By the Nutriveda editorial team · Updated 5 November 2026
Seaweeds are large marine algae, grouped by colour into brown seaweeds (kelp, kombu, wakame), red seaweeds (nori, dulse) and green seaweeds (sea lettuce). Of about 11,000 species, around 145 are eaten as food, mostly in Asia. Fresh seaweeds are low in energy, 35-45 kcal per 100 g, and give some calcium, magnesium and folate. The one thing that really matters is iodine, and it differs more than a hundredfold between types: a gram of dried kombu can carry more than the 600 µg a day that EFSA sets as the safe upper limit for adults, while nori has one of the lowest ranges. Spirulina, often sold next to seaweed, is a microscopic cyanobacterium, not a seaweed.
Types of seaweed: brown, red and green
- Brown seaweeds (about 2,000 species): kelp and kombu (Laminaria, Saccharina), wakame (Undaria pinnatifida), bladderwrack and hijiki. They have the highest iodine.
- Red seaweeds (about 7,500 species, the largest group): nori, also called laver (Porphyra, Pyropia), and dulse.
- Green seaweeds (about 1,500 species): sea lettuce (Ulva). Green algae also grow in fresh water.
- Not a seaweed: spirulina is a microscopic cyanobacterium, often called blue-green alga, sold dried as powder or tablets.
Species numbers from Guiry (seaweed.ie, citing AlgaeBase); the 145 food species from the BfR.
Seaweed nutrition per 100 g: kelp, nori, wakame and spirulina
| Nutrient | Kelp, raw | Nori (laver), raw | Wakame, raw | Spirulina, dried |
|---|---|---|---|---|
| Energy | 43 kcal | 35 kcal | 45 kcal | 290 kcal |
| Protein | 1.7 g | 5.8 g | 3.0 g | 57.5 g |
| Fat | 0.6 g | 0.3 g | 0.6 g | 7.7 g |
| Carbohydrate | 9.6 g | 5.1 g | 9.1 g | 23.9 g |
| Fibre | 1.3 g | 0.3 g | 0.5 g | 3.6 g |
| Calcium | 168 mg | 70 mg | 150 mg | 120 mg |
| Magnesium | 121 mg | 2 mg | 107 mg | 195 mg |
| Iron | 2.9 mg | 1.8 mg | 2.2 mg | 28.5 mg |
| Potassium | 89 mg | 356 mg | 50 mg | 1,360 mg |
| Sodium | 233 mg | 48 mg | 872 mg | 1,050 mg |
| Vitamin B12 | 0 µg | 0 µg | 0 µg | 0 µg |
| Iodine | no USDA data | no USDA data | no USDA data | no USDA data |
Source: USDA FoodData Central, SR Legacy: “Seaweed, kelp, raw” (FDC 168457), “Seaweed, laver, raw” (FDC 168458), “Seaweed, wakame, raw” (FDC 170496) and “Seaweed, spirulina, dried” (FDC 170495), checked 5 November 2026. Values rounded.
Read the columns with care. Kelp, laver and wakame are fresh, with 80-85% water, while spirulina is a dry powder, so per 100 g it looks far richer. In real life you eat a few grams of any of them: USDA’s portion for the fresh seaweeds is 10 g (2 tablespoons), and for spirulina a 7 g tablespoon. None of the four USDA records has an iodine value.
Iodine by type of seaweed
| Dried seaweed | Group | Iodine, µg per gram |
|---|---|---|
| Laminaria digitata (finger kelp, Atlantic kombu) | brown | 2,000-10,000 |
| Saccharina japonica (kombu, kelp) | brown | 1,500-8,000 |
| Saccharina latissima | brown | 1,000-5,000 |
| Wakame (Undaria pinnatifida) | brown | 50-1,500 |
| Bladderwrack | brown | 200-900 |
| Dulse | red | 50-1,000 |
| Nori (Pyropia) | red | 10-400 |
| Sea lettuce (Ulva) | green | 50-300 |
Source: BfR Opinion No. 044/2026, Table 1 (given in mg/kg dry weight, which equals µg per gram). Spirulina is not in the table. NIH reports a similar spread for commercial whole or sheet seaweeds: 16 to 2,984 µg per gram.
Adults need 150 µg of iodine a day (NIH; also the EU reference intake), and EFSA’s upper limit for adults is 600 µg a day from all foods. By the BfR ranges, a gram of dried kombu gives 2.5 to 13 times that limit, while a gram of nori gives at most 400 µg. Soaking, rehydrating, blanching or cooking lowers the iodine in seaweed, by up to 80-90% in studies on Saccharina latissima, but the BfR says the effect is hard to predict.
Glycemic index
No seaweed is listed in the international GI tables (Atkinson 2008). Fresh seaweeds have 5-10 g of carbohydrate per 100 g and are eaten by the gram, so their effect on blood sugar is negligible. More in What is the glycemic index? (with food table).
Health benefits: what the evidence says
- Iodine. NIH calls seaweed one of the best food sources of iodine, and under EU rules iodine contributes to the normal production of thyroid hormones and normal thyroid function (Regulation (EU) No 432/2012). The difficulty is getting a steady amount: with nori and wakame in small portions that is easier than with kelp.
- Useful for people who eat no seafood or dairy. NIH notes that vegans and others who eat little or no seafood, eggs or dairy might not get enough iodine. Seaweed can fill that gap, but with the ranges above, a labelled product or advice from your doctor is a safer plan than guessing.
- Minerals with little energy. Per 100 g, fresh kelp and wakame give 150-170 mg of calcium and 107-121 mg of magnesium for about 45 kcal (USDA). In gram-sized portions the contribution is modest.
- Spirulina is a different case. It is valued for protein and iron per spoonful, not for iodine; see the spirulina entry for its benefits and contamination risks.
How to eat it
- Start with nori and wakame: nori sheets for sushi or as a crumbled topping, dried wakame rehydrated for miso soup or a salad with cucumber and sesame.
- Use kombu sparingly: a small piece is enough for a pot of dashi stock; do not make it a daily habit.
- Soak or blanch brown seaweed: pour away the soaking water. It lowers the iodine, though not by a predictable amount.
- Do not stack seaweeds: kombu stock, wakame and nori in one meal add up.
- Buy labelled products: the BfR recommends that ready-to-eat seaweed products state their iodine content; prefer those that do.
- Storage: keep dried seaweed sealed, cool and dry.
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 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, children and kidney disease. The BfR lists pregnant and breastfeeding women, babies, young children and people with impaired kidney function as vulnerable groups. EFSA’s upper limits for children start at 200 µg a day at ages 1-3.
- Thyroid medicines. NIH warns that high iodine intakes with antithyroid drugs such as methimazole can add up and cause hypothyroidism.
- Hijiki and arsenic. The Canadian Food Inspection Agency advises consumers to avoid hijiki because of its inorganic arsenic. Samples of dulse, nori and kombu tested low.
- Salt. Fresh wakame has 872 mg of sodium per 100 g, about 2.2 g of salt. For ready-made seaweed salads and snacks, check the salt on the label.
- Seaweed supplements. NIH notes that kelp supplements are sold and that the label shows the iodine dose. Compare it with 150 µg and 600 µg a day, and talk to your doctor first if you have a thyroid condition.
Traditional view (Ayurveda)
In Usha and Vasant Lad’s food guidelines, seaweed is favourable for Vata and okay in moderation for Pitta and Kapha; kelp is on the lists to avoid for Pitta and Kapha. This is a traditional system, not clinical evidence.
Related entries and articles
- Kelp – kombu and other kelps, the seaweeds with the most iodine
- Nori (laver) – the sushi seaweed, with the lowest iodine range
- Wakame – the brown seaweed in miso soup
- Spirulina – a blue-green microalga, not a seaweed
- Miso and tofu – classic partners for seaweed
- Chlorella: benefits, side effects and chlorella vs spirulina
- What is a vegan diet: benefits and risks
FAQ
Which seaweed has the least iodine?
Of the seaweeds in the BfR table, nori (10-400 µg per gram dried) and sea lettuce (50-300 µg) have the lowest ranges. The kelps, including kombu, are the highest, at 1,000-10,000 µg per gram.
Is seaweed good for your thyroid?
In small amounts it can supply the iodine the thyroid needs. In large amounts, especially of kelp or kombu, it can harm the thyroid: NIH explains that too much iodine can cause goitre, hypothyroidism or hyperthyroidism. If you have a thyroid condition, agree on seaweed with your doctor.
Is spirulina a seaweed?
No. Seaweeds are large algae that grow in the sea. Spirulina is a microscopic cyanobacterium, a blue-green alga, that is cultivated and sold dried. Its nutrients and its risks are different from those of seaweed.
Sources
- USDA FoodData Central, SR Legacy: Seaweed, kelp, raw (FDC 168457). fdc.nal.usda.gov
- USDA FoodData Central, SR Legacy: Seaweed, laver, raw (FDC 168458). fdc.nal.usda.gov
- USDA FoodData Central, SR Legacy: Seaweed, wakame, raw (FDC 170496). fdc.nal.usda.gov
- USDA FoodData Central, SR Legacy: Seaweed, spirulina, dried (FDC 170495). fdc.nal.usda.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
- NIH Office of Dietary Supplements: Iodine, Fact Sheet for Health Professionals (updated 5 November 2024). ods.od.nih.gov
- 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.