with food table).
Health benefits: what the evidence says
- Iodine for the thyroid. Under EU rules, iodine contributes to the normal production of thyroid hormones and normal thyroid function (Regulation (EU) No 432/2012). Kelp is a very rich source, but because its iodine varies so much, it is a hard way to get a steady daily amount.
- Minerals and folate. Per 100 g, fresh kelp gives 168 mg of calcium, 121 mg of magnesium and 180 µg of folate (USDA). In the small amounts people use, these add up to a modest contribution.
- Very low in energy. 43 kcal per 100 g and about 4 kcal in a 10 g portion, with almost no fat.
How to eat it
- Kombu stock: a small piece of dried kombu simmered in water is the base of Japanese dashi and of miso soup. Use it occasionally and keep the piece small.
- Soak or blanch first: when you eat kelp itself, in salads or stir-fries, soak or blanch it and pour away the water. It lowers the iodine, though not by a predictable amount.
- Kelp flakes and powder: use them by the pinch, as a seasoning, and choose a product that states its iodine content per serving.
- Storage: keep dried kelp in a closed container in a cool, dry place.
Who should be careful
- Thyroid conditions. NIH notes that people with autoimmune thyroid disease may have adverse effects at iodine intakes that are safe for others. The BfR also names people with autonomous thyroid nodules and Graves’ disease. Too much iodine can cause goitre, hypothyroidism or hyperthyroidism, and high intakes are linked to thyroiditis and papillary thyroid cancer (NIH).
- Pregnancy, breastfeeding and children. The BfR counts pregnant and breastfeeding women, babies and young children among the vulnerable groups. EFSA’s upper limits for children are much lower than for adults: 200 µg a day at ages 1-3, 250 µg at 4-6, 300 µg at 7-10, 450 µg at 11-14 and 500 µg at 15-17.
- Kidney disease. People with impaired kidney function are also on the BfR list of vulnerable groups.
- Thyroid medicines. NIH warns that high doses of iodine together with antithyroid drugs such as methimazole can have an additive effect and cause hypothyroidism.
- Kelp tablets. NIH notes that kelp supplements are sold and that the label shows the iodine dose. Compare it with 150 µg a day and the 600 µg limit, and do not take kelp for your thyroid without your doctor.
- Hijiki. Hijiki is a different brown seaweed. The Canadian Food Inspection Agency advises consumers to avoid it because of its inorganic arsenic, while samples of kombu, nori and dulse tested low.
Traditional view (Ayurveda)
In Usha and Vasant Lad’s food guidelines, kelp is favourable for Vata and on the lists to avoid for Pitta and Kapha. This is a traditional system, not clinical evidence.
Related entries and articles
- Seaweed – brown, red and green seaweeds compared, with iodine per type
- Nori (laver) – the sushi seaweed, with the lowest iodine range
- Wakame – the brown seaweed in miso soup
- Spirulina – a microalga, not a seaweed
- Miso
- What is a vegan diet: benefits and risks
FAQ
Is kelp the same as kombu?
Kombu is a kind of kelp. The BfR lists Saccharina japonica under the trade names kombu and kelp, and Laminaria digitata as finger kelp or Atlantic kombu. Both are brown seaweeds with very high iodine.
How much kelp is safe to eat?
There is no safe gram amount we can give, because dried kelp carries anywhere from about 1,000 to 10,000 µg of iodine per gram (BfR), and EFSA’s limit for adults is 600 µg a day from all foods. Use it in small amounts and not every day, soak or blanch it, and choose products that state their iodine content.
Are kelp supplements good for the thyroid?
Not on your own. The thyroid needs iodine, but NIH explains that too much iodine can cause the same problems as too little, including goitre and hypothyroidism, and also hyperthyroidism. If you have a thyroid condition or take thyroid medicine, ask your doctor before taking kelp.
Sources
- USDA FoodData Central, SR Legacy: Seaweed, kelp, raw (FDC 168457). fdc.nal.usda.gov
- NIH Office of Dietary Supplements: Iodine, Fact Sheet for Health Professionals (updated 5 November 2024). ods.od.nih.gov
- German Federal Institute for Risk Assessment (BfR): Binding maximum iodine levels in edible seaweeds may help reduce health-relevant intake levels. Opinion No. 044/2026. doi:10.17590/20260921-134034-0. bfr.bund.de
- Guiry MD: Seaweeds, seaweed.ie. seaweed.ie
- Canadian Food Inspection Agency: Inorganic arsenic and hijiki seaweed consumption. inspection.gc.ca
- Regulation (EU) No 432/2012, authorised health claims (iodine). legislation.gov.uk
- Regulation (EU) No 1169/2011, Annex XIII (reference intakes). legislation.gov.uk
- Atkinson FS, Foster-Powell K, Brand-Miller JC. International Tables of Glycemic Index and Glycemic Load Values: 2008. Diabetes Care 2008;31(12):2281-2283. PMC
- Lad U, Lad V. Food Guidelines for Basic Constitutional Types, from Ayurvedic Cooking for Self-Healing, 2nd ed. The Ayurvedic Institute. ayurveda.com
This page is for information only and is not medical or dietary advice.