Seaweed & Algae · Seaweed
Nori (Laver)
Pyropia and Porphyra species · also called Laver, purple laver, gim
Nori is the thin, dark sheet of dried red seaweed used for sushi rolls and rice balls. It is lower in iodine than kombu on average, but products vary a lot, and some are high. One small 4-week trial in 30 vegetarians found better vitamin B12 blood markers with roasted nori. The main cautions are iodine, cadmium, shellfish allergy, pregnancy and young children.
General information, not medical advice. Last substantive update · Not medically reviewed. A named clinician has not signed off on this page. How this page is made
About Nori (Laver)
Nori is the thin, dark sheet made from dried red seaweed of the Pyropia and Porphyra group. It is used for sushi rolls, rice balls and as flakes. It is a food, not a proven remedy. It carries vitamin B12 and folate by lab tests, but its iodine, cadmium and shellfish-allergy risks are worth knowing first.
What it is
Nori is made from red seaweeds of the genus Pyropia, formerly Porphyra, such as Pyropia yezoensis and Porphyra tenera. GBIF lists Pyropia as an accepted genus, and the species names have changed over time, so studies use several names for the same kind of food. It is a seaweed, not a plant.
USDA data for raw (wet) laver per 100 g: 35 kcal, 5.81 g protein, 0.3 g fibre, 48 mg sodium, 356 mg potassium, 70 mg calcium, 1.8 mg iron, 0.988 mg manganese, 39 mg vitamin C, 146 microg folate and 260 microg vitamin A. The USDA household portion of 10 sheets (26 g) gives about 9 kcal, 1.5 g protein, 10 mg vitamin C and 38 microg folate. USDA lists no iodine value and lists vitamin B12 as 0 for this raw item. Dried sheets are lighter and more concentrated per gram and were not read.
Vitamin B12 and folate
Lab tests found true vitamin B12 in dried purple laver: about 30 to 60 microg per 100 g dry weight, with folate at about 880 to 1300 microg per 100 g, more than in kombu, hijiki or wakame. A 2023 review of 25 studies gave 1.03 to 2.68 microg of real B12 per nori sheet. Much of this work comes from the same research group.
In a 4-week open-label trial in 30 vegetarians, roasted nori went with higher serum B12 and holoTC and lower homocysteine than no nori. MMA did not differ between groups, and the higher amount did no better than the lower one. It was small and not repeated, and it measured blood markers, not health problems. Nori is not shown to prevent or treat B12 deficiency. Ask a clinician or dietitian about B12 if you eat a plant-based diet.
Iodine
Nori is lower in iodine than kombu on average. A 2004 test found 16 microg per gram, the lowest of 12 seaweeds. But products vary: a 2022 Hong Kong test of 27 ready-to-eat seaweeds found 1,030 to 34,600 microg per 100 g, with the highest in a nori for rice balls, where one 1.3 g serving gave 450 microg. The American Thyroid Association says more than 1,100 microg of iodine a day is not recommended and may cause thyroid dysfunction.
A 2023 EFSA report found mean iodine intakes above 20 microg per kg of body weight a day among European consumers of kombu and laver. Iodine also dissolves in cooking water and may be lost in humid storage, so what ends up in a dish is hard to estimate.
What the research shows
Beyond B12, human research on nori itself is thin. A 2025 review of 29 trials found edible algae lowered blood pressure by about 2 mmHg overall, but it pools many algae, spirulina worked best, and the abstract gives no nori-only result. A 12-week pilot in 24 older women eating dried laver found more diverse gut bacteria, but with no control group and no health outcome.
No good evidence was found for weight loss, cholesterol, blood sugar, cancer, detox or thyroid-support claims in the sources read.
Food versus supplements
The B12 trial used roasted nori and the gut study used dried laver, both as food. Kelp tablets and iodine pills are different products and can contain far more iodine. ATA advises against iodine and kelp supplements containing more than 500 microg of iodine a day and says many contain up to several thousand times the upper limit.
At a glance
Traditional uses history, not proof
- Eaten as dried sheets for rolls and rice balls, and as flakes, in Japan, Korea, China and elsewhere
- Long use as food is not proof that it treats any condition
Studied for research
- Nutrients in raw laver, high certainty for the USDA values
- Iodine content, high certainty that it varies widely by product
- Vitamin B12 and folate in dried nori, moderate certainty (lab measurements)
- Vitamin B12 blood markers in vegetarians, low certainty (one small open-label trial)
- Blood pressure, very low certainty for nori (pooled algae trials)
- Gut bacteria, insufficient evidence (small pilot, no control)
Main compounds
- Iodine
- Vitamin B12 and folate
- EPA (an omega-3 fat)
- Cadmium and arsenic (contaminants taken up from sea water)
Common forms
- Raw (wet) laver (USDA values read)
- Dried and roasted sheets (the form used in the B12 trial)
- Seasoned snack sheets and nori for rice balls (iodine measured in a 2022 test)
- Nori flakes and kelp tablets are not covered
Research links: 2024 trial: roasted nori and vitamin B12 status in vegetarians · 2022 Hong Kong test: iodine and metals in 27 seaweed products · 2026 analysis: minerals, cadmium, arsenic and iodine in seaweeds · American Thyroid Association statement on excess iodine · Research Library · Genes & Biomarkers Library
What it is and its history
- Accepted scientific name
- Pyropia / Porphyra species (nori is the food name, not one species)
- Naming authority
- GBIF: Pyropia J.Agardh is an accepted genus; Porphyra tenera is listed as accepted
- Synonyms
- Neopyropia yezoensis (a GBIF synonym of Pyropia yezoensis); Porphyra spp. (older genus name still used in studies)
- Common names
- Nori (English); Laver (English); Purple laver (English)
- Family or group
- Bangiaceae (the red seaweed family that nori comes from), per GBIF.
- Classification
- Plantae > Rhodophyta > Bangiophyceae > Bangiales > Bangiaceae > Pyropia > Pyropia yezoensis, Porphyra tenera and others
- Native range
- Not verified from a source read.
- Habitat
- Not verified from a source read.
- What it looks like
- Roots: Not verified.; Stems: Not verified.; Leaves: Not verified.
- Cultivation
- Amphipods living on laver culture platforms can end up in sheets (one source); farming otherwise not read.
- Harvest and processing
- Sold as dried or roasted sheets, flakes or seasoned snack sheets according to the existing project summary; processing steps were not read from a source.
- Look-alikes and adulterants
- Name: Ao-nori (green laver); Note: A green seaweed with its own page; not the same food.; Name: Dulse; Note: Another red seaweed, eaten differently and with its own page.; Name: Kombu, wakame; Note: Brown seaweeds; usually higher in iodine than nori on average.
Photographs
What is inside, and how it may work
What it contains and how those compounds act in cells and animals. Laboratory and animal mechanisms are not proven effects in people.
Main compounds
| Compound | Family | Where it is concentrated | Activity studied | Level of evidence |
|---|---|---|---|---|
| Iodine | mineral (halogen) | Whole seaweed 16 microg/g in one nori type (2004); up to 34,600 microg/100 g in one nori for rice balls (Hong Kong, 2022). | Needed for thyroid hormones. Too much may disturb the thyroid. 12 16 14 | Human data |
| Vitamin B12 (cobalamin) | vitamin | Dried purple laver About 30 to 60 microg/100 g dry weight (2023 analysis); 1.03 to 2.68 microg per sheet (2023 review). | Needed for blood and nerves. Nori B12 raised blood markers in one small trial. 6 7 4 | Human data |
| Folate (mainly 5-methyltetrahydrofolate) | vitamin | Dried purple laver About 880 to 1300 microg/100 g dry weight (2023 analysis). | A B vitamin; measured, not tested as a treatment. 6 | Laboratory |
| EPA (an omega-3 fatty acid) | fatty acid | Whole seaweed Not given as numbers in the abstract. | Measured only; the 2026 analysis found red algae, especially nori, rich in EPA. 9 | Laboratory |
| Cadmium and arsenic | toxic elements (contaminants) | Whole seaweed All 35 market nori samples above 1.2 microg/g cadmium in one study. | Contaminants taken up from sea water; a safety topic, not a benefit. 2 9 | Laboratory |
| Pheophorbide a and pyropheophorbide a | chlorophyll breakdown products | Dried laver 851 to 906 and 5,460 to 5,624 microg/g in one product linked to an outbreak. | Light-activated compounds that can cause skin reactions; found at high levels in one implicated product. 5 | Human data |
Mechanisms, receptors and pathways
| Target or pathway | Gene or receptor | What was found | Level |
|---|---|---|---|
| Thyroid hormone production | Not specified in the sources read | Iodine is needed to make thyroid hormones; high intake may cause thyroid dysfunction (ATA). 14 | Human data |
| Vitamin B12 status | Not specified in the sources read | Nori contains B12 by lab methods and went with higher blood B12 and holoTC and lower homocysteine in a small trial; the mechanism was not tested. 4 6 | Human data |
| Cadmium handling | Not specified | In mice, nori cadmium built up in tissue, while nori or kelp with cadmium-containing rice reduced rice cadmium in tissue by 25.8% to 48.1%, with more Bacteroides and more fecal output. 2 | Animal research |
Absorption, metabolism and elimination
- Absorption
- The 2024 trial set out to study B12 bioavailability from nori; B12 markers rose and plateaued at the lower amount, with no extra gain at the higher amount. Absorption was not measured directly in the abstract.
- Distribution
- Not found.
- Metabolism
- Not found.
- Elimination
- Not found.
- Bioavailability
- Suggested by blood markers only, in 30 vegetarians. Not established for ordinary use.
Isolated compounds versus whole extracts
The B12 trial and the gut bacteria pilot used whole dried nori or laver as food. Fucoidan-like extracts, porphyran or other seaweed extracts, and kelp tablets are different products and were not covered. Kelp supplements can carry far more iodine than food; ATA advises against iodine and kelp supplements over 500 microg of iodine a day.
Nutritional composition
Nutrition from USDA FoodData Central (SR Legacy, April 2018 release; FDC 168458 'Seaweed, laver, raw'), read 2026-10-26 from the saved copy in the project. This is raw (wet) laver, not the dried sushi sheets, snack sheets or flakes, which are lighter and more concentrated per gram. Per 100 g raw laver: 35 kcal, 5.81 g protein, 0.28 g fat, 5.11 g carbohydrate, 0.3 g fibre, 48 mg sodium, 356 mg potassium, 70 mg calcium, 1.8 mg iron, 2 mg magnesium, 1.05 mg zinc, 0.988 mg manganese, 0.264 mg copper, 39 mg vitamin C, 146 microg folate (DFE), 0.446 mg riboflavin, 260 microg vitamin A (RAE) and 4 microg vitamin K. USDA lists 0 microg vitamin B12 and no iodine value for this entry. The USDA household portion is 10 sheets (26 g), which by simple arithmetic from the 100 g values gives about 9 kcal, 1.5 g protein, 12 mg sodium, 93 mg potassium, 18 mg calcium, 0.47 mg iron, 0.26 mg manganese, 10 mg vitamin C, 38 microg folate and 1 microg vitamin K. USDA gives 2 tablespoons as 10 g. Dried, roasted, seasoned and flaked nori were not read.
2 high certainty1 moderate certainty1 low certainty1 very low certainty1 not enough evidence
What was looked at
The 6 benefits and uses checked for Nori (Laver), each with how strong the evidence is. Tap one to open it.
- Everyday nutrients in laverHigh certainty
- Iodine supply (nutrient content)High certainty
- Vitamin B12 and folate content of dried noriModerate certainty
- Vitamin B12 blood markers in vegetariansLow certainty
- Blood pressure (pooled seaweed trials)Very low certainty
- Gut bacteriaNot enough evidence
Lab markers tracked in the studies
Serum vitamin B12Holotranscobalamin (holoTC)HomocysteineMethylmalonic acid (MMA)Systolic blood pressureDiastolic blood pressure
What is Nori (Laver) good for?
The main studied uses and how strong the human evidence is for each. Most nori research is lab measurement of nutrients; there is one small trial of nori itself.
Benefits, one by one
Each card is a research question, not a promise. Tap a goal to filter. Open a card to see the studies, doses and limits.
Everyday nutrients in laver
High certaintyUSDA lists raw (wet) laver with 5.81 g protein, 146 microg folate, 39 mg vitamin C, 0.988 mg manganese and 260 microg vitamin A per 100 g. Dried sushi sheets are lighter and more concentrated, so these numbers do not carry over to a sheet. This is a nutrient fact, not a treatment.
2 studiesRaw (wet) laver, per USDA.
Studies, doses and limits
What the trials found
Not a study of people. USDA values for raw laver (FDC 168458): 35 kcal, 5.81 g protein, 0.3 g fibre, 356 mg potassium, 70 mg calcium, 1.8 mg iron, 0.988 mg manganese, 0.264 mg copper, 39 mg vitamin C, 146 microg folate, 0.446 mg riboflavin, 260 microg vitamin A and 4 microg vitamin K per 100 g. USDA lists 0 for vitamin B12 and no iodine value. A nori-specific lab analysis (2026) found red algae, especially nori, rich in EPA.
Results
Raw laver is a modest source of several nutrients per 100 g, but a portion is small: the USDA household portion of 10 sheets is 26 g, which gives about 9 kcal and 1.5 g protein.
Effect sizes as reported
Per 100 g raw laver: protein 5.81 g, folate 146 microg, vitamin C 39 mg, manganese 0.988 mg. Per 26 g: protein 1.51 g, folate 38 microg, vitamin C 10 mg, manganese 0.26 mg.
Why this rating
High that these are the USDA values for raw laver. They say little about dried sheets as sold.
Form and dose studied
Raw (wet) laver, per USDA.
Who was studied
Adults.
Limits
One USDA entry for the raw form, 2018 release. Dried sheets are far more concentrated per gram and were not read. The USDA vitamin B12 value of 0 is not used here to say nori has no B12; lab studies below measured B12 in dried nori.
Why researchers looked
People ask what a thin seaweed sheet adds to a meal.
Traditional use
Nori is eaten as a food in Japan, Korea, China and elsewhere, mostly as dried sheets for rolls and rice balls. Long use as food is not proof that it treats any condition.
Proposed mechanism Laboratory measurement
These are measured amounts of nutrients in the raw food.
A proposed mechanism does not show that the effect happens in people.
Iodine, heavy metals and allergy come first; see the Safety tab.
Iodine supply (nutrient content)
High certaintyNori is lower in iodine than kombu on average, but real products range a lot. A 2004 lab test found 16 microg per gram in one nori type. A 2022 Hong Kong test found up to 34,600 microg per 100 g in a nori for rice balls. Iodine is needed for the thyroid, but too much can harm it.
4 studiesNori as food unless stated.
Studies, doses and limits
What the trials found
No trial tested nori to correct iodine shortage. In 2004, nori (Porphyra tenera) had 16 microg/g, the lowest of 12 seaweeds tested. In 2022, Hong Kong's Consumer Council measured 1,030 to 34,600 microg per 100 g across 27 ready-to-eat seaweeds, with the highest in a nori for rice balls (one 1.3 g serving gave 450 microg). A 2023 EFSA report found mean iodine intakes above 20 microg per kg of body weight a day among European consumers of kombu and laver.
Results
Nori is a real iodine source whose level depends heavily on the product. This is a measurement, not a health benefit.
Effect sizes as reported
2004: nori 16 microg/g. 2022: up to 34,600 microg per 100 g (346 microg/g) in one nori for rice balls; one 1.3 g serving 450 microg. ATA adult upper limit 1,100 microg a day.
Why this rating
High that nori contains iodine and that the amount varies widely between products. Exact amounts for any one pack are unknown.
Form and dose studied
Nori as food unless stated.
Who was studied
Adults.
Limits
A 2004 lab sample, a 2022 consumer test of 27 products (8 nori wrappers) and a modelling report. The EFSA figure groups laver with other seaweed foods.
Why researchers looked
Seaweed is a major iodine source, and iodine intake that is too high or too low affects the thyroid.
Traditional use
Nori is eaten as a food in Japan, Korea, China and elsewhere, mostly as dried sheets for rolls and rice balls. Long use as food is not proof that it treats any condition.
Proposed mechanism Laboratory measurement
These are measured amounts of iodine in the food. Iodine is needed to make thyroid hormones.
A proposed mechanism does not show that the effect happens in people.
Iodine, heavy metals and allergy come first; see the Safety tab.
Vitamin B12 and folate content of dried nori
Moderate certaintyLab tests found true vitamin B12 in dried purple laver: about 30 to 60 microg per 100 g dry weight in one analysis, and 1 to 2.7 microg per sheet in a review. Folate was also high. These are measurements in the product, not proof of what a person's body gets.
3 studiesDried nori sheets.
Studies, doses and limits
What the trials found
A 2023 HPLC analysis of commercial seaweeds found dried purple laver (Neopyropia yezoensis) had about 30 to 60 microg B12 and 880 to 1300 microg folate per 100 g dry weight, more than kombu, hijiki or wakame. A 2023 systematic review of 25 studies gave real B12 for nori of 1.03 to 2.68 microg per sheet. A 2014 review by the same research group called nori the most suitable plant-derived B12 source for vegetarians.
Results
Dried nori contains measurable vitamin B12 and folate by lab methods. How much a person absorbs is a separate question.
Effect sizes as reported
B12 about 30 to 60 microg/100 g dry weight (2023 analysis); real B12 1.03 to 2.68 microg per sheet (2023 review).
Why this rating
Moderate: several analyses agree nori has B12 and folate, but two of the three sources come from one research group and sheet sizes vary.
Form and dose studied
Dried nori sheets.
Who was studied
Adults, vegetarians.
Limits
Lab content only. The USDA value for raw laver lists B12 as 0, which is a different form. Sample numbers are not in the abstracts.
Why researchers looked
B12 comes mostly from animal foods, so people on plant-based diets look for other sources.
Traditional use
Nori is eaten as a food in Japan, Korea, China and elsewhere, mostly as dried sheets for rolls and rice balls. Long use as food is not proof that it treats any condition.
Proposed mechanism Laboratory measurement
Not a mechanism study. The review notes that newer mass-spectrometry methods can tell real B12 from look-alike compounds.
A proposed mechanism does not show that the effect happens in people.
Iodine, heavy metals and allergy come first; see the Safety tab.
Vitamin B12 blood markers in vegetarians
Low certaintyIn one small 4-week trial in 30 vegetarians, people who ate roasted nori had higher blood B12 and holoTC and lower homocysteine than people who ate none. MMA did not differ. It was open-label, short, and measured blood markers, not health problems.
1 studiesRoasted dried nori sheets.
Studies, doses and limits
What the trials found
A randomised, open-label, three-group trial in 30 vegetarians in Taiwan who did not use supplements: no nori, 5 g of roasted nori a day, or 8 g a day, for 4 weeks. After adjusting for baseline, serum B12, holoTC, homocysteine and a combined score improved. The 8 g group did not do better than the 5 g group. NCT05614960.
Results
Nori went with better B12 blood markers over 4 weeks. It does not show that nori prevents or treats B12 deficiency.
Effect sizes as reported
5 g group vs control: serum B12 +59 pmol/L (95% CI 25 to 93), holoTC +28.2 pmol/L (10.1 to 46.3), homocysteine -3.7 micromol/L (-6.8 to -0.6). MMA: no significant difference between groups.
Why this rating
Low: one small open-label trial, 4 weeks, blood markers only, one MMA marker did not change, and no clinical outcome.
Form and dose studied
Roasted dried nori sheets.
Who was studied
30 vegetarians (Taiwan).
Limits
Not blinded, 30 people, short, not repeated. Authors report no conflict of interest, but two are among the researchers who have measured nori B12 for years. Health bodies still point vegans to fortified foods or supplements; that advice was not re-read for this page.
Why researchers looked
Vegetarians and vegans are at risk of low B12, so researchers tested whether nori improves B12 status.
Traditional use
Nori is eaten as a food in Japan, Korea, China and elsewhere, mostly as dried sheets for rolls and rice balls. Long use as food is not proof that it treats any condition.
Proposed mechanism Author interpretation
Not shown. The authors link the result to B12 in nori being available to the body.
A proposed mechanism does not show that the effect happens in people.
Iodine, heavy metals and allergy come first; see the Safety tab.
Blood pressure (pooled seaweed trials)
Very low certaintyA 2025 review pooled 29 trials of many edible algae, mostly not nori, and found blood pressure about 2 mmHg lower. The abstract says nori and kelp are rich in nitrate but gives no nori-only result. For nori, the evidence is very thin.
1 studiesMany edible algae; whole and extracts.
Studies, doses and limits
What the trials found
A systematic review and meta-analysis of 29 randomised trials (1,583 adults, at least 4 weeks) of edible algae of many kinds and extracts. Systolic BP was lower by 2.05 mmHg and diastolic by 1.87 mmHg overall. Heterogeneity was high. Spirulina had the largest effect, and whole algae did better than extracts. No nori-only trial result is given in the abstract.
Results
A small average drop across many algae, driven largely by other algae. This does not show that eating nori lowers blood pressure.
Effect sizes as reported
Pooled all algae: SBP -2.05 mmHg (95% CI -3.80 to -0.31); DBP -1.87 mmHg (-3.10 to -0.64); I2 75% and 68%.
Why this rating
Very low for nori: the pooled result is for many algae and extracts, heterogeneity is high, and the abstract gives no nori-specific trial.
Form and dose studied
Many edible algae; whole and extracts.
Who was studied
Adults, healthy or with cardiometabolic risk.
Limits
Nori's share of the 29 trials is not in the abstract. Results may differ by species, dose, health status and duration.
Why researchers looked
Algae contain potassium, peptides, nitrate and fibre, and researchers asked whether eating them changes blood pressure.
Traditional use
Nori is eaten as a food in Japan, Korea, China and elsewhere, mostly as dried sheets for rolls and rice balls. Long use as food is not proof that it treats any condition.
Proposed mechanism Author interpretation
Not shown. The authors point to nitrate, potassium, peptides and polyphenols.
A proposed mechanism does not show that the effect happens in people.
Iodine, heavy metals and allergy come first; see the Safety tab.
Gut bacteria
Not enough evidenceIn a 12-week Korean pilot, 24 older women all ate dried laver and had a more diverse mix of gut bacteria afterwards. There was no comparison group and no health outcome. It is a hint, not a finding.
1 studiesDried laver, 12 weeks.
Studies, doses and limits
What the trials found
An exploratory single-group study of 24 older women in Korea (13 with metabolic syndrome, 11 without), all given dried laver (Porphyra tenera) for 12 weeks. Gut bacteria diversity rose in both groups; the Firmicutes/Bacteroidetes ratio fell; four genera differed.
Results
Changes in bacteria counts and diversity; no change in any symptom or disease was reported in the abstract.
Effect sizes as reported
Alpha diversity up (p < 0.001 Chao1; p < 0.05 Fisher); F/B ratio 0.81-fold (controls) and 0.54-fold (metabolic syndrome group).
Why this rating
Insufficient: tiny pilot with no comparison group and no health outcome.
Form and dose studied
Dried laver, 12 weeks.
Who was studied
Older Korean women.
Limits
Women only, 24 people, one product, no control, bacteria counts only. Funding not stated in the abstract.
Why researchers looked
Researchers asked whether eating laver changes the mix of bacteria in the gut.
Traditional use
Nori is eaten as a food in Japan, Korea, China and elsewhere, mostly as dried sheets for rolls and rice balls. Long use as food is not proof that it treats any condition.
Proposed mechanism Author interpretation
Not shown. The authors suggest laver changes the gut microbiota.
A proposed mechanism does not show that the effect happens in people.
Iodine, heavy metals and allergy come first; see the Safety tab.
Connections to the rest of health
How Nori (Laver) connects to the rest of our health knowledge graph. Every link says what kind of connection it is. A safety link (for example to the liver) is a warning, not a benefit. A study that merely mentions a topic never creates a benefit link.
Explore every connection: goals, body systems, organs, symptoms, genes
A. Wellness goals
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Daily Vitality | High certainty | Everyday nutrients in laver: USDA lists raw (wet) laver with 5.81 g protein, 146 microg folate, 39 mg vitamin C, 0.988 mg manganese and 260 microg vitamin A per 100 g. Dried sushi sheets are lighter and more concentrated, so these numbers do not carry over to a sheet. This is a nutrient fact, not a treatment. | 18 9 | |
| Thyroid Support | High certainty | Iodine supply (nutrient content): Nori is lower in iodine than kombu on average, but real products range a lot. A 2004 lab test found 16 microg per gram in one nori type. A 2022 Hong Kong test found up to 34,600 microg per 100 g in a nori for rice balls. Iodine is needed for the thyroid, but too much can harm it. | 12 16 3 14 | |
| Nutrient Support | Moderate certainty | Vitamin B12 and folate content of dried nori: Lab tests found true vitamin B12 in dried purple laver: about 30 to 60 microg per 100 g dry weight in one analysis, and 1 to 2.7 microg per sheet in a review. Folate was also high. These are measurements in the product, not proof of what a person's body gets. | 6 7 13 | |
| Nutrient Support | Low certainty | Vitamin B12 blood markers in vegetarians: In one small 4-week trial in 30 vegetarians, people who ate roasted nori had higher blood B12 and holoTC and lower homocysteine than people who ate none. MMA did not differ. It was open-label, short, and measured blood markers, not health problems. | 4 | |
| Blood Pressure Support | Very low certainty | Blood pressure (pooled seaweed trials): A 2025 review pooled 29 trials of many edible algae, mostly not nori, and found blood pressure about 2 mmHg lower. The abstract says nori and kelp are rich in nitrate but gives no nori-only result. For nori, the evidence is very thin. | 1 | |
| Gut Health | Not enough evidence | Gut bacteria: In a 12-week Korean pilot, 24 older women all ate dried laver and had a more diverse mix of gut bacteria afterwards. There was no comparison group and no health outcome. It is a hint, not a finding. | 10 |
B. Body systems
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Metabolic System | Biological association | High certainty | Studied for: Everyday nutrients in laver Studied for: Vitamin B12 and folate content of dried nori | 18 9 6 7 13 4 |
| Endocrine System | Biological association | High certainty | Studied for: Iodine supply (nutrient content) | 12 16 3 14 |
| Cardiovascular System | Biological association | Very low certainty | Studied for: Blood pressure (pooled seaweed trials) | 1 |
| Digestive System | Biological association | Not enough evidence | Studied for: Gut bacteria | 10 |
C. Body parts and organs
G. Root causes and contributing factors
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Heavy metal exposure | Adverse effect or risk | Documented risk | Nearly all commercial seaweed samples, nori among them, were above a seaweed-institute guide for cadmium and arsenic in a 2026 analysis. Preparation or compound: Seaweed as food |
9 2 |
| Nutrient Deficiency | Preliminary | Vitamin B12 blood markers were better with roasted nori in one 4-week open-label trial in 30 vegetarians; not repeated. Preparation or compound: Roasted nori |
4 |
I. Biomarkers
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Serum vitamin B12 | Biological association | Low certainty | Measured in trials: raised. A lab change is not the same as a health benefit. | 4 |
| Holotranscobalamin (holoTC) | Biological association | Low certainty | Measured in trials: raised. A lab change is not the same as a health benefit. | 4 |
| Homocysteine | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 4 |
| Methylmalonic acid (MMA) | Biological association | Low certainty | Measured in trials: no-change. A lab change is not the same as a health benefit. | 4 |
| Systolic blood pressure | Biological association | Very low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 1 |
| Diastolic blood pressure | Biological association | Very low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 1 |
K. Biological pathways
L. Active compounds
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Iodine | Biological association | Human | Needed for thyroid hormones. Too much may disturb the thyroid. Preparation or compound: Whole seaweed |
12 16 14 |
| Vitamin B12 (cobalamin) | Biological association | Human | Needed for blood and nerves. Nori B12 raised blood markers in one small trial. Preparation or compound: Dried purple laver |
6 7 4 |
| Folate (mainly 5-methyltetrahydrofolate) | Biological association | Laboratory | A B vitamin; measured, not tested as a treatment. Preparation or compound: Dried purple laver |
6 |
| EPA (an omega-3 fatty acid) | Biological association | Laboratory | Measured only; the 2026 analysis found red algae, especially nori, rich in EPA. Preparation or compound: Whole seaweed |
9 |
| Cadmium and arsenic | Biological association | Laboratory | Contaminants taken up from sea water; a safety topic, not a benefit. Preparation or compound: Whole seaweed |
2 9 |
| Pheophorbide a and pyropheophorbide a | Biological association | Human | Light-activated compounds that can cause skin reactions; found at high levels in one implicated product. Preparation or compound: Dried laver |
5 |
Each connection is stored with the ingredient, the connected record, the kind of connection, the evidence status, the supporting studies, the preparation or compound involved, and a review date (last reviewed October 26, 2026).
Evidence at a glance: what changed in 6 lab markers
Tap a number to show only those markers. Counts are markers, not people.
The evidence
The research centre for Nori (Laver). Every record here was checked against PubMed or the publisher's page; we never list a study we have not verified.
Evidence by outcome
Bars show how many studies we found for each outcome, not how well it works. People are added up across trials only (reviews are not added, to avoid double counting).
How we rate evidence
High certainty
Several well-run trials or reviews agree; further research is unlikely to change the picture much.
Moderate certainty
Good trials point the same way, but some doubt remains.
Low certainty
Small, short or inconsistent trials. The finding could change with better research.
Very low certainty
Very little or very weak human data. Treat as a possibility only.
Mixed findings
Trials or reviews disagree with each other.
Not enough evidence
Not enough human research to say either way.
Evidence against
Good research found no benefit, or found harm.
We take the reviewers' own risk-of-bias and GRADE statements where they exist, and otherwise judge from study size, design, consistency, directness and whether a product matches what is sold. Many publications do not make evidence high. Method note: We started from our existing nori summary, re-checked the nutrition against saved USDA FoodData Central data, read the American Thyroid Association, NHS and Hong Kong Consumer Council pages, checked names in GBIF, and re-read PubMed abstracts for the studies. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.
Study library: every study we checked 18
| Study | Design | People | Product, dose, length | Result |
|---|---|---|---|---|
| Casas-Agustench 2025 Edible Algae Reduce Blood Pressure in Humans: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. J Hum Nutr Diet · PMID 40726022 · DOI |
Systematic review meta analysis n/a |
1583 29 randomised trials, 1,583 adults, healthy or with a cardiometabolic condition, at least 4 weeks |
Edible algae of many kinds (spirulina was most studied) and extracts Whole algae and extracts Varied At least 4 weeks · vs Placebo or control |
Edible algae lowered systolic BP by 2.05 mmHg and diastolic BP by 1.87 mmHg overall, with high heterogeneity (I2 75% and 68%). Spirulina had the largest effect. Whole algae did better than extracts. The abstract notes nori and kelp are rich in nitrate but gives no nori-only result. SBP -2.05 mmHg; DBP -1.87 mmHg (SBP 95% CI -3.80 to -0.31; DBP -3.10 to -0.64) Methods, limits and fundingOutcomes: Systolic and diastolic blood pressure Limitations: Pools many different algae, mostly not nori. High heterogeneity. Nori-specific results are not in the abstract. Funding and conflicts: Authors declare no conflicts of interest. Read: abstract |
| Duan 2024 Cadmium bioavailability in market nori and kelp: A comparison with rice and mechanisms underlying reduction in rice cadmium bioavailability with nori and kelp consumption. Sci Total Environ · PMID 39488285 · DOI |
Animal study with lab analysis n/a |
35 35 market nori samples and 24 kelp samples measured; mouse feeding study for 14 days |
Market nori and kelp mixed into mouse feed Whole seaweed 5% of the feed by weight (mice) 14 days · vs Cadmium-free rice |
All 35 nori samples had cadmium above 1.2 microg/g, while most kelp samples (18 of 24) were below 0.5 microg/g. Mice fed nori at 1.30 and 1.67 microg/g built up cadmium in tissue. The authors say caution is advised for nori, but cadmium from nori was less available than from rice, and nori or kelp mixed with cadmium-containing rice reduced the rice cadmium in mouse tissue by 25.8% to 48.1%. Nori cadmium above 1.2 microg/g in all 35 samples (Not applicable) Methods, limits and fundingOutcomes: Cadmium in seaweed; cadmium in mouse liver and kidney Limitations: A mouse study plus lab analysis. Does not show harm in people. Sample origin not stated in the abstract. Funding and conflicts: Authors declare no competing interests. Read: abstract |
| European Food Safety Authority (EFSA); Dujardin 2023 Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European population. EFSA J · PMID 36742462 · DOI |
Exposure assessment n/a |
468 468 subjects in 19 European countries with 697 seaweed eating occasions (consumption database); 2011-2021 analytical data |
Seaweed and halophyte foods Whole seaweed and related foods |
For adults who eat seaweed, cadmium exposure estimates were within the range of earlier whole-diet estimates; inorganic arsenic and lead were between 10% and 30% of earlier whole-diet exposures. Mean iodine intakes above 20 microg/kg body weight a day were found in consumers of kombu and laver. Mean iodine above 20 microg per kg body weight per day among consumers of laver (and kombu) (Not applicable) Methods, limits and fundingOutcomes: Dietary exposure to arsenic, cadmium, lead, mercury and iodine Limitations: A modelling report from a small European database. The abstract does not separate nori from other laver foods. Funding and conflicts: Official EU agency report. Read: abstract |
| Huang 2024 Effect of roasted purple laver (nori) on vitamin B(12) nutritional status of vegetarians: a dose-response trial. Eur J Nutr · PMID 39352476 · DOI |
RCT n/a |
30 30 vegetarians who did not use supplements |
Roasted dried purple laver (nori) Whole dried seaweed sheets 5 g or 8 g of nori a day (aiming for 2.4 or 4 microg of vitamin B12) 4 weeks · vs No nori (control group) |
After adjusting for baseline, nori went with significant improvement in serum B12 (P = 0.0029), holoTC (P = 0.0127), homocysteine (P = 0.0225) and the combined score (P = 0.0094). MMA did not differ between groups. The 5 g group did about as well as the 8 g group. Open-label. 5 g group vs control: serum B12 +59 pmol/L, holoTC +28.2 pmol/L, homocysteine -3.7 micromol/L, 4cB12 +0.67 (5 g group vs control, 95% CI: B12 25 to 93; holoTC 10.1 to 46.3; homocysteine -6.8 to -0.6; 4cB12 0.24 to 1.09) Methods, limits and fundingOutcomes: Serum vitamin B12, holotranscobalamin (holoTC), homocysteine, methylmalonic acid (MMA) and a combined score of the four (4cB12) Limitations: Small (30 people), open-label (no blinding), 4 weeks, blood markers only. No clinical outcome. Registered as NCT05614960. Funding and conflicts: No conflict of interest reported (per the abstract). Read: abstract |
| Hwang 2005 HPLC determination of pheophorbide a and pyropheophorbide a in dried laver product implicated in food poisoning. Shokuhin Eiseigaku Zasshi · PMID 16018590 · DOI |
Outbreak report n/a |
4 Four people with a skin reaction after eating one dried laver (nori) product; five volunteers tested on the skin |
Extract of the implicated nori product Dried laver |
Four people had allergic-like symptoms (inflammation and a red rash on the face, mouth and belly). Three of five volunteers had a slight reaction when the extract on the skin was exposed to sunlight. Pheophorbide a and pyropheophorbide a were high in the product (851 to 906 and 5,460 to 5,624 microg/g), and the authors concluded they were the causes. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Symptoms, and chlorophyll-breakdown compounds measured by HPLC Limitations: A single outbreak with one product, in Taiwan. It shows this can happen, not how often. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Koseki 2023 Determination of Vitamin B(12) and Folate Compounds in Commercially Available Edible Seaweed Products. Front Biosci (Elite Ed) · PMID 37369571 · DOI |
Laboratory analysis n/a |
Commercially available edible seaweed products (dried purple laver, kombu, hijiki, wakame) | Seaweed products measured by HPLC Whole seaweed products |
Dried purple laver (Neopyropia yezoensis) products had higher vitamin B12 (about 30 to 60 microg/100 g dry weight) and folate compounds (about 880 to 1300 microg/100 g dry weight) than kombu, hijiki and wakame. 5-methyltetrahydrofolate was the main folate form. B12 about 30 to 60 microg/100 g dry weight; folate about 880 to 1300 microg/100 g dry weight (Not applicable) Methods, limits and fundingOutcomes: Vitamin B12 and folate compounds per 100 g dry weight Limitations: A lab measurement of products, not a test in people. Dry weight, not as eaten. Number of samples not in the abstract. Same research group as the 2024 trial and 2014 review (Watanabe, Koseki). Funding and conflicts: No conflict of interest reported (per the abstract). Read: abstract |
| Marques de Brito 2023 Vitamin B12 sources in non-animal foods: a systematic review. Crit Rev Food Sci Nutr · PMID 35343314 · DOI |
Systematic review n/a |
25 25 studies measuring B12 in seaweeds, mushrooms, plants and fermented foods |
Non-animal foods measured for B12 Foods |
Real B12 ranged from 3 x 10-3 microg per 7 g portion of hijiki to 1.03 to 2.68 microg per sheet of nori. The review says daily B12 needs may be met by a varied diet with non-animal B12 sources. Nori 1.03 to 2.68 microg real B12 per sheet (Not applicable) Methods, limits and fundingOutcomes: Real vitamin B12 content (by the more specific mass-spectrometry methods) Limitations: Reviews food content, not B12 status in people. Sheet sizes vary. Earlier studies used methods that can count B12-like compounds that do not work as B12. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Motoyama 2007 Allergenicity and allergens of amphipods found in nori (dried laver). Food Addit Contam · PMID 17691004 · DOI |
Laboratory immunoassay n/a |
Serum samples from crustacean-allergic patients tested against small crustaceans (amphipods) found in nori | Extracts of amphipods that can end up in nori sheets Amphipod extracts · vs Prawn and spiny lobster extracts |
Amphipods can be found in nori sheets because they live on the laver culture nets. Sera from crustacean-allergic patients reacted to amphipod extracts, less strongly than to prawn or lobster. The main allergen was tropomyosin (37 kDa). Not applicable (Not applicable) Methods, limits and fundingOutcomes: IgE reactivity (ELISA, immunoblot) Limitations: A lab test on blood samples, not a food challenge. It does not say how often amphipods occur in nori sold today. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Nava 2026 Commercial Edible Seaweeds: Characterization by Mineral Content and Fatty Acid Composition. Foods · PMID 42794005 · DOI |
Laboratory analysis n/a |
Commercial edible seaweeds: dulse, wakame, kombu, arame, nori and spirulina | Commercial seaweeds measured for minerals, toxic elements and fatty acids Whole seaweed |
Nearly all samples exceeded the CEVA guide of 0.5 mg/kg cadmium and 3 mg/kg total arsenic. Lead and mercury stayed below regulatory limits. Kombu was extremely high in iodine. Red algae, especially nori, were rich in EPA. Not given for nori alone in the abstract (Not applicable) Methods, limits and fundingOutcomes: Minerals, toxic elements (Pb, Hg, Cd, As), iodine and fatty acids Limitations: Abstract does not give nori-only numbers or the number of samples. CEVA limits are a seaweed-institute guide, not a law. Funding and conflicts: Authors declare no conflicts of interest. Read: abstract |
| Shin 2026 Dietary Dried Laver (Porphyra tenera) Modulates Gut Microbiota Composition and Diversity in Older Women with and Without Metabolic Syndrome: An Exploratory Pilot Study. Nutrients · PMID 42196995 · DOI |
Pilot single group n/a |
24 24 older women in Korea: 13 with metabolic syndrome and 11 without |
Dried laver (Porphyra tenera) Whole dried seaweed 4 g a day 12 weeks |
Firmicutes/Bacteroidetes ratio fell 0.81-fold in the control group and 0.54-fold in the metabolic syndrome group. Alpha diversity (Chao1 and Fisher) rose significantly in both groups. Four genera differed significantly. The authors call the results preliminary and ask for larger randomised trials. F/B ratio 0.81-fold and 0.54-fold; diversity up (p < 0.001 and p < 0.05) (Not stated in the abstract) Methods, limits and fundingOutcomes: Gut bacteria (16S rRNA sequencing): diversity and composition Limitations: Very small, no comparison group, bacteria counts only, no health outcome. Funding not stated. Funding and conflicts: Authors declare no conflicts of interest. Read: abstract |
| Taylor 2026 What Endocrinologists Should Know About Iodine: Population Deficiency, Individual Excess and Misinformation in the United Kingdom. Clin Endocrinol (Oxf) · PMID 42144726 · DOI |
Narrative review n/a |
UK iodine status and patients with iodine problems | Review of iodine deficiency and excess |
Mild iodine deficiency has returned in the UK, particularly in women of reproductive age, vegans and people avoiding dairy. A growing minority of patients have iodine-induced thyroid problems linked to kelp-based supplements with very high iodine. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Iodine status and thyroid effects Limitations: A review. It does not discuss nori by name in the abstract. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Teas 2004 Variability of iodine content in common commercially available edible seaweeds. Thyroid · PMID 15588380 · DOI |
Laboratory analysis n/a |
12 species of dietary seaweed from US commercial sources plus samples from Canada, Tasmania and Namibia | Edible seaweeds measured for iodine Whole seaweed |
Iodine ranged from 16 microg/g (+/-2) in nori (Porphyra tenera) to over 8165 microg/g in processed kelp granules. Iodine dissolves in cooking water and may be lost in humid storage, so prepared dishes are hard to estimate. Nori 16 microg/g (Not applicable) Methods, limits and fundingOutcomes: Iodine content per gram Limitations: Lab measurement of one nori sample type from 2004, not a survey of today's nori products. Later product tests (Hong Kong, 2022) found far higher values in some nori for rice balls. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Watanabe 2014 Vitamin B12-containing plant food sources for vegetarians. Nutrients · PMID 24803097 · DOI |
Narrative review n/a |
Plant foods surveyed for vitamin B12 | Plant foods measured for B12 Foods |
The authors' survey suggested dried purple laver (nori) is the most suitable plant-derived B12 source currently available for vegetarians, and that it also has iron and n-3 fats. Not given as numbers in the abstract (Not applicable) Methods, limits and fundingOutcomes: Vitamin B12 content of plant foods Limitations: A narrative review by researchers who have studied nori B12 for years; abstract gives no numbers. An opinion on suitability, not a test in people. Funding and conflicts: Not stated in the abstract. Read: abstract |
| s-ata-2013 American Thyroid Association: ATA statement on the potential risks of excess iodine ingestion and exposure · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-gbif-pyropia GBIF Backbone: Pyropia J.Agardh (genus, usageKey 4908866) and related names · Source |
Reference database n/a |
Methods, limits and fundingRead: full-text | ||
| s-hk-consumer-2022 Hong Kong Consumer Council (CHOICE): Healthy seaweed cannot be consumed without limit, especially for children (seaweed products test, no. 552) · Source |
Consumer body product test n/a |
Methods, limits and fundingRead: full-text | ||
| s-nhs-iodine NHS: Vitamins and minerals: Iodine · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-usda-fdc-168458 USDA FoodData Central: Seaweed, laver, raw (SR Legacy, FDC 168458) · Source |
Reference database n/a |
Methods, limits and fundingRead: saved-copy |
What we do not know yet
- Only one trial of nori itself for B12 was found: 30 vegetarians for 4 weeks, open-label. No trial in people with diagnosed B12 deficiency was found.
- No study of nori in people with thyroid disease or on thyroid medicine, or with warfarin, was found.
- Nutrient values for dried sushi sheets, snack sheets and flakes were not read; USDA lists raw (wet) laver.
- The blood pressure review pools many algae and gives no nori-only result in its abstract.
- Official pregnancy advice about nori in particular, children's iodine limits from an official table, how common nori allergy is, choking risk, history and farming were not researched.
How new research is added
New studies are found by a saved PubMed search and placed in a review queue before the page changes.
Lab markers
What changed in blood or saliva tests during trials. A changed number is not the same as feeling or being healthier, and we give no "normal ranges".
Other 6
Serum vitamin B12
Raised in trials
The amount of vitamin B12 in a blood sample. It is the usual first test of B12 status.
Low certaintyResults and what it means
Why it was measured
Low B12 can cause anaemia and nerve problems.
Statistical vs. clinical meaning
A higher blood level in a small open-label trial; not a health outcome.
Safety implication
None found for this measure.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Higher with nori than with no nori after adjusting for baseline (among-group P = 0.0029); +59 pmol/L (95% CI 25 to 93) in the 5 g group vs control. | 30 vegetarians, 4 weeks | Roasted nori, 5 g or 8 g a day, vs none |
Holotranscobalamin (holoTC)
Raised in trials
The part of blood B12 that is ready for cells to use (active B12).
Low certaintyResults and what it means
Why it was measured
It is considered an early marker of B12 status.
Statistical vs. clinical meaning
A higher active-B12 marker in one small trial.
Safety implication
None found for this measure.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Higher with nori (P = 0.0127 among groups); +28.2 pmol/L (95% CI 10.1 to 46.3) in the 5 g group vs control. | 30 vegetarians, 4 weeks | Roasted nori vs none |
Homocysteine
Lowered in trials
An amino acid in the blood that rises when B12 or folate is low.
Low certaintyResults and what it means
Why it was measured
High levels are linked to a higher risk of heart disease; low B12 is one cause of a high level.
Statistical vs. clinical meaning
A lower marker in one small trial; not shown to change heart disease.
Safety implication
None found for this measure.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Lower with nori (P = 0.0225 among groups); -3.7 micromol/L (95% CI -6.8 to -0.6) in the 5 g group vs control. | 30 vegetarians, 4 weeks | Roasted nori vs none |
Methylmalonic acid (MMA)
No clear change in trials
A substance that builds up in blood when B12 is short; another B12 status test.
Low certaintyResults and what it means
Why it was measured
It is a sensitive marker of B12 shortage.
Statistical vs. clinical meaning
One of the four markers did not show a group difference, which is why the trial is read with caution.
Safety implication
None found for this measure.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Changes did not differ significantly between groups, though the low-dose group improved before to after (median -339 nmol/L). | 30 vegetarians, 4 weeks | Roasted nori vs none |
Systolic blood pressure
Lowered in trials
The top number in a blood pressure reading, the pressure when the heart beats.
Very low certaintyResults and what it means
Why it was measured
High blood pressure raises the risk of heart disease and stroke.
Statistical vs. clinical meaning
A small pooled change for algae in general, not for nori specifically.
Safety implication
None found for this measure.
| Study | Result | People | Product |
|---|---|---|---|
| 1 | Lower by 2.05 mmHg (95% CI -3.80 to -0.31) across all edible algae pooled; high heterogeneity; no nori-only result given. | 29 randomised trials, 1,583 adults | Edible algae of many kinds, at least 4 weeks |
Diastolic blood pressure
Lowered in trials
The bottom number in a blood pressure reading, the pressure between beats.
Very low certaintyResults and what it means
Why it was measured
It is the second half of a blood pressure reading.
Statistical vs. clinical meaning
A small pooled change for algae in general, not for nori specifically.
Safety implication
None found for this measure.
| Study | Result | People | Product |
|---|---|---|---|
| 1 | Lower by 1.87 mmHg (95% CI -3.10 to -0.64) across all edible algae pooled; no nori-only result given. | 29 randomised trials, 1,583 adults | Edible algae of many kinds, at least 4 weeks |
Safety
What studies and official sources say about side effects, medicines and who should check first. Each point shows what kind of evidence it comes from.
Main points to know 4
- Iodine: lower than kelp on average, but nori products vary a lot. A 2004 lab test found 16 microg of iodine per gram in nori, the lowest of 12 seaweeds. A 2022 Hong Kong test found up to 34,600 microg per 100 g in a nori for rice balls, where one 1.3 g serving gave 450 microg. The American Thyroid Association says more than 1,100 microg a day is not recommended and may cause thyroid dysfunction. Labels often do not state iodine. Laboratory measurement and authoritative guidance 12 16 14
- Cadmium and arsenic. A 2026 lab study found nearly all commercial seaweed samples, nori among six types tested, above a French seaweed-institute guide of 0.5 mg/kg cadmium and 3 mg/kg total arsenic. A mouse study with 35 market nori samples warned that nori carries cadmium. A Hong Kong test found cadmium and inorganic arsenic in all 27 seaweed samples but within the standards it applied. Laboratory analysis and animal 9 2 16
- Allergy and skin reaction cases. Small crustaceans (amphipods) can end up in nori sheets, and blood from crustacean-allergic patients reacted to them in the lab. One outbreak in Taiwan of rash after a nori product was traced to chlorophyll breakdown compounds that react to sunlight. Both are rare reports, not common problems. Laboratory immunoassay and outbreak report 8 5
- Thyroid disease, pregnancy, young children and kidney disease need extra care. People with thyroid disease, people who are pregnant or breastfeeding, and young children should ask a clinician before eating nori often, especially nori snacks and nori for rice balls. The Hong Kong Consumer Council also advises people with chronic kidney disease, or on warfarin, to talk to a doctor or dietitian before eating seaweed. Expert guidance 14 16
Side effects 4
| Effect | How often | Severity and recovery | Evidence |
|---|---|---|---|
| Too much iodine can disturb the thyroid (thyroid dysfunction) ATA: more than 1,100 microg a day may cause thyroid dysfunction. NHS: high doses of iodine for long periods could change how the thyroid works. A 2026 UK review describes patients with iodine-induced thyroid problems linked to kelp-based supplements, not nori. | Not found for nori as food. | can be serious over time Not described in the sources read. | Authoritative guidance 14 17 11 |
| Allergic-like skin reaction with sunlight (rash on face, mouth and belly) after one nori product Three of five volunteers had a slight skin reaction in sunlight to an extract of the product. The authors blamed pheophorbide a and pyropheophorbide a, which were very high in that product. A single outbreak from 2005. | Four people in one outbreak in Taiwan; no rate found. | rash and inflammation in the report; a slight reaction in volunteers Not stated in the abstract. | Outbreak report 5 |
| Allergic reaction in people allergic to shellfish, from amphipods mixed into nori sheets The abstract says amphipods in nori may cause allergic reactions in sensitized patients. Tests used blood samples, not people eating nori. Cross-reaction with tropomyosin in prawn and lobster. | Not found. | not stated Not stated | Laboratory immunoassay 8 |
| High iodine in snack and rice-ball nori, especially for small children The Council said two servings of that product exceeded the upper level for children aged 1 to 10. | One toddler-labelled product tested had 171 microg iodine per serving. | depends on how much is eaten Not stated | Consumer product test 16 |
Liver
No liver injury from eating nori was found in the sources read. A liver-safety source such as LiverTox was not searched for nori.
How often. No frequency was found.
Monitoring. No agency monitoring advice was found.
This is a safety connection. It is not evidence that Nori (Laver) supports liver health.
Medicines and supplements 3
| With | Why it matters | Importance | Evidence | What to do |
|---|---|---|---|---|
| Thyroid medicines (levothyroxine, antithyroid drugs) and thyroid disease prescription | Iodine acts on the thyroid itself; whether nori changes how thyroid medicine works was not found. | No source on nori with thyroid medicine was read. Together with the iodine facts above, this is a question for a clinician. | None found Not enough evidence 14 | If you have thyroid disease or take thyroid medicine, ask your clinician before eating nori often. |
| Warfarin and other blood thinners prescription | Vitamin K helps blood clot. USDA lists 4 microg of vitamin K per 100 g of raw laver, a small amount. | The Hong Kong Consumer Council advises people on warfarin to talk to a doctor or dietitian before eating seaweed. No nori-specific interaction study was read. | Consumer body advice Not enough evidence 16 18 | If you take warfarin, ask your pharmacist or doctor and keep your seaweed eating steady. |
| Kelp tablets, iodine pills and iodine-containing supplements supplement | Iodine adds up from all sources. | ATA advises against iodine and kelp supplements containing more than 500 mcg iodine daily and says many contain up to several thousand times the upper limit. | Authoritative guidance Moderate certainty 14 11 | Count iodine from every source, and ask a pharmacist or clinician before combining nori with kelp or iodine supplements. |
Who should check first 5
| Situation | Advice | Evidence |
|---|---|---|
| Thyroid disease, or taking thyroid medicine | Ask your clinician before eating nori often. This page does not tell anyone to eat or avoid it for a thyroid problem. | Expert guidance 14 17 |
| Pregnancy or breastfeeding | ATA says recommended iodine intakes are higher in pregnancy (220 to 250 microg a day) and breastfeeding (250 to 290 microg), while the 1,100 microg adult upper limit still applies. No nori-specific advice was found. Ask your midwife or doctor. | Authoritative guidance 14 |
| Young children | The Hong Kong Consumer Council notes that infants and young children have a lower iodine upper level than adults, and found a product labelled for toddlers whose two servings exceeded it. Ask a clinician before giving children nori snacks often. Choking risk from sheets was not researched. | Consumer body advice 16 |
| Chronic kidney disease | The Hong Kong Consumer Council advises people with chronic kidney disease to talk to a doctor or dietitian before eating seaweed. The reason for nori in particular was not stated in the page read. | Consumer body advice 16 |
| Shellfish (crustacean) allergy | Amphipods can be mixed into nori sheets, and tests with blood from shellfish-allergic patients showed a reaction in the lab. No official guidance for nori was read. Ask your allergy doctor. | Laboratory immunoassay 8 |
Overdose, product quality and regulators
Summary. The safety points for nori as food are iodine, cadmium and arsenic, allergy to hitchhiking crustaceans and rare skin reactions to chlorophyll breakdown products. No other toxicity from ordinary nori was found in the sources read.
Animal. In a 14-day mouse study, market nori with 1.30 and 1.67 microg/g cadmium led to cadmium building up in liver and kidney; the authors said caution is advised for nori, though cadmium in nori was less available than in rice.
High dose or long term signals. NHS says high doses of iodine for long periods could change how the thyroid works. ATA says more than 1,100 microg a day may cause thyroid dysfunction.
Overdose response. Not found.
Genotoxicity. Not researched.
Nori takes up minerals and metals from sea water, and sheets are made on culture nets in the sea.
Cadmium and arsenic (lab analysis). In a 2026 analysis of six commercial seaweed types including nori, nearly all samples exceeded the CEVA guide limits of 0.5 mg/kg cadmium and 3 mg/kg total arsenic. Lead and mercury stayed below regulatory limits. The guide is not a law, and nori-only numbers are not in the abstract. 9
Cadmium in market nori (lab and mouse). All 35 market nori samples in a study had cadmium above 1.2 microg/g, compared with most kelp samples under 0.5 microg/g. In mice, nori cadmium built up in tissue, but less than rice with similar cadmium; the authors advised caution for nori. 2
Hong Kong product test. Cadmium and inorganic arsenic were found in all 27 ready-to-eat seaweed samples, including 8 nori wrappers. The Council said amounts did not exceed the standards it applied and would probably not pose risk in regular portions. 16
EU exposure estimates. EFSA found that for adults who eat seaweed, cadmium exposure was within the range of whole-diet estimates, and inorganic arsenic and lead were 10% to 30% of earlier whole-diet exposures. 3
Crustaceans mixed into sheets. Amphipods living on laver culture nets are occasionally found in nori sheets. 8
Iodine labelling. The Hong Kong test found iodine varied almost 33 times between products and that iodine is not required on labels there; for most products, other nutrients on the label also differed from test results. 16
| Body | Action | Date |
|---|---|---|
| American Thyroid Association (United States) | Statement: more than 1,100 mcg of iodine a day (Tolerable Upper Limit) is not recommended and may cause thyroid dysfunction; advises against iodine and kelp supplements containing more than 500 mcg iodine daily. source | 2013-06-05 |
| NHS (United Kingdom) | Says taking high doses of iodine for long periods could change how the thyroid gland works; the page does not mention seaweed. source | 2020-08-03 |
| European Food Safety Authority (EFSA) (European Union) | Scientific report on exposure to arsenic, cadmium, lead, mercury and iodine from seaweeds; found mean iodine intakes above 20 microg/kg body weight a day among consumers of kombu and laver. source | 2023-01-31 |
| Hong Kong Consumer Council (Hong Kong) | Consumer product test: iodine in 27 ready-to-eat seaweeds ranged 1,030 to 34,600 microg per 100 g; advises people with chronic kidney disease or on warfarin to talk to a doctor or dietitian before eating seaweed. Not a regulator. source | 2022 |
When to get medical help
Stop using the product and contact a clinician or emergency service. In the US, Poison Control is 1-800-222-1222.
General information only, not medical advice. Ask a clinician or pharmacist before use, especially with medicines, in pregnancy or with a health condition. See the Natural Remedy Safety guide.
Frequently asked questions
What is nori?
Nori is the thin, dark sheet made from dried red seaweed of the Pyropia and Porphyra group. It is used for sushi, rice balls and as flakes.
Does nori have vitamin B12?
Can nori replace a B12 supplement for vegans?
That is not shown. One 4-week open-label trial in 30 vegetarians found better B12 blood markers with roasted nori, but MMA did not differ and it was not repeated. Ask a clinician or dietitian about B12 on a plant-based diet.
Is nori high in iodine?
Is nori good for the thyroid?
Does nori lower blood pressure?
A 2025 review of 29 trials found edible algae lowered blood pressure by about 2 mmHg overall, but it pools many algae and the abstract gives no nori-only result.
Is nori safe in pregnancy?
Is nori safe for children?
A Hong Kong test found a product labelled for toddlers whose two servings exceeded the upper level for ages 1 to 10. Choking risk was not researched. Ask a clinician before giving children nori snacks often.
Does nori contain cadmium and arsenic?
Can nori cause an allergic reaction?
Small crustaceans can end up in nori sheets, and blood from shellfish-allergic patients reacted to them in the lab. If you have a shellfish allergy, ask your allergy doctor. For trouble breathing or throat swelling, call emergency services.
Does nori interact with medicines?
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Editorial transparency
- Written by
- AlternativeMed editorial team. Research compiled with AI assistance, then checked against the sources listed.
- Medical review
- None. This page has not been reviewed by a physician or pharmacist.
- First published
- 2026-10-26
- Last substantive update
- 2026-10-26
- How we research
- We started from our existing nori summary, re-checked the nutrition against saved USDA FoodData Central data, read the American Thyroid Association, NHS and Hong Kong Consumer Council pages, checked names in GBIF, and re-read PubMed abstracts for the studies. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.
- How we rate evidence
- See "How we rate evidence" in the Evidence tab.
- Independence
- No product is promoted on this page. No affiliate links are shown. Funding is not stated in the abstracts of most studies used.
Sources 18
Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.
- Casas-Agustench P, Minguez S, Brookes Z, Bescos R. Edible Algae Reduce Blood Pressure in Humans: A Systematic Review and Meta-Analysis of Randomised Controlled Trials.. J Hum Nutr Diet. 2025. PMID 40726022. DOI 10.1111/jhn.70095. (abstract)
- Duan X, Zhou L, Chen SZ, Lin XY, Xue RY, Xue XM, Zhou D, Li HB. Cadmium bioavailability in market nori and kelp: A comparison with rice and mechanisms underlying reduction in rice cadmium bioavailability with nori and kelp consumption.. Sci Total Environ. 2024. PMID 39488285. DOI 10.1016/j.scitotenv.2024.177329. (abstract)
- European Food Safety Authority (EFSA); Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA. Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European population.. EFSA J. 2023. PMID 36742462. DOI 10.2903/j.efsa.2023.7798. (abstract)
- Huang QN, Watanabe F, Koseki K, He RE, Lee HL, Chiu THT. Effect of roasted purple laver (nori) on vitamin B(12) nutritional status of vegetarians: a dose-response trial.. Eur J Nutr. 2024. PMID 39352476. DOI 10.1007/s00394-024-03505-9. (abstract)
- Hwang DF, Tsai YS, Chou SS, Liu SM, Wu JT, Lin SJ, Tu WC. HPLC determination of pheophorbide a and pyropheophorbide a in dried laver product implicated in food poisoning.. Shokuhin Eiseigaku Zasshi. 2005. PMID 16018590. DOI 10.3358/shokueishi.46.45. (abstract)
- Koseki K, Yoshimura R, Ido K, Katsuura K, Bito T, Watanabe F. Determination of Vitamin B(12) and Folate Compounds in Commercially Available Edible Seaweed Products.. Front Biosci (Elite Ed). 2023. PMID 37369571. DOI 10.31083/j.fbe1502010. (abstract)
- Marques de Brito B, Campos VM, Neves FJ, Ramos LR, Tomita LY. Vitamin B12 sources in non-animal foods: a systematic review.. Crit Rev Food Sci Nutr. 2023. PMID 35343314. DOI 10.1080/10408398.2022.2053057. (abstract)
- Motoyama K, Hamada Y, Nagashima Y, Shiomi K. Allergenicity and allergens of amphipods found in nori (dried laver).. Food Addit Contam. 2007. PMID 17691004. DOI 10.1080/02652030701305454. (abstract)
- Nava V, Potorti AG, Albergamo A, Sturniolo R, Spano IM, Di Bella G. Commercial Edible Seaweeds: Characterization by Mineral Content and Fatty Acid Composition.. Foods. 2026. PMID 42794005. DOI 10.3390/foods15183219. (abstract)
- Shin D, Lee S, So B, Kang C, Lee KJ. Dietary Dried Laver (Porphyra tenera) Modulates Gut Microbiota Composition and Diversity in Older Women with and Without Metabolic Syndrome: An Exploratory Pilot Study.. Nutrients. 2026. PMID 42196995. DOI 10.3390/nu18101535. (abstract)
- Taylor PN, Prentice M, Bath S, Combet E, Schoenmakers N, Smith VE, Gibb FW, Zammitt N, Okosieme O, Boelaert K, Lazarus J, Vanderpump M. What Endocrinologists Should Know About Iodine: Population Deficiency, Individual Excess and Misinformation in the United Kingdom.. Clin Endocrinol (Oxf). 2026. PMID 42144726. DOI 10.1111/cen.70162. (abstract)
- Teas J, Pino S, Critchley A, Braverman LE. Variability of iodine content in common commercially available edible seaweeds.. Thyroid. 2004. PMID 15588380. DOI 10.1089/thy.2004.14.836. (abstract)
- Watanabe F, Yabuta Y, Bito T, Teng F. Vitamin B12-containing plant food sources for vegetarians.. Nutrients. 2014. PMID 24803097. DOI 10.3390/nu6051861. (abstract)
- . American Thyroid Association: ATA statement on the potential risks of excess iodine ingestion and exposure. . . (full-text)
- . GBIF Backbone: Pyropia J.Agardh (genus, usageKey 4908866) and related names. . . (full-text)
- . Hong Kong Consumer Council (CHOICE): Healthy seaweed cannot be consumed without limit, especially for children (seaweed products test, no. 552). . . (full-text)
- . NHS: Vitamins and minerals: Iodine. . . (full-text)
- . USDA FoodData Central: Seaweed, laver, raw (SR Legacy, FDC 168458). . . (saved-copy)
Where to buy
Check the label first. Whatever you buy, look for these:
- The iodine amount: seaweed can be very high in iodine, which matters for thyroid conditions
- Heavy metal testing information from the maker
- Origin and harvest area
- Organic or tested labeling
Talk to a pharmacist or doctor before you start any supplement, especially if you take medicines. See the Safety tab for interactions.
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