Seaweed & Algae · Seaweed
Kombu
Laminaria japonica (Saccharina japonica) · also called Konbu, kelp, haidai
Kombu is a thick, dark brown seaweed used mostly to make Japanese stock. It is one of the highest-iodine foods measured, and too much iodine can disturb the thyroid. One 12-week trial of kombu powder in healthy adults found lower body fat in men and lower blood pressure, but it has not been repeated. The main cautions are iodine, thyroid disease, pregnancy, young children, and arsenic and cadmium.
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 Kombu
Kombu is a thick, dark brown seaweed, mostly Laminaria japonica (also called Saccharina japonica). It is sold dried in strips or sheets and used mainly to make stock. It is a food, not a proven remedy. Its iodine content is so high and so variable that iodine is the first thing to know about it.
What it is
Kombu is a kelp, a large brown seaweed. GBIF lists Laminaria japonica as the accepted name and Saccharina japonica as a synonym. It is a seaweed, not a plant. It is eaten in Japanese and other East Asian cooking, mostly to flavour stock.
USDA data for raw kelp (not dried kombu) per 100 g: 43 kcal, 1.68 g protein, 1.3 g fibre, 233 mg sodium, 168 mg calcium, 2.85 mg iron, 121 mg magnesium, 180 microg folate and 66 microg vitamin K. The USDA household portion is 2 tablespoons (10 g), which gives about 4 kcal, 17 mg calcium and 12 mg magnesium. USDA lists no iodine value for this item. The dried kombu in packets is more concentrated.
Iodine comes first
Kombu is among the highest-iodine foods measured. A Norwegian survey of 96 products found a mean of 2,276 microg per gram for kombu, and a 2026 analysis found kombu samples above 2000 mg/kg. The 2021 abstract does not say whether the figure is dry or wet weight. The American Thyroid Association says more than 1,100 microg of iodine a day is not recommended and may cause thyroid dysfunction. By simple arithmetic, one gram at the Norwegian mean is about twice that limit.
Iodine varies by species, harvest and processing. In the Norwegian survey, 54 of 96 seaweed products would pass the upper limit in one portion or dose, and labelling of iodine was inadequate for several. Iodine also dissolves in cooking water, so the amount in a finished dish is hard to estimate.
What the research shows
The only trial of kombu itself was in healthy Japanese adults for 12 weeks, with cookies made with boiled kombu powder or cellulose. Body fat was lower in men, systolic blood pressure was lower in men and women, adiponectin was higher in men, and thyroid hormones did not rise at about 420 microg of iodine a day. Several gut bacteria groups increased. It was one trial, with no numbers in the abstract, and it has not been repeated.
Studies of people who eat seaweed regularly show high iodine exposure. In 44 Norwegian eaters, median urinary iodine was 1200 microg/L. In 49 eaters, estimated intake fell from 658 to 189 microg a day after six weeks without seaweed, and median TSH decreased. In 2,845 Japanese adults, estimated intake averaged about 400 microg a day and all thyroid values were in the reference range. These are blood-test changes, not proof of disease. Estimates of Japanese intake differ with the method: a 2011 literature analysis put it at 1,000 to 3,000 microg a day, largely from seaweed.
No good evidence was found for weight loss, detox, cancer or thyroid-support claims.
Food versus supplements
The trial results are for boiled kombu powder in cookies. The iodine uptake study used a kombu supplement in a meal. 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 a sea vegetable and used to make stock in Japanese cooking
- Long use as food is not proof that it treats any condition
Studied for research
- Iodine content, high certainty that kombu is a very concentrated source, which is also its main risk
- Nutrients in raw kelp, high certainty for the USDA values
- Thyroid blood tests in seaweed eaters, low certainty (small, mixed studies)
- Body fat, low certainty (one trial, men only)
- Blood pressure, low certainty (one trial)
- Gut bacteria, insufficient evidence for health outcomes
- Iodine uptake from kombu, low certainty (one small study)
Main compounds
- Iodine
- Alginate (a fibre from the cell wall)
- Arsenic and cadmium (contaminants taken up from sea water)
- Magnesium, calcium, folate and vitamin K (small amounts in raw kelp)
Common forms
- Dried kombu strips or sheets, the usual form, used for stock (nutrient values not read)
- Raw kelp (USDA values read)
- Boiled kombu powder baked into cookies (used in a 2025 trial)
- Kelp tablets and capsules are supplements and are not covered
Research links: 2021 survey: iodine in 96 seaweed products (kombu 2,276 microg/g) · 2025 trial: kombu, body fat, blood pressure and gut bacteria · 2026 analysis: kombu iodine, cadmium and arsenic · American Thyroid Association statement on excess iodine · Research Library · Genes & Biomarkers Library
What it is and its history
- Accepted scientific name
- Laminaria japonica J.E.Areschoug
- Naming authority
- GBIF accepted name (the species most kombu comes from)
- Synonyms
- Saccharina japonica (Aresch.) C.E.Lane, C.Mayes, Druehl & G.W.Saunders
- Common names
- Kombu (English); Konbu (English (alternative spelling)); Kelp (English); Haidai (Chinese (pinyin)); Kombu (Spanish)
- Family or group
- Laminariaceae (the kelp family of brown seaweeds), as given by GBIF. Kombu is a seaweed (a brown alga), not a plant.
- Classification
- Chromista > Ochrophyta > Phaeophyceae > Laminariales > Laminariaceae > Laminaria > Laminaria japonica
- 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
- Not verified from a source read.
- Harvest and processing
- Sold dried in strips or sheets according to the existing site page; a 2025 trial used boiled kombu powder in cookies. How kombu is farmed, harvested and dried was not read from a source.
- Look-alikes and adulterants
- Name: Wakame, arame and hijiki; Note: Other brown seaweeds with their own pages; they differ in iodine and metal content.; Name: Kelp tablets and capsules; Note: Supplements, a different product from the food, which can carry very high iodine.; Name: Kombucha; Note: A fermented tea drink; unrelated despite the similar name.
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 Mean 2,276 microg/g in kombu (Norwegian survey, dry or wet not stated); above 2000 mg/kg in a 2026 analysis; 50.9% to 55.0% of it as inorganic iodide in one study | Needed for thyroid hormones. Too much may disturb the thyroid. 3 8 7 12 | Human data |
| Alginate | polysaccharide (fibre) | Cell wall Not given | A 2025 trial's authors describe boiled kombu powder as high in alginate and link the body-fat and gut-bacteria changes to it; the trial did not test alginate alone. 4 | Human data |
| Arsenic and cadmium | toxic elements | Whole seaweed Above guide limits of 3 mg/kg total arsenic and 0.5 mg/kg cadmium in nearly all commercial samples (2026) | Contaminants taken up from sea water. Boiling reduced arsenic by 74.7% in a lab model. 8 6 | Lab |
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 according to ATA. TSH was higher in high-iodine seaweed eaters and fell after stopping seaweed in one study. 12 2 | Human data |
| Gut bacteria (alginate) | Not applicable | The authors of a 12-week trial suggest the alginate in kombu powder changes gut bacteria and body fat. This is their interpretation. 4 | Human data |
Absorption, metabolism and elimination
- Absorption
- In 18 adults, iodine from a meal with a kombu supplement was recovered at 29.5% and 23.7% in two sessions, against 83.5% and 63.7% from potassium iodide. In a lab cell model, iodine transport from kelp fell as the concentration rose.
- Distribution
- Not found.
- Metabolism
- Not found.
- Elimination
- Iodine leaves in urine; kombu iodine rose slowly in urine, potassium iodide quickly (one study).
- Bioavailability
- Lower than potassium iodide in one small study; not established for ordinary dried kombu.
Isolated compounds versus whole extracts
The trial results are for boiled kombu powder baked into cookies. The iodine uptake study used a kombu supplement in a meal. Kelp tablets and iodine pills are different products and can contain far more iodine; ATA advises against supplements over 500 microg of iodine a day.
Nutritional composition
Nutrition from USDA FoodData Central (SR Legacy, April 2018 release; FDC 168457 'Seaweed, kelp, raw'), read 2026-10-26 from the saved copy in the project. This is raw, wet kelp, not the dried kombu in packets, which is more concentrated. Per 100 g raw kelp: 43 kcal, 1.68 g protein, 0.56 g fat, 9.57 g carbohydrate, 1.3 g fibre, 233 mg sodium, 168 mg calcium, 2.85 mg iron, 121 mg magnesium, 42 mg phosphorus, 89 mg potassium, 1.23 mg zinc, 180 microg folate (DFE) and 66 microg vitamin K. The USDA household portion is 2 tablespoons (10 g): 4.3 kcal, 17 mg calcium, 12 mg magnesium, 0.29 mg iron, 18 microg folate, 6.6 microg vitamin K, 23 mg sodium (100 g values divided by 10). USDA lists no iodine value for this item; iodine comes from the product surveys. Dried, stock, shredded or seasoned kombu were not read.
2 high certainty5 low certainty1 very low certainty1 not enough evidence
What was looked at
The 9 benefits and uses checked for Kombu, each with how strong the evidence is. Tap one to open it.
- Iodine supply (nutrient content)High certainty
- Minerals and vitamins in raw kelp (USDA nutrient values)High certainty
- Thyroid blood tests in regular seaweed eatersLow certainty
- Thyroid hormones in a kombu trialLow certainty
- Body fatLow certainty
- Blood pressureLow certainty
- Gut bacteriaNot enough evidence
- How much kombu iodine the body takes upLow certainty
- Boiling and the iodine and arsenic in kelp (lab)Very low certainty
Lab markers tracked in the studies
Urinary iodineTSH (thyroid stimulating hormone)Systolic blood pressureBody fat content and percentage
What is Kombu good for?
The main studied uses and how strong the human evidence is for each. Most results come from one small trial of boiled kombu powder, not from kombu as it is usually eaten.
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.
Iodine supply (nutrient content)
High certaintyKombu is one of the most iodine-rich foods measured. A Norwegian survey of 96 seaweed products found kombu averaged 2,276 microg of iodine per gram, and a 2026 lab study found kombu samples above a seaweed-institute limit of 2000 mg/kg. Iodine is needed for the thyroid, but too much can harm it.
4 studiesKombu as food unless stated.
Studies, doses and limits
What the trials found
No trial tested kombu to correct iodine shortage. Product surveys measured iodine content: 16 microg/g in nori up to over 8165 microg/g in one processed kelp product (2004), and kombu at a mean of 2,276 microg/g in 96 Norwegian products (2021). A 2026 analysis found kombu above 2000 mg/kg. For 54 of 96 Norwegian products, one portion or dose would exceed the upper limit. The upper limit set by the American Thyroid Association is 1,100 microg a day.
Results
Kombu is a very concentrated source and the amount varies a lot between products. A little can supply, or pass, an adult's daily limit. This is a measurement, not a health benefit.
Effect sizes as reported
Kombu mean 2,276 microg/g (Norwegian survey). Other seaweeds: nori 16 microg/g; processed kelp granules over 8165 microg/g (US market, 2004). By simple arithmetic, 1 g of kombu at the Norwegian mean is about twice the 1,100 microg adult limit.
Why this rating
High that kombu is a very high-iodine food, because several independent product measurements agree. The exact amount in any one pack is unknown, and the 2021 abstract does not say whether the figure is for dry or wet weight.
Form and dose studied
Kombu as food unless stated.
Who was studied
Adults.
Limits
Product surveys from Norway and the US, a small number of samples for kombu, and one lab sample. Iodine is lost in cooking water, so the amount in a prepared dish is hard to estimate.
Why researchers looked
Seaweed is a major iodine source in East Asian diets, and iodine intake that is too high or too low affects the thyroid.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. 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 comes first; see the Safety tab.
Minerals and vitamins in raw kelp (USDA nutrient values)
High certaintyBy USDA data, 100 g of raw kelp has 168 mg of calcium, 121 mg of magnesium, 2.85 mg of iron, 180 microg of folate and 66 microg of vitamin K. A USDA portion of 10 g gives a small fraction of each.
1 studiesRaw kelp (Seaweed, kelp, raw) from USDA tables. Dried, stock or seaso…
Studies, doses and limits
What the trials found
No trial was read showing that eating kombu corrects a low mineral or vitamin level. The USDA item is raw, wet kelp (FDC 168457), not the dried kombu in packets, which is more concentrated. USDA lists no iodine value for this item.
Results
Raw kelp is a modest source of several minerals on paper. A usual kombu portion is small, so these amounts are small too.
Effect sizes as reported
USDA, raw kelp per 100 g: 43 kcal, 1.68 g protein, 1.3 g fibre, 233 mg sodium, 168 mg calcium, 121 mg magnesium, 2.85 mg iron, 89 mg potassium, 180 microg folate (DFE), 66 microg vitamin K. Per 10 g portion: 4.3 kcal, 17 mg calcium, 12 mg magnesium, 0.29 mg iron, 18 microg folate.
Why this rating
High that raw kelp contains these amounts (USDA data). Not tested as a treatment for any shortfall, and the dried product differs.
Form and dose studied
Raw kelp (Seaweed, kelp, raw) from USDA tables. Dried, stock or seasoned kombu were not read.
Who was studied
Adults.
Limits
A nutrient table, not trials. Values vary with species, harvest place and processing.
Why researchers looked
People eat sea vegetables for minerals as well as flavour.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Nutrient database
These are nutrient amounts in the food, from USDA tables for raw kelp.
A proposed mechanism does not show that the effect happens in people.
Iodine comes first; see the Safety tab.
Thyroid blood tests in regular seaweed eaters
Low certaintyIn small studies, people who ate seaweed often had very high urinary iodine and slightly higher TSH, and TSH fell when they stopped seaweed for six weeks. Values stayed in the normal range in a large Japanese survey. These are blood-test changes, not a proven benefit or a proven disease.
3 studiesHabitual seaweed eating in general (mostly kelp, dulse and laver in N…
Studies, doses and limits
What the trials found
In 44 Norwegian seaweed eaters, median urinary iodine was 1200 microg/L, which the authors called excessive, and TSH was within reference values but higher than in comparison groups. In 49 habitual seaweed eaters, estimated iodine intake fell from 658 to 189 microg a day after six weeks without seaweed, and median TSH decreased significantly. In 2,845 Japanese adults, estimated intake averaged about 400 microg a day and the only significant link was a higher TSH with higher urinary iodine, with all values in the reference range.
Results
High-iodine seaweed eating goes with higher TSH within normal ranges. Nobody has shown from these studies that it causes thyroid disease, or that it improves thyroid health.
Effect sizes as reported
Urinary iodine 1200 microg/L (median) in Norwegian seaweed eaters; intake 658 to 189 microg/day and a significant fall in TSH after stopping (49 people); Japan: median urinary iodine 295 microg/L, intake about 400 microg/day.
Why this rating
Low: small, non-randomised or cross-sectional studies, mixed seaweed species (not kombu alone), blood tests rather than disease. Direction of the finding is a caution, not a benefit.
Form and dose studied
Habitual seaweed eating in general (mostly kelp, dulse and laver in Norway); the Japanese survey measured the whole diet.
Who was studied
Adult seaweed eaters in Norway; healthy adults across Japan.
Limits
No control group in the before-after study. Convenience samples. Not specific to kombu.
Why researchers looked
Seaweed is the main iodine source for many eaters, and researchers want to know whether that much iodine changes thyroid tests.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Observational
Not shown. Iodine is needed to make thyroid hormones, and the studies read look only at blood and urine tests.
A proposed mechanism does not show that the effect happens in people.
Iodine comes first; see the Safety tab.
Thyroid hormones in a kombu trial
Low certaintyIn a 12-week trial of healthy Japanese adults, blood thyroid hormones did not rise when they ate boiled kombu powder cookies giving about 420 microg of iodine a day.
1 studiesBoiled kombu powder baked into cookies. Not dried kombu as sold, not …
Studies, doses and limits
What the trials found
In a randomised, double-blind, placebo-controlled trial, healthy Japanese adults ate cookies with boiled kombu powder (Laminaria japonica) or cellulose for 12 weeks. Serum thyroid hormones did not increase at about 420 microg of iodine a day.
Results
No rise in thyroid hormones in healthy adults at that intake. It does not say what happens at higher intakes, for longer, or in people with thyroid disease.
Effect sizes as reported
Not given as numbers in the abstract.
Why this rating
Low: one trial, healthy adults only, a safety measure rather than the main outcome, funding and numbers of participants not given in the abstract.
Form and dose studied
Boiled kombu powder baked into cookies. Not dried kombu as sold, not stock.
Who was studied
Healthy Japanese adults, men and women.
Limits
Does not cover thyroid disease. Intake of 420 microg iodine a day is well under the 1,100 microg limit; real kombu servings can be much higher.
Why researchers looked
Researchers checked thyroid hormones as a safety measure in a trial of kombu for body fat.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Human trial
Not tested. This was a safety measure.
A proposed mechanism does not show that the effect happens in people.
Iodine comes first; see the Safety tab.
Body fat
Low certaintyIn one 12-week trial of healthy Japanese adults, body fat content and percentage were significantly lower in men who ate boiled kombu powder cookies than in men given cellulose cookies. Women were not reported to have the same change.
1 studiesBoiled kombu powder in cookies.
Studies, doses and limits
What the trials found
A randomised, double-blind, placebo-controlled study in healthy Japanese adults compared cookies with boiled kombu powder (Laminaria japonica) and cookies with cellulose over 12 weeks. Body fat content and percentage were significantly lower in the male kombu group. Serum adiponectin was higher in men. The abstract gives no numbers and does not report the same fat change in women.
Results
A body-fat difference in men in one trial. It was not repeated and cannot be taken as a weight-loss effect of kombu.
Effect sizes as reported
Not given as numbers in the abstract.
Why this rating
Low: one trial, healthy people, a result in men only, participant numbers and funding not in the abstract. Kombu as a powder in cookies, not as ordinarily eaten.
Form and dose studied
Boiled kombu powder in cookies.
Who was studied
Healthy Japanese adults.
Limits
Not repeated. Does not show any effect on weight or health outcomes.
Why researchers looked
Kombu is high in alginate, a fibre from the seaweed cell wall, and researchers asked whether it could reduce fat build-up.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Author interpretation
Not shown. The authors link the result to alginate and to gut bacteria (Agathobacter) but this is their interpretation.
A proposed mechanism does not show that the effect happens in people.
Iodine comes first; see the Safety tab.
Blood pressure
Low certaintyIn the same 12-week trial, systolic blood pressure was significantly lower in both men and women who ate boiled kombu powder cookies than with the cellulose placebo.
2 studiesBoiled kombu powder in cookies (about 420 microg iodine a day).
Studies, doses and limits
What the trials found
In the randomised, double-blind, placebo-controlled trial in healthy Japanese adults, systolic blood pressure was significantly lower in the kombu group in men and in women. The abstract does not give the size of the change.
Results
A lower systolic reading in one 12-week trial in healthy people. Not repeated.
Effect sizes as reported
Not given as numbers in the abstract.
Why this rating
Low: a single trial, healthy people, no numbers in the abstract, funding not stated.
Form and dose studied
Boiled kombu powder in cookies (about 420 microg iodine a day).
Who was studied
Healthy Japanese adults.
Limits
Not repeated. Raw kelp carries sodium (233 mg per 100 g in USDA data); the trial abstract says nothing about salt. Not evidence for kelp pills.
Why researchers looked
Blood pressure was one of the measures in the kombu trial.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Not tested
Not shown in the abstract.
A proposed mechanism does not show that the effect happens in people.
Iodine comes first; see the Safety tab.
Gut bacteria
Not enough evidenceThe same trial found that Faecalibacterium, Bacteroides, Alistipes and Agathobacter increased in the kombu group compared with placebo. These are counts of bacteria, not health outcomes.
1 studiesBoiled kombu powder in cookies.
Studies, doses and limits
What the trials found
In the 12-week randomised trial, the abundance of several bacterial groups was higher in the kombu group than in placebo. In men, the fall in body fat correlated with Agathobacter.
Results
A change in bacteria counts. No effect on digestion or symptoms was reported in the abstract.
Effect sizes as reported
Not given as numbers in the abstract.
Why this rating
Insufficient: bacteria counts only, one trial, and no health outcome was reported.
Form and dose studied
Boiled kombu powder in cookies.
Who was studied
Healthy Japanese adults.
Limits
The meaning of the bacterial changes for health is unknown.
Why researchers looked
Alginate is a fibre that gut bacteria can use, so the trial counted gut bacteria.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Author interpretation
The authors suggest that the alginate in kombu changes the gut bacteria. This is their interpretation; the abstract does not test it.
A proposed mechanism does not show that the effect happens in people.
Iodine comes first; see the Safety tab.
How much kombu iodine the body takes up
Low certaintyIn 18 young adults, the iodine from a meal with a kombu supplement was taken up less than iodine from potassium iodide: about 29.5% versus 83.5% in one session and 23.7% versus 63.7% in another.
1 studiesA kombu supplement (not ordinary kombu) in a meal, compared with pota…
Studies, doses and limits
What the trials found
Twenty Chinese participants (18 finished) were allocated to two parallel groups. One ate a meal with a kombu supplement giving 993 and 1735 microg of iodine, the other potassium iodide giving 547 and 1263 microg, at two sessions, with urine collected for 24 and 48 hours.
Results
Less of the iodine was recovered in urine after kombu than after potassium iodide. Lower uptake is not a reason to eat more kombu.
Effect sizes as reported
Bioavailability 29.5% vs 83.5% (first session) and 23.7% vs 63.7% (second), p<0.001 for both.
Why this rating
Low: very small study of young adults, a kombu supplement in a meal rather than ordinary dried kombu, and the amounts of iodine given were far above the 1,100 microg limit.
Form and dose studied
A kombu supplement (not ordinary kombu) in a meal, compared with potassium iodide.
Who was studied
Young Chinese adults.
Limits
Two sessions with very large iodine amounts. Does not show that high amounts are safe.
Why researchers looked
Researchers wanted to know whether the iodine in kombu is absorbed as fully as iodine in a pill or salt.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Human trial
Not shown. The authors report that about half of the iodine in kombu was in inorganic iodide forms and rose gradually in urine, unlike a quick surge after potassium iodide.
A proposed mechanism does not show that the effect happens in people.
The iodine amounts given in this study were larger than the 1,100 microg a day upper limit. Do not read it as safety evidence.
Boiling and the iodine and arsenic in kelp (lab)
Very low certaintyIn a lab study of Saccharina japonica kelp, boiling for 10 minutes cut iodine by 94.4% and arsenic by 74.7%. This was a lab model, not people, and it tested the kelp, not a stock.
2 studiesRaw and cooked Saccharina japonica (kombu kelp) in a lab model.
Studies, doses and limits
What the trials found
No study in people. In the lab, 10 minutes of boiling produced the largest drops in iodine and arsenic content. The authors concluded risk from cooked kelp is extremely low; that is their interpretation of a lab model.
Results
Cooking in water can remove much of the iodine, which moves into the water. The 2004 paper also notes that iodine is water-soluble in cooking.
Effect sizes as reported
Iodine -94.4%, arsenic -74.7% (10 minutes boiling).
Why this rating
Very low: a laboratory model on one species, not people, so it cannot show real-world safety.
Form and dose studied
Raw and cooked Saccharina japonica (kombu kelp) in a lab model.
Limits
Lab only. It tested kelp pieces, not kombu stock; the iodine that leaves the kelp goes into the water, so a stock may contain a lot.
Why researchers looked
Kelp has high iodine and arsenic, so researchers tested what cooking does.
Traditional use
Kombu is eaten as a food in Japan and other East Asian cuisines, mostly as a stock ingredient. Long use as food is not proof that it treats any condition.
Proposed mechanism Laboratory
Lab digestion and Caco-2 cell model. Iodine and arsenic move into the cooking water.
A proposed mechanism does not show that the effect happens in people.
Iodine comes first; see the Safety tab.
Connections to the rest of health
How Kombu 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 |
|---|---|---|---|---|
| Blood Pressure Support | Preliminary | Systolic blood pressure was lower with boiled kombu powder in one 12-week trial; not repeated. Preparation or compound: Boiled kombu powder |
4 | |
| Thyroid Support | High certainty | Iodine supply (nutrient content): Kombu is one of the most iodine-rich foods measured. A Norwegian survey of 96 seaweed products found kombu averaged 2,276 microg of iodine per gram, and a 2026 lab study found kombu samples above a seaweed-institute limit of 2000 mg/kg. Iodine is needed for the thyroid, but too much can harm it. | 10 3 8 12 | |
| Preventive Wellness | High certainty | Minerals and vitamins in raw kelp (USDA nutrient values): By USDA data, 100 g of raw kelp has 168 mg of calcium, 121 mg of magnesium, 2.85 mg of iron, 180 microg of folate and 66 microg of vitamin K. A USDA portion of 10 g gives a small fraction of each. | 16 | |
| Thyroid Support | Low certainty | Thyroid blood tests in regular seaweed eaters: In small studies, people who ate seaweed often had very high urinary iodine and slightly higher TSH, and TSH fell when they stopped seaweed for six weeks. Values stayed in the normal range in a large Japanese survey. These are blood-test changes, not a proven benefit or a proven disease. | 1 2 5 | |
| Thyroid Support | Low certainty | Thyroid hormones in a kombu trial: In a 12-week trial of healthy Japanese adults, blood thyroid hormones did not rise when they ate boiled kombu powder cookies giving about 420 microg of iodine a day. | 4 | |
| Weight Management | Low certainty | Body fat: In one 12-week trial of healthy Japanese adults, body fat content and percentage were significantly lower in men who ate boiled kombu powder cookies than in men given cellulose cookies. Women were not reported to have the same change. | 4 | |
| Blood Pressure Support | Low certainty | Blood pressure: In the same 12-week trial, systolic blood pressure was significantly lower in both men and women who ate boiled kombu powder cookies than with the cellulose placebo. | 4 16 | |
| Microbiome Balance | Not enough evidence | Gut bacteria: The same trial found that Faecalibacterium, Bacteroides, Alistipes and Agathobacter increased in the kombu group compared with placebo. These are counts of bacteria, not health outcomes. | 4 | |
| Thyroid Support | Low certainty | How much kombu iodine the body takes up: In 18 young adults, the iodine from a meal with a kombu supplement was taken up less than iodine from potassium iodide: about 29.5% versus 83.5% in one session and 23.7% versus 63.7% in another. | 7 | |
| Preventive Wellness | Very low certainty | Boiling and the iodine and arsenic in kelp (lab): In a lab study of Saccharina japonica kelp, boiling for 10 minutes cut iodine by 94.4% and arsenic by 74.7%. This was a lab model, not people, and it tested the kelp, not a stock. | 6 10 |
B. Body systems
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Endocrine System | Biological association | High certainty | Studied for: Iodine supply (nutrient content) Studied for: Thyroid blood tests in regular seaweed eaters | 10 3 8 12 1 2 5 4 7 |
| Musculoskeletal System | Biological association | High certainty | Studied for: Minerals and vitamins in raw kelp (USDA nutrient values) | 16 |
| Metabolic System | Biological association | Low certainty | Studied for: Body fat | 4 |
| Cardiovascular System | Biological association | Low certainty | Studied for: Blood pressure | 4 16 |
| Digestive System | Biological association | Not enough evidence | Studied for: Gut bacteria Studied for: Boiling and the iodine and arsenic in kelp (lab) | 4 6 10 |
C. Body parts and organs
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Thyroid | Adverse effect or risk | Documented risk | Kombu has so much iodine that intake can pass the 1,100 microg adult upper limit, which may cause thyroid dysfunction. Kombu iodine can pass the 1,100 microg adult upper limit, which may cause thyroid dysfunction. Preparation or compound: Iodine in kombu |
12 3 9 |
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, kombu included, were above a seaweed-institute guide for cadmium and arsenic in a 2026 lab analysis. Nearly all commercial seaweed samples, kombu included, were above a seaweed-institute guide for cadmium and arsenic in a 2026 analysis. Preparation or compound: Seaweed as food |
8 |
I. Biomarkers
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Urinary iodine | Biological association | Low certainty | Measured in trials: raised. A lab change is not the same as a health benefit. | 1 2 5 |
| TSH (thyroid stimulating hormone) | Biological association | Mixed findings | Measured in trials: mixed. A lab change is not the same as a health benefit. | 2 1 5 4 |
| Systolic blood pressure | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 4 |
| Body fat content and percentage | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 4 |
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 |
3 8 7 12 |
| Alginate | Biological association | Human | A 2025 trial's authors describe boiled kombu powder as high in alginate and link the body-fat and gut-bacteria changes to it; the trial did not test alginate alone. Preparation or compound: Cell wall |
4 |
| Arsenic and cadmium | Biological association | Lab | Contaminants taken up from sea water. Boiling reduced arsenic by 74.7% in a lab model. Preparation or compound: Whole seaweed |
8 6 |
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).
The evidence
The research centre for Kombu. 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 kombu summary, re-checked the nutrition against saved USDA FoodData Central data, read the American Thyroid Association and NHS pages, checked the species name 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 16
| Study | Design | People | Product, dose, length | Result |
|---|---|---|---|---|
| Aakre 2020 Iodine Status and Thyroid Function in a Group of Seaweed Consumers in Norway. Nutrients · PMID 33202773 · DOI |
Observational n/a |
44 44 habitual seaweed (macroalgae) consumers in Norway; thyroid blood tests in a subgroup of 19 |
Habitual seaweed eating (sugar kelp, winged kelp, dulse and laver most common) Whole seaweeds Habitual Cross-sectional |
Median urinary iodine was 1200 microg/L, which the authors called excessive iodine status. Seaweed was the major contributor to iodine intake. TSH was within reference values but higher than in comparison groups. Labelling of iodine content was lacking for a large share of products. Median urinary iodine 1200 microg/L (p25-p75 370 to 2850) (Not applicable) Methods, limits and fundingOutcomes: Urinary iodine, TSH, fT4, fT3, TPO antibodies Limitations: Small, cross-sectional. Seaweed species were mostly not kombu. It shows an association and blood-test values, not disease. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Aakre 2026 Impact of habitual seaweed consumption on iodine nutrition and thyroid function: a non-randomized pre-post clinical study. Eur J Nutr · PMID 41543576 · DOI |
Non randomized pre post n/a |
49 49 habitual seaweed consumers in the two largest cities in Norway |
Stopping seaweed (habitual species not limited to kombu) Whole seaweeds Six-week break from seaweed 6 weeks · vs Their own habitual intake (before-after) |
Median estimated iodine intake fell from 658 to 189 microg a day and median urinary iodine from 270 to 87 microg/L. Before stopping, median intake was above the tolerable upper intake level. Median TSH decreased significantly after stopping, most in those with the highest intake. Intake 658 to 189 microg/day; TSH decreased (value not in the abstract) (Not stated) Methods, limits and fundingOutcomes: Urinary iodine, estimated iodine intake, TSH, fT3, fT4, TPO antibodies Limitations: No control group. Convenience sample. Mixed seaweed species, not kombu alone. It shows blood-test changes, not thyroid disease. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Aakre I et al. 2021 Aakre I et al. Commercially available kelp and seaweed products: valuable iodine source or risk of excess intake? Food Nutr Res 2021 Food Nutr Res · Source |
Laboratory analysis Laminaria (kombu), per the abstract's species label |
96 96 macroalgae products from the Norwegian market (43 whole-food seaweed products, 39 foods containing seaweed, 14 supplements) |
Seaweed products measured for iodine (ICP-MS) Whole seaweeds, seaweed foods and supplements |
Iodine in one portion of whole-food seaweed products ranged from 128 to 62,400 microg. Oarweed, sugarkelp and kombu had the highest mean iodine, 7,800, 4,469 and 2,276 microg/g. For 54 products one portion or dose would exceed the tolerable upper intake level. Labelling of species and iodine content was inadequate or inaccurate for several products. Kombu mean 2,276 microg/g (Not applicable) Methods, limits and fundingOutcomes: Iodine per portion and per gram Limitations: Norwegian market products only; the abstract does not say whether the kombu figure is dry or wet weight. Not a trial. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Aoe 2025 Effects of Daily Kombu (Laminaria japonica) Intake on Body Composition, Blood Pressure, and Fecal Microbiota in Healthy Adult Japanese: A Randomized, Double-Blind Study. Food Sci Nutr · PMID 40390848 · DOI |
RCT n/a |
Healthy Japanese adults, men and women | Cookies containing boiled kombu powder (Laminaria japonica), high in alginate Whole kombu, boiled and powdered, as a food ingredient About 420 microg iodine a day from kombu (the amount of kombu is not given in the abstract) 12 weeks · vs Cookies containing cellulose (placebo) |
Body fat content and percentage were significantly lower in the men in the kombu group than in placebo. Systolic blood pressure was significantly lower in men and women. Serum adiponectin was higher in men. Serum thyroid hormones did not increase. Faecalibacterium, Bacteroides, Alistipes and Agathobacter increased in the kombu group. Not given as numbers in the abstract (Not stated in the abstract) Methods, limits and fundingOutcomes: Body fat, visceral fat, blood pressure, adiponectin, thyroid hormones, gut bacteria Limitations: One 12-week trial, not repeated. Participant number and funding not in the abstract. Healthy people; it does not cover thyroid disease. Body fat fell in men only. Boiled kombu powder, not kombu as sold dried or in stock. Funding and conflicts: Funding is not stated in the abstract. Read: abstract |
| Fuse 2026 Nationwide survey on iodine nutritional status and thyroid function in Japanese adults. Endocr J · PMID 41905952 · DOI |
Cross sectional survey n/a |
2845 2,845 healthy adults (mean age 45.6) across 20 regions of Japan, 2016 to 2023 |
Habitual Japanese diet (iodine intake) Estimated average about 400 microg iodine a day |
Median urinary iodine was 295 microg/L, within the WHO adequacy range. Median TSH, FT4 and FT3 were within reference ranges in every region. The only significant correlation was a positive one between median urinary iodine and median TSH. Estimated average intake was about 400 microg a day. Positive correlation between urinary iodine and TSH (Not stated) Methods, limits and fundingOutcomes: Urinary iodine, TSH, FT4, FT3, thyroid antibodies Limitations: A cross-sectional survey of the whole diet, not of seaweed or kombu. It shows an association only. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Li 2024 Evaluation of the Bioavailability of Iodine and Arsenic in Raw and Cooked Saccharina japonica Based on Simulated Digestion/Caco-2 Cell Model. Foods · PMID 39335793 · DOI |
Laboratory in vitro Saccharina japonica |
Saccharina japonica (kombu kelp), raw and cooked, in a lab digestion and Caco-2 cell model | Cooking treatments on kelp Whole kelp Boiling for 10 minutes was the strongest treatment Lab · vs Raw kelp |
Boiling for 10 minutes gave the largest reductions: iodine by 94.4% and arsenic by 74.7%. In the gastric phase, iodine bioaccessibility fell from 3188.2 to 317.0 microg/L after boiling. Iodine transport across the cell layer fell as the concentration rose. The authors concluded the risk of excess iodine and arsenic from cooked kelp is extremely low. Iodine -94.4%, arsenic -74.7% with 10 minutes of boiling (Not applicable) Methods, limits and fundingOutcomes: Iodine and arsenic content, bioaccessibility and cell transport Limitations: A lab model on one species, not people, and it tested the kelp, not a stock or the cooking water. The authors' conclusion about low risk is theirs and conflicts with the product surveys, so it is not relied on for safety. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Liu 2025 The Iodine Bioavailability and Its Metabolic Response for Kombu and Potassium Iodide Intake in Chinese Adults. Food Sci Nutr · PMID 40918173 · DOI |
Controlled intervention n/a |
20 20 Chinese participants (mean age 19.6), 18 completed; two parallel groups |
A meal with a kombu supplement (993 and 1735 microg iodine at the two sessions) Kombu supplement in a meal 993 and 1735 microg iodine (kombu); 547 and 1263 microg (potassium iodide) Two sessions, 48 hours of urine follow-up · vs Potassium iodide |
Iodine bioavailability was lower with kombu than with potassium iodide: 29.5% vs 83.5% at the first session and 23.7% vs 63.7% at the second. Kombu iodine rose gradually, while potassium iodide gave a steep surge. Between 50.9% and 55.0% of the iodine in kombu was in inorganic iodide forms. 29.5% vs 83.5%; 23.7% vs 63.7% (Not stated) Methods, limits and fundingOutcomes: Iodine bioavailability from 24-hour urine, urinary iodine excretion Limitations: Very small study of young adults. The product was called a kombu supplement, not ordinary dried kombu. Absorption of iodine does not mean it is safe at high amounts. 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 dulse, wakame, kombu, arame, nori and spirulina samples | Commercial edible seaweeds measured for minerals and fatty acids Whole dried seaweeds |
Lead and mercury stayed below regulatory limits. Nearly all samples exceeded the recommended limits of 0.5 mg/kg for cadmium and 3 mg/kg for total arsenic set by the French seaweed institute CEVA. Kombu samples had extremely high iodine, above the CEVA upper limit of 2000 mg/kg. Brown algae had the highest levels of potassium and sodium. Kombu iodine above 2000 mg/kg (Not applicable) Methods, limits and fundingOutcomes: Minerals, toxic elements, fatty acids Limitations: A lab analysis of commercial samples; the abstract does not give sample numbers or say which kombu product or origin. The limits named are a seaweed-institute guide, not a law. Funding and conflicts: Not stated in the abstract. 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 |
Review of UK iodine status and thyroid health | Iodine intake from diet and kelp supplements Kelp-based supplements described as delivering far more iodine than recommended |
Mild to moderate iodine deficiency has re-emerged in the UK, especially in women of reproductive age. Social-media 'thyroid detox' or 'iodine reset' trends and kelp-based supplements deliver iodine far above recommended intake, and a growing minority of patients present with iodine-induced thyroid dysfunction. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Iodine deficiency and excess Limitations: A narrative review (not a systematic one). About kelp supplements, not kombu as food, and about the UK. 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 |
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, including kelp |
Iodine ranged from 16 microg/g in nori to over 8165 microg/g in one sample of processed Laminaria digitata kelp granules sold as a salt substitute. In two Namibian kelps, bleached blades had 514 microg/g and fresh juvenile blades 6571 microg/g. Iodine dissolves in cooking water and may be lost in humid storage, so iodine in prepared food is hard to estimate. Some Asian seaweed dishes may exceed the upper limit of 1100 microg a day. 16 to over 8165 microg/g (Not applicable) Methods, limits and fundingOutcomes: Iodine content per gram Limitations: A lab measurement of products, not kombu specifically (kombu is not named in the abstract). Not a trial. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Zava 2011 Assessment of Japanese iodine intake based on seaweed consumption in Japan: A literature-based analysis. Thyroid Res · PMID 21975053 · DOI |
Literature based analysis n/a |
Japanese adults, from published dietary records, surveys and urine iodine data | Seaweed iodine intake estimate Edible seaweeds |
Estimated Japanese iodine intake, largely from seaweeds, averages 1,000 to 3,000 microg a day. The authors say researchers often over- or under-estimate intake because iodine differs by species and cooking. 1,000 to 3,000 microg/day (estimate) (Not applicable) Methods, limits and fundingOutcomes: Estimated iodine intake Limitations: An estimate pooled from other studies, not a measurement in a trial. Seaweed in general, not kombu alone. 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-laminaria-japonica GBIF Backbone: Laminaria japonica J.E.Areschoug (usageKey 5422368) · Source |
Reference database n/a |
Methods, limits and fundingRead: full-text | ||
| s-healthcanada-dri Health Canada: Dietary reference intakes tables: reference values for elements (iodine upper limits) · Source |
Health agency n/a |
Methods, limits and fundingRead: not-re-read | ||
| s-nhs-iodine NHS: Vitamins and minerals: Iodine · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-usda-fdc-168457 USDA FoodData Central: Seaweed, kelp, raw (SR Legacy, FDC 168457) · Source |
Reference database n/a |
Methods, limits and fundingRead: saved-copy |
What we do not know yet
- Only one trial of kombu itself was found, and it was a 12-week study of boiled kombu powder in healthy Japanese adults; the abstract gives no numbers or participant count.
- No study of kombu in people with thyroid disease or on thyroid medicine, or with warfarin, was found.
- Whether the 2,276 microg/g kombu figure is dry or wet weight is not stated in the abstract.
- Nutrient values for dried kombu and stock, seaweed allergy, children's iodine limits (the Health Canada table could not be re-read), history and farming were not researched.
- How much iodine moves from kombu into stock was not found in any source read.
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".
Hormonal 1
TSH (thyroid stimulating hormone)
Mixed results in trials
A hormone from the brain that tells the thyroid how much to work. A higher level can be an early sign of a sluggish thyroid.
Mixed findingsResults and what it means
Why it was measured
It is the usual first blood test for thyroid function.
Statistical vs. clinical meaning
TSH rises a little with high seaweed iodine in observational data and a before-after study, but stays in the normal range; the kombu trial at about 420 microg of iodine showed no rise in thyroid hormones.
Safety implication
People with thyroid disease or on thyroid medicine were not the subject of these studies; ask a clinician.
| Study | Result | People | Product |
|---|---|---|---|
| 2 | Median TSH decreased significantly after seaweed was stopped for six weeks, most in those with the highest estimated intake | 49 habitual seaweed consumers | Stopping seaweed for 6 weeks |
| 1 | TSH within reference values but higher than in other comparable groups | 19 seaweed eaters with thyroid tests | Habitual seaweed eating |
| 5 | Higher urinary iodine went with higher median TSH; all values in the reference range | 2,845 Japanese adults | Usual diet |
| 4 | Serum thyroid hormones did not increase at about 420 microg iodine a day | Healthy Japanese adults in a 12-week randomised trial | Boiled kombu powder cookies vs cellulose placebo |
Other 3
Urinary iodine
Raised in trials
The amount of iodine in urine. It is the usual measure of recent iodine intake.
Low certaintyResults and what it means
Why it was measured
It shows whether a diet gives too little, enough or too much iodine.
Statistical vs. clinical meaning
Seaweed eaters can have very high iodine levels; this is a measure of exposure, not of health.
Safety implication
High urinary iodine in seaweed eaters is the reason for the iodine caution on this page.
| Study | Result | People | Product |
|---|---|---|---|
| 1 | Median 1200 microg/L (p25-p75 370 to 2850), called excessive by the authors | 44 Norwegian habitual seaweed eaters | Habitual seaweed eating |
| 2 | Median fell from 270 to 87 microg/L after six weeks without seaweed | 49 habitual seaweed consumers in Norway | Stopping seaweed for 6 weeks |
| 5 | Median 295 microg/L, within the WHO adequacy range | 2,845 Japanese adults | Usual Japanese diet |
Systolic blood pressure
Lowered in trials
The top number in a blood pressure reading, the pressure when the heart beats.
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 lower reading in one trial in healthy people; not repeated.
Safety implication
None found for this measure.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Significantly lower in men and in women than with placebo (size not in the abstract) | Healthy Japanese adults, 12-week randomised trial | Cookies with boiled kombu powder vs cellulose |
Body fat content and percentage
Lowered in trials
How much of the body is fat, measured in the trial with body scans.
Low certaintyResults and what it means
Why it was measured
Extra body fat, especially around the organs, is linked to metabolic problems.
Statistical vs. clinical meaning
A change in men in one trial, not repeated and not in women.
Safety implication
None found.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Significantly lower in men than with placebo (size not in the abstract) | Healthy Japanese men, 12-week randomised trial | Cookies with boiled kombu powder vs cellulose |
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 3
- Iodine comes first. Kombu is among the highest-iodine foods measured: a Norwegian survey found a mean of 2,276 microg per gram. The American Thyroid Association says more than 1,100 microg of iodine a day is not recommended and may cause thyroid dysfunction. By simple arithmetic, one gram of kombu at that mean is about twice the limit. Real products vary a lot and the label often does not say. Laboratory measurement and authoritative guidance 3 12 8
- Thyroid disease, pregnancy and young children need extra care. People with thyroid disease or on thyroid medicine, people who are pregnant or breastfeeding, and young children should ask a clinician before eating kombu often. No kombu-specific advice was found in the sources read for any of these groups. Expert guidance 12 15 9
- Arsenic and cadmium. A 2026 lab study found nearly all commercial seaweed samples, kombu included, above a French seaweed-institute guide of 0.5 mg/kg cadmium and 3 mg/kg total arsenic. Lead and mercury stayed below regulatory limits. Laboratory analysis 8
Side effects 3
| 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 a growing minority of patients with iodine-induced thyroid dysfunction, linked to kelp-based supplements. | Not found for kombu as food. | can be serious over time Not described in the sources read. | Authoritative guidance 12 15 9 |
| Higher TSH, still in the normal range, in regular seaweed eaters Blood-test values, mostly in seaweed in general, not kombu alone. | Seen in small studies of seaweed eaters; no rate found. | not serious in the studies read In 49 habitual seaweed eaters, median TSH decreased significantly after six weeks without seaweed. | Observational 1 2 5 |
| Symptoms of thyroid problems (fast heartbeat, a swelling in the neck, new tiredness) Listed as signs to act on; not taken from a source read for this page. | Not found. | not stated Not stated | Expert opinion not verified |
Liver
No liver injury from eating kombu was found in the sources read. A liver-safety source (such as LiverTox) was not searched for kombu.
How often. No frequency was found.
Monitoring. No agency monitoring advice was found.
This is a safety connection. It is not evidence that Kombu 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 kombu changes how thyroid medicine works was not found. | No source on kombu with thyroid medicine was read. Taken together with the iodine facts above, this is a question for a clinician. | None found Not enough evidence 12 | If you have thyroid disease or take thyroid medicine, ask your clinician before adding kombu. |
| Warfarin and other vitamin K-sensitive blood thinners prescription | Vitamin K is a factor in blood clotting. USDA lists 66 microg of vitamin K per 100 g of raw kelp, about 6.6 microg in a 10 g portion. | No kombu-specific warfarin source was read. | None found Not enough evidence 16 | If you take a blood thinner, ask your pharmacist and keep what you eat 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. A 2026 UK review describes kelp supplements delivering iodine far above recommended intake. | Authoritative guidance Moderate certainty 12 9 | Count iodine from every source, and ask a pharmacist or clinician before combining kombu with kelp or iodine supplements. |
Who should check first 4
| Situation | Advice | Evidence |
|---|---|---|
| Thyroid disease, or taking thyroid medicine | Ask your clinician before adding kombu. This page does not tell anyone to eat or avoid it for a thyroid problem. | Expert guidance 12 15 |
| Pregnancy or breastfeeding | The American Thyroid Association 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 kombu-specific advice was found. Ask your midwife or doctor. | Authoritative guidance 12 |
| Young children | Children's upper limits for iodine are lower than adults'; a Health Canada table cited on the existing page lists 200 microg for ages 1 to 3 and 300 microg for ages 4 to 8, but that table could not be re-read for this page. Ask a clinician before giving kombu or strong kombu stock. | Authoritative guidance not re read 14 |
| Known seaweed allergy | No source on seaweed allergy was read, so how common it is cannot be stated. Dried kombu is often plain, but seasoned or flavoured products may contain other allergens: read the label. | None found |
Overdose, product quality and regulators
Summary. The safety points for kombu as food are iodine, arsenic and cadmium. No other toxicity from ordinary kombu was found in the sources read.
Animal. Not reviewed.
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.
Kombu and other seaweeds take up minerals and metals from sea water.
Cadmium and arsenic. In a 2026 lab analysis, nearly all commercial seaweed samples, kombu among them, exceeded the CEVA guide limits of 0.5 mg/kg for cadmium and 3 mg/kg for total arsenic. Lead and mercury stayed below limits. The limits are a seaweed-institute guide, not a law. 8
Boiling. In a lab model on Saccharina japonica, boiling for 10 minutes cut iodine by 94.4% and arsenic by 74.7%. The authors concluded that risk from cooked kelp is low. This was a lab result on one species and it tested the kelp, not a stock. 6
Iodine labelling. In a Norwegian survey, labelling of species and iodine content was inadequate or inaccurate for several products; in another study, labelling of iodine content was lacking for a large share of products eaten. 3 1
| 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) | High doses of iodine for long periods could change how the thyroid gland works; 0.5 mg or less a day of iodine supplements is unlikely to cause harm. source | 2020-08-03 |
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 kombu?
Kombu is a thick, dark brown seaweed, mostly Laminaria japonica, sold dried in strips or sheets. It is used in Japanese cooking, mostly to make stock.
Is kombu high in iodine?
Is kombu good for the thyroid?
Does boiling kombu remove iodine?
Can kombu help with weight loss?
One 12-week trial found lower body fat in men who ate boiled kombu powder cookies than in men given placebo cookies. It was one small trial, not repeated, and not a weight-loss result.
Does kombu lower blood pressure?
In the same trial, systolic blood pressure was lower in men and women with kombu powder than with placebo. It was one trial in healthy people and has not been repeated.
Is kombu safe in pregnancy?
Is kombu safe for children?
No kombu-specific advice for children was found. Limits for iodine are lower for young children than adults, so ask a clinician before giving children kombu or strong kombu stock.
Does kombu contain arsenic and cadmium?
Nearly all commercial seaweed samples in a 2026 analysis, kombu included, were above a seaweed-institute guide of 0.5 mg/kg for cadmium and 3 mg/kg for total arsenic. Lead and mercury stayed below limits. The guide is not a law.
Is kombu the same as kelp tablets?
Does kombu interact with medicines?
Does kombu help with detox or cancer?
No good evidence was found for detox, cancer or thyroid-support claims in the sources read.
Explore by health goal
Thyroid SupportPreventive WellnessWeight ManagementBlood Pressure SupportMicrobiome Balance
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 kombu summary, re-checked the nutrition against saved USDA FoodData Central data, read the American Thyroid Association and NHS pages, checked the species name 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 the studies used.
Sources 16
Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.
- Aakre I, Tveito Evensen L, Kjellevold M, et al.. Iodine Status and Thyroid Function in a Group of Seaweed Consumers in Norway.. Nutrients. 2020. PMID 33202773. DOI 10.3390/nu12113483. (abstract)
- Aakre I, Vogt EC, Myrmel LS, et al.. Impact of habitual seaweed consumption on iodine nutrition and thyroid function: a non-randomized pre-post clinical study.. Eur J Nutr. 2026. PMID 41543576. DOI 10.1007/s00394-025-03813-8. (abstract)
- Aakre I et al.. Aakre I et al. Commercially available kelp and seaweed products: valuable iodine source or risk of excess intake? Food Nutr Res 2021. Food Nutr Res. 2021. (abstract)
- Aoe S, Ohtoshi H, Nakamura F. Effects of Daily Kombu (Laminaria japonica) Intake on Body Composition, Blood Pressure, and Fecal Microbiota in Healthy Adult Japanese: A Randomized, Double-Blind Study.. Food Sci Nutr. 2025. PMID 40390848. DOI 10.1002/fsn3.70298. (abstract)
- Fuse Y, Kunii Y, Tsukada N, et al.. Nationwide survey on iodine nutritional status and thyroid function in Japanese adults.. Endocr J. 2026. PMID 41905952. DOI 10.1507/endocrj.EJ25-0603. (abstract)
- Li N, Geng Z, Guo Y, et al.. Evaluation of the Bioavailability of Iodine and Arsenic in Raw and Cooked Saccharina japonica Based on Simulated Digestion/Caco-2 Cell Model.. Foods. 2024. PMID 39335793. DOI 10.3390/foods13182864. (abstract)
- Liu X, Wang J, Li Y, et al.. The Iodine Bioavailability and Its Metabolic Response for Kombu and Potassium Iodide Intake in Chinese Adults.. Food Sci Nutr. 2025. PMID 40918173. DOI 10.1002/fsn3.70818. (abstract)
- Nava V, Potorti AG, Albergamo A, et al.. Commercial Edible Seaweeds: Characterization by Mineral Content and Fatty Acid Composition.. Foods. 2026. PMID 42794005. DOI 10.3390/foods15183219. (abstract)
- Taylor PN, Prentice M, Bath S, et al.. 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. (abstract)
- Zava TT, Zava DT. Assessment of Japanese iodine intake based on seaweed consumption in Japan: A literature-based analysis.. Thyroid Res. 2011. PMID 21975053. DOI 10.1186/1756-6614-4-14. (abstract)
- . American Thyroid Association: ATA statement on the potential risks of excess iodine ingestion and exposure. . . (full-text)
- . GBIF Backbone: Laminaria japonica J.E.Areschoug (usageKey 5422368). . . (full-text)
- . Health Canada: Dietary reference intakes tables: reference values for elements (iodine upper limits). . . (not-re-read)
- . NHS: Vitamins and minerals: Iodine. . . (full-text)
- . USDA FoodData Central: Seaweed, kelp, raw (SR Legacy, FDC 168457). . . (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.
Support our free guides: as an Amazon Associate we earn from qualifying purchases. Full disclosure.
