Sea Lettuce: Nutrients, What Research Shows and Safety
Sea lettuce (Ulva) as a food: Japanese dried "aosa" nutrient values, high iodine per gram, pollutant questions, what small studies of extracts show, and who should be careful.
Content last updated . This page has not been medically reviewed.
Sea lettuce is the common name for green seaweeds of the genus Ulva (family Ulvaceae), including Ulva lactuca. They are thin, bright green, ruffled sheets that grow in marine, brackish and even fresh water. [10] People eat them dried and crumbled in soups and salads, and in Japan the dried form is sold as "aosa." [1][3] This page keeps the whole seaweed as food apart from extracts and from hype.
Read this first
- Iodine is the main safety point. The Japanese food composition table lists 2,200 micrograms (mcg) of iodine in 100 g of sun-dried aosa (sea lettuce). [1] That is about 22 mcg in each gram. Ten grams would be 220 mcg, and 50 g would reach the 1,100 mcg adult upper limit quoted by the American Thyroid Association (ATA). [1][6] These are arithmetic from one table, not advice.
- People with thyroid disease, and pregnant or breastfeeding women, should ask a clinician before eating seaweed often. The ATA advises against iodine and kelp supplements above 500 mcg of iodine a day. [6] In 49 regular Norwegian seaweed eaters, thyroid-stimulating hormone (TSH) fell after six weeks without seaweed. The seaweed species were not named. [5]
- Pollutants: a 2025 review says Ulva can accumulate pollutants, so it needs monitoring when used as food. [10] We did not find a measured heavy metal value for edible Ulva lactuca in the sources we read.
- Dried flakes are salty by weight: the Japanese table lists 9.9 g of salt equivalent per 100 g (about 0.1 g per gram). [1]
- Evidence in one line: most health claims for sea lettuce come from the lab, animals or one very small trial of an extract. Proper evidence not found in the sources we read for weight loss, detox, thyroid support or disease prevention.
What is in it (USDA values)
No USDA entry for this food: USDA FoodData Central (SR Legacy) has no sea lettuce item. The values below come from the Japanese Standard Tables of Food Composition (8th revised, 2023 supplement), item 09001 "aosa, sun-dried" (genus Ulva, mainly one species called anaaosa). [1][3] The values are for dried seaweed (16.9 g water per 100 g), not fresh and not rehydrated. [1] The table lists no portion size, so the "per 1 g" column is our simple division by 100, shown only to make a pinch of flakes easier to picture. Percentages use FDA 2016 Daily Values, calculated by us.
| Nutrient | Per 100 g dried | Per 1 g (our arithmetic) | % Daily Value in 1 g |
|---|---|---|---|
| Energy | 201 kcal | 2.0 kcal | not set |
| Protein | 22.1 g | 0.22 g | 0%-1% |
| Dietary fiber (total) | 29.1 g | 0.29 g | 1% |
| Iodine | 2,200 mcg | 22 mcg | 15% |
| Magnesium | 3,200 mg | 32 mg | 8% |
| Manganese | 17.0 mg | 0.17 mg | 7% |
| Potassium | 3,200 mg | 32 mg | 1% |
| Calcium | 490 mg | 4.9 mg | under 1% |
| Iron | 5.3 mg | 0.05 mg | under 1% |
| Sodium | 3,900 mg | 39 mg | 2% |
| Vitamin K | 5 mcg | 0.05 mcg | under 1% |
| Beta-carotene | 2,500 mcg | 25 mcg | no DV |
The table also lists vitamin B12 at 37.2 mcg per 100 g. [1] We do not use that number as a B12 claim; see the B12 point below. Values vary with species, harvest place and processing. [3]
Plant compounds: the Japanese table measured beta-carotene (2,500 mcg per 100 g). [1] Lab studies describe polysaccharides from Ulva prolifera, a different species, but that work used cells and zebrafish embryos, not people. [14]
What research says
| Health goal | What the sources found | Strength of evidence | What was studied |
|---|---|---|---|
| Iodine intake and thyroid tests | In 49 habitual seaweed eaters (species not named), median iodine intake was 658 mcg a day before and 189 mcg after six weeks without seaweed. TSH fell significantly after stopping. [5] | Mixed (observational pre-post; shows seaweed iodine can move thyroid tests, not that sea lettuce does) | Regular seaweed eaters in Norway |
| Antioxidant and inflammation blood markers | In a randomized, double-blind, placebo-controlled trial, 300 mg a day of an Ulva lactuca extract for 28 days was linked with changes in several blood markers, including a CRP drop of 0.91 mg/L. 20 adults enrolled and 18 finished. The authors call it a pilot. A company grant (BluGen LLC) is listed. [12] | Promising but limited (markers only, very small, company-funded) | Extract capsule-type dose, healthy adults aged 20-35 |
| Liver protection | A methanol extract given for four weeks at 1,000 mg per kg body weight improved liver and oxidative stress markers in a chemical liver injury model. [13] | Lab or animal only | Extract; animal species not named in the abstract |
| Antioxidant effect of polysaccharides | Fractions from Ulva prolifera protected rat liver cells and zebrafish embryos from hydrogen peroxide damage. [14] | Lab or animal only | Processed polysaccharide fractions, another Ulva species |
| Vitamin B12 source | The one B12 review we read found dried purple laver (nori) to be the most suitable plant source and did not discuss sea lettuce. [8] | Insufficient evidence for sea lettuce | Nori only |
| Weight loss, detox, cancer, diabetes | Proper evidence not found in the sources we read | No good evidence of benefit found | Not applicable |
Eaten as food vs taken as an extract. The trial and the animal work above used extracts, which are not the same as sprinkling dried flakes. We found no human trial of whole eaten sea lettuce.
How much is a normal serving
No official serving exists in the sources we read. USDA has no entry, and the Japanese table gives per 100 g values only. [1] So there is no official figure to quote. For scale, 1 g of dried aosa is about 22 mcg of iodine and 2 kcal by simple division. [1] Fresh sea lettuce is mostly water, so its iodine per gram is lower, but we did not find a measured iodine value for fresh Ulva lactuca. The ATA says intake above 1,100 mcg of iodine a day is not recommended. [6] NHS says adults need 140 mcg a day. [7] This is information, not a prescription.
Safety, allergy and medicines
- Iodine and thyroid: high or repeated iodine intake can change thyroid function. NHS says taking high doses of iodine for long periods could change how the thyroid works. [7] The ATA advises against iodine and kelp supplements above 500 mcg a day and says intake above 1,100 mcg a day is not recommended. [6] If you have thyroid disease or take thyroid medicine, ask your clinician or pharmacist about seaweed. We did not read a source on specific medicine interactions for sea lettuce.
- Pregnancy and breastfeeding: the ATA encourages 220-250 mcg a day in pregnancy and notes limits for supplements. [6] We did not read a source that gives a safe amount of sea lettuce in pregnancy. See seaweed in pregnancy.
- Pollutants and metals: Ulva can accumulate pollutants. [10] In a 2026 study of Ulva reticulata, seven metals (arsenic, cadmium, tin, mercury, lead, nickel, chromium) were measured. Microwave and ultrasound treatment lowered all of them, up to 88% with microwave, to within the Indian food authority's limits. The abstract gives no concentrations. [11] Where the seaweed grew matters; for hijiki and arsenic see the metals guide.
- Sodium: dried aosa lists 3,900 mg sodium (9.9 g salt equivalent) per 100 g. [1] This matters only when large amounts are used or in salted mixes.
- Vitamin K and warfarin: the Japanese table lists only 5 mcg vitamin K per 100 g dried, which is low. [1] We did not read a source on warfarin and sea lettuce, so ask your clinician if you take warfarin.
- Allergy: seaweed is not fish or shellfish. We did not read a source on sea lettuce allergy. For trouble breathing or swelling of the face or throat after any food, call 911 (or your local emergency number). See food allergy basics.
- Vibrio and bacteria in fresh seaweed salads: Proper evidence not found in the sources we read for sea lettuce.
Choosing, storing and food safety
- We did not read a source on sea lettuce storage or on foraging wild sea lettuce or "green tides" (large algae blooms). Given the pollutant point above, [10] water quality matters if you gather it yourself.
- For buying and storing dried seaweed in general, see how to buy, prepare and store seaweed.
- Check the label: Japanese shops sometimes sell aosa as "aonori," and they are different entries in the Japanese table. [2][3] See ao-nori.
Related pages
Health connections for Sea Lettuce
What research links to eating or drinking this food, and how strong the evidence is. Many studies are observational or use juice, powder, oil or extracts rather than the food as eaten; each card says which.
Nutrients listed for dried sea lettuce
Established roleThe Japanese table lists magnesium (3,200 mg), manganese (17.0 mg), potassium, fiber (29.1 g) and beta-carotene per 100 g dried; a pinch supplies little.
Supplements: Composition only; no health benefit is claimed.
Sea lettuce extract, antioxidant and inflammation markers
Preliminary or mixedIn a randomized placebo-controlled pilot trial (18 finishers, 28 days, 300 mg a day of an Ulva lactuca extract, company grant) several blood markers changed.
Supplements: Extract, markers only, very small trial; not the food.
Sea lettuce in the lab and animals: liver and antioxidant studies
Lab or animal research onlyExtracts protected against chemical liver injury in an animal model, and Ulva prolifera polysaccharides protected rat liver cells and zebrafish embryos from damage.
Supplements: Not shown in people.
Sea lettuce and vitamin B12 claims
Not enough evidenceThe Japanese table lists 37.2 mcg B12 per 100 g dried, but the B12 review we read found purple laver, not sea lettuce, useful for vegetarians.
Supplements: No evidence that sea lettuce corrects B12 status.
Connected topics
Open a group to see how each topic connects and how strong the evidence is.
Body systems (1)
- Endocrine SystemDocumented risk
Iodine from seaweed can affect thyroid hormone balance when intake is high.
Body parts (1)
- ThyroidDocumented risk
Dried sea lettuce is a notable iodine source per gram. Too much iodine can change how the thyroid works; ask a clinician if you have thyroid disease. This is about excess, not thyroid support.
Root causes (1)
- Heavy Metal ExposureDocumented risk
A 2025 review says Ulva can accumulate pollutants, and a 2026 study measured seven metals in another Ulva species. Where it grew matters. We found no measured values for edible Ulva lactuca.
Biomarkers (1)
- TSHPreliminary or mixed
In 49 regular seaweed eaters (species not named), TSH fell after six weeks without seaweed. This is not specific to this seaweed and is not proof of harm.
Documented risks and interactions (5)
- Sea lettuce and iodine intake. Dried sea lettuce (aosa) is listed at 2,200 mcg iodine per 100 g, about 22 mcg per gram. The ATA says intake above 1,100 mcg a day is not recommended.
- Habitual seaweed eating, iodine and thyroid tests. In 49 regular seaweed eaters (species not named), TSH fell after six weeks without seaweed. People with thyroid disease should ask a clinician before eating seaweed often.
- Pollutants and metals in Ulva. Ulva can accumulate pollutants, so water quality and monitoring matter. In one Ulva species, microwave or ultrasound treatment lowered seven measured metals.
- Sodium in dried sea lettuce flakes. Dried aosa lists 9.9 g salt equivalent per 100 g (about 0.1 g per gram), which matters mainly in large amounts or salted mixes.
- Pregnancy, breastfeeding and seaweed iodine. The ATA encourages 220-250 mcg iodine a day in pregnancy and advises against iodine or kelp supplements above 500 mcg. We found no safe amount of sea lettuce for pregnancy.
Frequently asked questions
Is sea lettuce high in iodine?
In the Japanese food composition table, dried aosa (sea lettuce) has 2,200 mcg of iodine per 100 g, about 22 mcg per gram. [1] That is far less than kombu-type kelp, which the Japanese health ministry's table lists at 230,000 mcg per 100 g for shredded kelp. [9] In a Korean survey of five edible seaweeds, sea tangle (a kelp) averaged 2,432 mg of iodine per kg dry weight, the highest of the five. [4] Measured values vary by species and place, so treat it as a moderate-to-high iodine seaweed, not a safe-by-default one.
Does sea lettuce have vitamin B12?
The Japanese table lists 37.2 mcg per 100 g dried. [1] But the B12 review we read found purple laver (nori), not sea lettuce, to be the useful plant source, and it did not say sea lettuce B12 is active in people. [8] Do not count on sea lettuce for B12. See the B12 claims guide.
Is sea lettuce extract proven to reduce inflammation?
No. One small randomized trial of an Ulva lactuca extract (18 completers, 28 days, company grant) saw changes in blood markers and called itself a pilot. [12] Other findings are from animals and cells. [13][14]
Who should be careful with sea lettuce?
People with thyroid disease, people taking thyroid medicine, and pregnant or breastfeeding women should ask a clinician before eating seaweed often, because of iodine. [6][7]
References
- Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Standard Tables of Food Composition in Japan, item 09001 aosa, sun-dried (8th revised, 2023 supplement). Read 2026-10-24.
- MEXT. Item 09002 aonori, sun-dried. 2023 supplement. Read 2026-10-24 (used to tell aosa and aonori apart).
- MEXT. Standard Tables of Food Composition in Japan 2020 (8th revised), explanatory notes on algae. 2020. Read 2026-10-24.
- Lee Y, et al. Analysis and Risk Assessment of Total Iodine Content in Edible Seaweeds in South Korea. Foods 2025. Read 2026-10-24 (abstract; sea tangle 2,432 mg/kg dry weight, the highest of five seaweeds).
- Aakre I, et al. Impact of habitual seaweed consumption on iodine nutrition and thyroid function. Eur J Nutr 2026. Read 2026-10-24.
- American Thyroid Association. Statement on the potential risks of excess iodine ingestion and exposure. 2013, page updated 2024. Read 2026-10-24.
- NHS. Vitamins and minerals: Iodine. Read 2026-10-24.
- Watanabe F, et al. Vitamin B12-containing plant food sources for vegetarians. Nutrients 2014. Read 2026-10-24 (abstract via Europe PMC).
- Japan Ministry of Health, Labour and Welfare (e-JIM). Iodine page, seaweed iodine table. Read 2026-10-24.
- Duong TT, et al. Advances in the Genus Ulva Research: From Structural Diversity to Applied Utility. Plants 2025. Read 2026-10-24 (abstract).
- Ashok A, et al. Impact of thermal treatments on heavy metals ... of edible green seaweed Ulva reticulata. Food Chem 2026. Read 2026-10-24 (abstract).
- Jäger R, et al. Effect of an Ulva lactuca Extract on Systemic Antioxidant and Inflammatory Biomarkers in Healthy Adults. Nutrients 2026. Read 2026-10-24 (abstract).
- Khan N, et al. Antioxidative and Hepatoprotective Activity of Ulva lactuca in a CCl4-Induced Liver Injury Model. Foods 2026. Read 2026-10-24 (abstract).
- Tan J, et al. Antioxidant properties of low-molecular-weight Ulva prolifera polysaccharides. Food Chem 2026. Read 2026-10-24 (abstract).