Seaweed & Algae · Seaweed
Hijiki
Sargassum fusiforme (also called Hizikia fusiformis) · also called Hiziki
Hijiki is a thin, black brown seaweed from East Asia, soaked and cooked in Japanese home cooking. It holds far more inorganic arsenic than other seaweeds, and the UK, Canada, Hong Kong and Australia and New Zealand have each advised people to avoid it. It is also high in iodine. No human trial shows a health benefit from eating it. Ask a doctor first if you are pregnant, feeding a child or have thyroid disease.
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 Hijiki
Hijiki is a thin, black brown seaweed, Sargassum fusiforme, sold dried and eaten after soaking and cooking, mostly in Japan. It is a food, not a proven remedy. It is the one common sea vegetable that several governments have told people to avoid, because of its inorganic arsenic. It is also rich in iodine.
What it is
Hijiki is a brown seaweed. GBIF lists Sargassum fusiforme as the accepted name; the older name Hizikia fusiformis still appears in many papers. It is a seaweed, not a plant. In Japanese home cooking it is soaked and simmered with vegetables and fried bean curd.
No reliable nutrient table was found for hijiki: it has no USDA FoodData Central entry we could find and retailer labels online disagreed, so none is shown. What was measured is below.
Inorganic arsenic
Arsenic in seaweed comes in two kinds. The organic kind is much less toxic. Hijiki holds mostly the inorganic kind. A 2007 UK survey of 31 dried seaweeds found inorganic arsenic only in the nine hijiki samples, at 67 to 96 mg per kg. The other seaweeds had under 0.3 mg per kg.
In Japan, rice and hijiki supplied virtually all the inorganic arsenic from cereals and algae, so hijiki matters most for people who eat it often. Cooked hijiki from 14 Japanese households had 0.4 to 2.8 mg per kg wet weight of inorganic arsenic.
Inorganic arsenic taken in over many years is linked to cancers of the skin, bladder and lung. For hijiki, the cancer risk is a model: one study calculated a skin cancer risk for cooked hijiki above the authors' acceptable level. No study has counted cancers in hijiki eaters.
What governments say
The UK Food Standards Agency (read through a news report), the Canadian Food Inspection Agency, Hong Kong's Centre for Food Safety and Food Standards Australia New Zealand have each advised people to avoid hijiki. Hong Kong says occasional small amounts are unlikely to cause harm, and the UK agency says occasional eaters have probably not raised their risk much. The Canadian agency says no illnesses have been linked to hijiki so far.
A Japanese government position from 2004 is said to be less strict, but we could not open it, so we do not report its details.
Iodine
Hijiki is also a high-iodine seaweed. In 87 Korean samples it averaged 527 mg per kg dry weight (range 159 to 1,548). That is far below sea tangle (2,432) and far above nori. Iodine is needed for the thyroid, but too much can disturb it. Iodine dissolves into cooking water, so a finished dish is hard to estimate.
What research shows
We found no human trial of hijiki as food for any health outcome. The health claims for hijiki come from mouse and lab work on extracts such as fucoidan. A 2019 review of edible seaweeds says more human studies are needed before benefits can be claimed.
The firm findings are measurements: arsenic, iodine and how much people take in.
At a glance
Traditional uses history, not proof
- Eaten as a sea vegetable in Japan, often simmered with vegetables and fried bean curd
- Long use as food is not proof that it is safe or that it treats any condition
Studied for research
- Inorganic arsenic exposure, high certainty that hijiki is a concentrated source (a risk, not a benefit)
- Modelled cancer risk from hijiki arsenic, very low certainty (a calculation, not a count of cases)
- Arsenic intake in pregnant women and children who eat hijiki, low certainty (one 3-day study)
- Iodine content, high certainty that it is high (also a risk)
- Thyroid blood tests in seaweed eaters, low certainty (mixed seaweeds, hijiki not named)
- Soaking and boiling to cut arsenic, very low certainty (lab only)
- Weight and blood sugar claims, insufficient evidence (mouse work on extracts)
Main compounds
- Inorganic arsenic (arsenate)
- Iodine
- Fucoidan and other seaweed sugars (studied in mice and cells)
Common forms
- Dried hijiki strands, soaked and cooked (the usual form)
- Cooked hijiki dishes with vegetables and fried bean curd (measured in a Japanese study)
- Extracts such as fucoidan are lab products and are not covered as food
Research links: UK arsenic survey of dried seaweeds (hijiki 67 to 96 mg/kg) · Canadian Food Inspection Agency: inorganic arsenic and hijiki · Hong Kong Centre for Food Safety: hijiki risk assessment · 2019 Japanese study: arsenic intake in children and pregnant women · Research Library · Genes & Biomarkers Library
What it is and its history
- Accepted scientific name
- Sargassum fusiforme (Harvey) Setchell, 1931
- Naming authority
- GBIF accepted name
- Synonyms
- Hizikia fusiformis (older name, appears in the sources read); Hijikia fusiforme (spelling in one source)
- Common names
- Hijiki (English); Hiziki (English (alternative spelling)); Hijiki (Spanish (same word; no separate common name found))
- Family or group
- Sargassaceae (the sargassum family of brown seaweeds), as given by GBIF. Hijiki is a seaweed, not a plant.
- Classification
- Chromista > Ochrophyta > Phaeophyceae > Fucales > Sargassaceae > Sargassum > Sargassum fusiforme
- Native range
- Our earlier summary says the coasts of East Asia; not re-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
- Korean surveys sampled both wild and farmed hijiki (53 wild and 34 farmed samples); farming methods were not researched.
- Harvest and processing
- Sold dried as thin black strands, then soaked and cooked (our earlier summary; Nakamura 2008 describes cooked hijiki boiled and fried with vegetables and fried bean curd).
- Look-alikes and adulterants
- Name: Arame, kombu, wakame; Note: Other brown seaweeds with their own pages. The UK survey found no inorganic arsenic above 0.3 mg/kg in the four other seaweeds it tested.
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 |
|---|---|---|---|---|
| Inorganic arsenic (arsenate) | metalloid compound | Whole seaweed 67 to 96 mg/kg in nine dried UK samples (2007); 0.4 to 2.8 mg/kg wet weight in cooked Japanese hijiki dishes | A genotoxic carcinogen according to EFSA. This is the reason for the agency advice. 17 14 6 | Human epidemiology and lab |
| Iodine | mineral (halogen) | Whole seaweed Mean 526.6 mg/kg dry weight (range 159 to 1,548) in 87 Korean samples | Needed for thyroid hormones. Too much may disturb the thyroid. 11 18 | Human data |
| Fucoidan and other seaweed polysaccharides | sulfated polysaccharide | Whole seaweed (extract) Not found | Studied in mice and cells for effects on insulin resistance and fat tissue inflammation. 12 | Animal and cell |
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 raised TSH in habitual seaweed eaters. 1 | Human data |
| Bile acid signalling (TUDCA, TGR5) after fucoidan | TGR5 | In obese mice, a Sargassum fusiforme fucoidan raised TUDCA 6.3-fold; the authors link this to less fat tissue inflammation. Not tested in people. 12 | Animal research |
Absorption, metabolism and elimination
- Absorption
- Not found in people.
- Distribution
- In mice fed hijiki, arsenic reached all tissues.
- Metabolism
- Not found.
- Elimination
- In mice, 66 to 92% of arsenic from a first dose of cooked hijiki was excreted within 3 days; about 5% of repeated doses remained.
- Bioavailability
- Not found in people.
Isolated compounds versus whole extracts
The fucoidan results are for an extract given to mice, not for hijiki as food. Supplements and concentrated extracts were not covered; they can differ widely from the food in iodine and arsenic.
Nutritional composition
No USDA FoodData Central entry for hijiki was found in the earlier summary, and a search for the Japanese standard food table did not reach it on 2026-10-26; retailer labels found online conflicted with each other, so no per-portion nutrient table is given here. What was measured: iodine averaged 526.6 mg per kg dry weight (60.5 mg per kg wet weight) in 87 Korean samples; cooked hijiki dishes in 14 Japanese households contained 0.4 to 2.8 mg per kg wet weight of inorganic arsenic, and the average estimated intake in those households was 6.5 g of cooked hijiki a day (information only, not advice).
2 high certainty2 low certainty2 very low certainty2 not enough evidence
What was looked at
The 8 benefits and uses checked for Hijiki, each with how strong the evidence is. Tap one to open it.
- Inorganic arsenic exposure from eating hijiki (a measured risk, not a benefit)High certainty
- Modelled cancer risk from hijiki arsenicVery low certainty
- Arsenic intake in children and pregnant women who eat hijikiLow certainty
- Soaking and boiling to cut arsenic (lab only)Very low certainty
- What happens to hijiki arsenic in the body (mice)Not enough evidence
- Iodine content (a nutrient fact and a risk)High certainty
- Thyroid blood tests in people who eat seaweed (all seaweeds)Low certainty
- Weight and blood sugar claims (mouse and lab work on extracts)Not enough evidence
Lab markers tracked in the studies
Urinary iodineThyroid-stimulating hormone (TSH)
What is Hijiki good for?
The main studied questions and how strong the evidence is for each. Most results are about risk, not benefit, and none come from a trial of eating hijiki for health.
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.
Inorganic arsenic exposure from eating hijiki (a measured risk, not a benefit)
High certaintyHijiki holds far more of the harmful kind of arsenic (inorganic arsenic) than other seaweeds. A UK survey found 67 to 96 mg per kg in nine dried hijiki samples, and under 0.3 mg per kg in the four other seaweeds tested. In Japan, hijiki supplies almost all the inorganic arsenic that comes from seaweed in the diet.
3 studiesDried hijiki as sold, and cooked hijiki dishes.
Studies, doses and limits
What the trials found
No trial. Measurements of the food (UK 2007, Japan 2014) and of what people ate (Japan 2019).
Results
Hijiki is the one common sea vegetable that stands out for inorganic arsenic. The measurements are solid. They show exposure, not illness.
Effect sizes as reported
UK dried hijiki 67 to 96 mg/kg inorganic arsenic; other seaweeds under 0.3 mg/kg. Japanese adults: about 5.7 microg a day of inorganic arsenic from algae, almost all from hijiki, out of about 24 microg a day in total.
Why this rating
High that the arsenic measurements are correct and repeated by labs in several countries. This is a measured fact, not a health outcome.
Form and dose studied
Dried hijiki as sold, and cooked hijiki dishes.
Who was studied
Retail products in London; Japanese adults.
Limits
Measures what is in the food. It does not count how many people have been harmed. Amounts vary with place and batch.
Why researchers looked
Governments noticed that hijiki stood out in arsenic surveys and asked how much it adds to what people eat.
Traditional use
Hijiki is a long-standing home-cooking ingredient in Japan, often simmered with vegetables and fried bean curd. Long use as food does not show it is free of risk.
Proposed mechanism Laboratory measurement
Hijiki takes up arsenic from sea water. Most of the arsenic in hijiki is the inorganic form (arsenate). A lab study found only arsenate and arsenobetaine among six arsenic compounds tested.
A proposed mechanism does not show that the effect happens in people.
Several governments advise avoiding hijiki. See the Safety tab.
Modelled cancer risk from hijiki arsenic
Very low certaintyOne Japanese study measured arsenic in cooked hijiki from 14 homes and calculated a skin cancer risk above the level the authors call acceptable. It is a calculation, not a count of people who got cancer. European food safety experts say inorganic arsenic from the whole diet is a health concern.
2 studiesCooked hijiki dishes.
Studies, doses and limits
What the trials found
No study counted cancers in hijiki eaters. The estimate comes from measured arsenic in cooked hijiki (0.4 to 2.8 mg/kg wet weight), an average of 6.5 g a day of cooked hijiki and a US EPA slope factor.
Results
A mean modelled skin cancer risk of 2.4 x 10^-4, above the acceptable level of 10^-5 the authors used. They add that the contribution of hijiki to other cancers (bladder, lung) may not be negligible.
Effect sizes as reported
Mean modelled skin cancer risk 2.4 x 10^-4 (range 1.6 x 10^-6 to 7.0 x 10^-4). EFSA 2024: margins of exposure for adults 2 to 0.4 (average) from the whole diet.
Why this rating
Very low for hijiki itself: a model from 14 households. The general link between inorganic arsenic and cancer is much better supported, but it concerns arsenic from all sources.
Form and dose studied
Cooked hijiki dishes.
Who was studied
14 Japanese households; the European population for the EFSA work.
Limits
A calculation with assumptions. Cooked-dish arsenic was low in some homes and not in others. EFSA's finding is for the whole diet.
Why researchers looked
Inorganic arsenic is linked to skin, bladder and lung cancers in people who take it in over many years, mostly from water and food. Researchers asked what hijiki adds.
Proposed mechanism Regulatory summary
EFSA calls inorganic arsenic a genotoxic carcinogen with additional epigenetic effects.
A proposed mechanism does not show that the effect happens in people.
This is the main reason agencies advise avoiding hijiki. See the Safety tab.
Arsenic intake in children and pregnant women who eat hijiki
Low certaintyA Japanese study had 104 children and 101 pregnant women save a copy of three days of food for lab testing. Those who ate hijiki often had higher total and inorganic arsenic intakes. No health effects were measured.
1 studiesHijiki eaten in ordinary meals.
Studies, doses and limits
What the trials found
One 3-day duplicate-diet study in two Japanese cities (205 people).
Results
Hijiki raised arsenic intake, in both groups. Rice was the usual main source of inorganic arsenic in Japan, so hijiki is an extra source on top of it.
Effect sizes as reported
Pregnant women: inorganic arsenic about 1.74 microg per kg body weight a week on average. Hijiki-eater differences are not given as numbers in the abstract.
Why this rating
Low: one observational study over three days, no health outcomes.
Form and dose studied
Hijiki eaten in ordinary meals.
Who was studied
Children and pregnant women in Shimotsuke and Asahikawa, Japan.
Limits
Three days. No outcomes such as birth weight or illness. Intake is not the same as harm.
Why researchers looked
Unborn babies and children are thought to be more sensitive to arsenic, so researchers asked how much of it they take in.
Traditional use
Hijiki is a normal part of some Japanese family cooking.
Proposed mechanism Observational
Not a mechanism study.
A proposed mechanism does not show that the effect happens in people.
Ask a doctor or midwife about hijiki in pregnancy. See the Safety tab.
Soaking and boiling to cut arsenic (lab only)
Very low certaintyIn a lab test on the whole hijiki plant, heating at 90 C for 5 minutes cut arsenic by about 33 to 80%, and a 2% salt-water soak cut it further. This is a laboratory result and does not change government advice.
2 studiesBoiled and soaked whole hijiki.
Studies, doses and limits
Results
Heat and salt-water lowered arsenic in the lab. The authors conclude boiling and soaking would lower intake.
Effect sizes as reported
About 33 to 80% lower after 5 minutes at 90 C; a further 5 to 20% with salt-water as well.
Why this rating
Very low for real kitchens: a lab test, no people, and the remaining arsenic can still be above what agencies accept.
Form and dose studied
Boiled and soaked whole hijiki.
Who was studied
Laboratory samples.
Limits
The UK, Canadian, Hong Kong and Australian advice is to avoid hijiki even though it is normally soaked and cooked.
Why researchers looked
Cooks soak hijiki before eating, so researchers asked how much arsenic that removes.
Traditional use
Dried hijiki is usually soaked and cooked before eating.
Proposed mechanism Laboratory
Arsenic dissolves into the water. The authors report only arsenate and arsenobetaine were present.
A proposed mechanism does not show that the effect happens in people.
Do not read this as a way to make hijiki safe. See the Safety tab.
What happens to hijiki arsenic in the body (mice)
Not enough evidenceIn mice fed hijiki, most of the arsenic left the body within three days and only about 5% remained after repeated feeding. This is animal evidence and may not hold for people.
1 studiesCooked or dried hijiki given to mice.
Studies, doses and limits
Results
66 to 92% of the arsenic was excreted within 3 days of a first dose; with repeated doses arsenic reached all tissues but about 5% of the dose remained.
Effect sizes as reported
66 to 92% excreted within 3 days.
Why this rating
Insufficient for people: mice only.
Form and dose studied
Cooked or dried hijiki given to mice.
Who was studied
Mice.
Limits
Not tested in people. Not a safety guarantee.
Why researchers looked
To see whether arsenic from hijiki builds up.
Proposed mechanism Animal research
Arsenic leaves in urine and faeces.
A proposed mechanism does not show that the effect happens in people.
See the Safety tab.
Iodine content (a nutrient fact and a risk)
High certaintyHijiki is a high-iodine seaweed, though well below sea tangle (a kombu-type kelp). A Korean survey of 87 hijiki samples averaged 527 mg of iodine per kg of dry seaweed. Iodine is needed for the thyroid, but too much can disturb it.
2 studiesDried hijiki.
Studies, doses and limits
What the trials found
Measurements of the food, and a risk calculation based on Korean eating data. No trial of hijiki iodine in people.
Results
Hijiki iodine ranged from 159 to 1,548 mg/kg dry weight (mean 526.6). For people who actually ate hijiki the Korean hazard index was above 1 (4.02 against the Korean limit), meaning intake could pass that limit. A wider survey found iodine varied from 16 microg/g in nori to over 8,165 microg/g in a kelp product, and that cooking changes the amount.
Effect sizes as reported
Hijiki mean 526.6 mg/kg dry weight (60.5 mg/kg wet weight). Korean consumer average of 18.3 g dry weight a day gave the hazard index above 1.
Why this rating
High that the measurements are right for those Korean samples. They are not a health outcome.
Form and dose studied
Dried hijiki.
Who was studied
Korean market and coastal samples (2020 to 2024).
Limits
Korean samples. Soaking and cooking change the iodine in the dish.
Why researchers looked
Seaweeds vary hugely in iodine, so surveys check which ones could push intake too high.
Proposed mechanism Established physiology
Iodine is a building block of thyroid hormones.
A proposed mechanism does not show that the effect happens in people.
Ask a clinician if you have thyroid disease. See the Safety tab.
Thyroid blood tests in people who eat seaweed (all seaweeds)
Low certaintyIn 49 Norwegian habitual seaweed eaters, a thyroid hormone signal (TSH) fell when they stopped seaweed for six weeks. Hijiki is not named in the abstract, so this describes high-iodine seaweed in general.
2 studiesMixed seaweed foods.
Studies, doses and limits
What the trials found
One non-randomised before-and-after study of 49 people.
Results
Estimated iodine intake before stopping was above the tolerable upper level, and well below it after. TSH fell significantly, most in those with the highest earlier intake.
Effect sizes as reported
Median estimated iodine intake 658 microg a day before, 189 after; median urinary iodine 270 before, 87 after.
Why this rating
Low: one study without a control group, mixed seaweeds, hijiki not named.
Form and dose studied
Mixed seaweed foods.
Who was studied
Adult habitual seaweed eaters in Bergen and Oslo.
Limits
No control group. Not specific to hijiki. Blood tests, not illness.
Why researchers looked
Researchers asked whether regular seaweed eating pushes iodine intake too high.
Proposed mechanism Author interpretation
Extra iodine can raise TSH.
A proposed mechanism does not show that the effect happens in people.
See the Safety tab.
Weight and blood sugar claims (mouse and lab work on extracts)
Not enough evidenceClaims about weight, blood sugar and similar benefits come from lab and animal work on extracts, such as a fucoidan from Sargassum fusiforme given to obese mice. We found no human trial of hijiki as food for any health outcome.
2 studiesFucoidan extract from Sargassum fusiforme (not the food).
Studies, doses and limits
Results
Interesting mouse findings with an extract, nothing in people. The food is not the same as the extract.
Effect sizes as reported
6.3-fold rise in serum TUDCA in mice.
Why this rating
Insufficient: mice and cells only, and an extract rather than the food.
Form and dose studied
Fucoidan extract from Sargassum fusiforme (not the food).
Who was studied
Obese mice; fat cells.
Limits
Extract, not food. Two authors work for a nutrition biotech company.
Why researchers looked
Brown seaweeds contain fucoidan and other plant sugars that labs test for effects on fat and sugar handling.
Proposed mechanism Animal and cell
In obese mice the extract raised a bile acid (TUDCA) 6.3-fold, which the authors linked to better insulin resistance and less fat tissue inflammation. It had no direct effect on fat cells in the lab.
A proposed mechanism does not show that the effect happens in people.
Do not take this as a reason to eat hijiki. See the Safety tab.
Connections to the rest of health
How Hijiki 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 |
|---|---|---|---|---|
| Detoxification Support | High certainty | Inorganic arsenic exposure from eating hijiki (a measured risk, not a benefit): Hijiki holds far more of the harmful kind of arsenic (inorganic arsenic) than other seaweeds. A UK survey found 67 to 96 mg per kg in nine dried hijiki samples, and under 0.3 mg per kg in the four other seaweeds tested. In Japan, hijiki supplies almost all the inorganic arsenic that comes from seaweed in the diet. | 17 15 16 | |
| Preventive Wellness | Very low certainty | Modelled cancer risk from hijiki arsenic: One Japanese study measured arsenic in cooked hijiki from 14 homes and calculated a skin cancer risk above the level the authors call acceptable. It is a calculation, not a count of people who got cancer. European food safety experts say inorganic arsenic from the whole diet is a health concern. | 14 6 | |
| Womens Wellness | Low certainty | Arsenic intake in children and pregnant women who eat hijiki: A Japanese study had 104 children and 101 pregnant women save a copy of three days of food for lab testing. Those who ate hijiki often had higher total and inorganic arsenic intakes. No health effects were measured. | 13 | |
| Detoxification Support | Very low certainty | Soaking and boiling to cut arsenic (lab only): In a lab test on the whole hijiki plant, heating at 90 C for 5 minutes cut arsenic by about 33 to 80%, and a 2% salt-water soak cut it further. This is a laboratory result and does not change government advice. | 16 17 | |
| Detoxification Support | Not enough evidence | What happens to hijiki arsenic in the body (mice): In mice fed hijiki, most of the arsenic left the body within three days and only about 5% remained after repeated feeding. This is animal evidence and may not hold for people. | 10 | |
| Thyroid Support | High certainty | Iodine content (a nutrient fact and a risk): Hijiki is a high-iodine seaweed, though well below sea tangle (a kombu-type kelp). A Korean survey of 87 hijiki samples averaged 527 mg of iodine per kg of dry seaweed. Iodine is needed for the thyroid, but too much can disturb it. | 11 18 | |
| Thyroid Support | Low certainty | Thyroid blood tests in people who eat seaweed (all seaweeds): In 49 Norwegian habitual seaweed eaters, a thyroid hormone signal (TSH) fell when they stopped seaweed for six weeks. Hijiki is not named in the abstract, so this describes high-iodine seaweed in general. | 1 5 | |
| Weight Management | Not enough evidence | Weight and blood sugar claims (mouse and lab work on extracts): Claims about weight, blood sugar and similar benefits come from lab and animal work on extracts, such as a fucoidan from Sargassum fusiforme given to obese mice. We found no human trial of hijiki as food for any health outcome. | 12 4 |
B. Body systems
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Digestive System | Biological association | High certainty | Studied for: Inorganic arsenic exposure from eating hijiki (a measured risk, not a benefit) Studied for: Soaking and boiling to cut arsenic (lab only) | 17 15 16 10 |
| Integumentary System | Biological association | Very low certainty | Studied for: Modelled cancer risk from hijiki arsenic | 14 6 |
| Reproductive System | Biological association | Low certainty | Studied for: Arsenic intake in children and pregnant women who eat hijiki | 13 |
| Endocrine System | Biological association | High certainty | Studied for: Iodine content (a nutrient fact and a risk) Studied for: Thyroid blood tests in people who eat seaweed (all seaweeds) | 11 18 1 5 12 4 |
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 | Hijiki holds far more inorganic arsenic than other seaweeds and several governments advise avoiding it. Hijiki holds far more inorganic arsenic than other seaweeds, and several governments advise avoiding it. Preparation or compound: Inorganic arsenic in hijiki |
17 2 |
I. Biomarkers
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Urinary iodine | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 1 |
| Thyroid-stimulating hormone (TSH) | Biological association | 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 |
|---|---|---|---|---|
| Inorganic arsenic (arsenate) | Biological association | Human epidemiology and lab | A genotoxic carcinogen according to EFSA. This is the reason for the agency advice. Preparation or compound: Whole seaweed |
17 14 6 |
| Iodine | Biological association | Human | Needed for thyroid hormones. Too much may disturb the thyroid. Preparation or compound: Whole seaweed |
11 18 |
| Fucoidan and other seaweed polysaccharides | Biological association | Animal and cell | Studied in mice and cells for effects on insulin resistance and fat tissue inflammation. Preparation or compound: Whole seaweed (extract) |
12 |
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 2 lab markers
Tap a number to show only those markers. Counts are markers, not people.
The evidence
The research centre for Hijiki. 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 hijiki summary and re-checked it: PubMed abstracts were re-read, the Korean iodine full text was read for the hijiki numbers, the species name was checked in GBIF, and the Canadian, Hong Kong and Australia and New Zealand agency pages were read. The UK advice was read through a news report. Items we could not verify are flagged in the research gaps.
Study library: every study we checked 19
| Study | Design | People | Product, dose, length | Result |
|---|---|---|---|---|
| Aakre I et al. 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 randomised pre post Mixed seaweeds |
49 49 habitual seaweed eaters in Norway |
Stopping seaweed for six weeks Seaweeds eaten as usual (species not named in the abstract) 6 weeks · vs Their own earlier intake |
Median urinary iodine fell from 270 to 87 microg/L and estimated iodine intake from 658 to 189 microg a day. TSH fell significantly after stopping seaweed, most in those with the highest earlier intake. Estimated intake above the upper level before; well below it after (Not stated in the abstract) Methods, limits and fundingOutcomes: Urinary iodine, iodine intake, TSH, fT3, fT4 Limitations: No control group. Hijiki is not named in the abstract. It tells us about high-iodine seaweed in general. Funding and conflicts: The authors declare none. Read: abstract |
| Canadian Food Inspection Agency 2019 Inorganic arsenic and hijiki seaweed consumption (fact sheet) · Source |
Regulator statement n/a |
Not applicable | Not applicable |
Based on Health Canada information, the CFIA advises people to avoid eating hijiki because it has much higher inorganic arsenic than other seaweeds, and a small amount could exceed the tolerable daily intake. No known illnesses have been linked to hijiki so far. People on a macrobiotic diet with large amounts of seaweed may be at greater risk. Dulse, nori and kombu have tested low in inorganic arsenic. The page does not mention pregnancy or children. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Official or reference statement Limitations: A summary page, not a study. Funding and conflicts: Public body or reference database. Read: full-text |
| Centre for Food Safety 2011 Risk assessment on hijiki and inorganic arsenic · Source |
Regulator statement n/a |
Not applicable | Not applicable |
Advises the public to avoid eating hijiki and avoid choosing it as an ingredient, and to read labels on pre-packed foods. Other seaweeds need not be avoided. A safety assessment found occasional eating of a small amount is unlikely to cause adverse effects, but frequent eaters are at greater risk. Hijiki samples taken in November 2004 exceeded the local limit and were withdrawn. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Official or reference statement Limitations: A summary page, not a study. Funding and conflicts: Public body or reference database. Read: full-text |
| Cherry 2019 Risks and benefits of consuming edible seaweeds Nutr Rev · PMID 30840077 · DOI |
Narrative review Edible seaweeds in general |
Human evidence on whole seaweeds and extracts | Edible seaweeds, extracts and seaweed foods Various |
The review says more evidence from human intervention studies is needed before benefits can be claimed, and that mechanistic evidence is needed to support health claims. It also covers excess iodine and arsenic as potential harms. Not applicable (Not stated in the abstract) Methods, limits and fundingOutcomes: Benefits and risks including iodine and arsenic Limitations: A narrative review, not a systematic one; one author's affiliation is a seaweed products company (VOYA). Funding and conflicts: One author is employed by VOYA Products Limited (seaweed products). Read: abstract |
| EFSA (Dujardin B et al.) 2023 Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European population EFSA J · PMID 36742462 · DOI |
Regulatory exposure assessment n/a |
468 468 European seaweed consumers in 697 eating occasions |
European data on metals and iodine in seaweed Seaweeds and seaweed products |
For adult seaweed consumers, cadmium exposure was within the range of whole-diet estimates; inorganic arsenic and lead were 10 to 30% of whole-diet estimates. Mean iodine intakes above 20 microg/kg body weight a day were found for consumers of kombu and laver. Hijiki is not broken out in the abstract. Not applicable (Not stated in the abstract) Methods, limits and fundingOutcomes: Exposure to arsenic, cadmium, lead, mercury and iodine Limitations: Hijiki is not named in the abstract. Funding and conflicts: EFSA; no outside funding stated. Read: abstract |
| EFSA CONTAM Panel (Schrenk D et al.) 2024 Update of the risk assessment of inorganic arsenic in food EFSA J · PMID 38239496 · DOI |
Regulatory risk assessment n/a |
European population, whole diet | Inorganic arsenic from food and drinking water (not hijiki alone) Whole diet |
Long-term inorganic arsenic intake is linked in epidemiological studies to cancers of the skin, bladder and lung. Inorganic arsenic is called a genotoxic carcinogen. Margins of exposure for adults were low (2 to 0.4 for average consumers) and raise a health concern. Margin of exposure 2 to 0.4 for mean adult consumers, 0.9 to 0.2 at the 95th percentile (Not stated in the abstract) Methods, limits and fundingOutcomes: Health concern for inorganic arsenic Limitations: About the whole diet, not hijiki specifically. Funding and conflicts: EFSA expert panel; declarations of interest are held by EFSA. Read: abstract |
| Food Safety News 2010 Food Standards Agency issues seaweed warning (news report of UK FSA advice) · Source |
News report of regulator statement n/a |
Not applicable | Not applicable |
Reports that the UK Food Standards Agency advises people to stop eating hijiki, that occasional eaters are unlikely to have raised their cancer risk significantly, and that arame, kombu, nori or wakame can be chosen instead because the FSA survey found no inorganic arsenic in them. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Official or reference statement Limitations: A summary page, not a study. Funding and conflicts: Public body or reference database. Read: full-text |
| Food Standards Australia New Zealand 2013 Survey of inorganic arsenic in seaweed · Source |
Regulator statement n/a |
Not applicable | Not applicable |
FSANZ issued a 2004 media release advising consumers to avoid hijiki because it may contain high levels of naturally occurring inorganic arsenic; this survey reviewed that advice. Most samples were under 1 mg/kg; one composite hijiki sample was over and was referred for follow-up. People with high arsenic exposure from other foods who eat hijiki regularly may face greater risk. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Official or reference statement Limitations: A summary page, not a study. Funding and conflicts: Public body or reference database. Read: full-text |
| GBIF 2026 GBIF Backbone: Sargassum fusiforme (Harvey) Setchell, 1931 (usageKey 3196917) · Source |
Reference database n/a |
Not applicable | Not applicable |
GBIF species match 2026-10-26: Sargassum fusiforme (Harvey) Setchell, 1931 is ACCEPTED (99 percent, exact). Chromista, Ochrophyta, Phaeophyceae, order Fucales, family Sargassaceae, genus Sargassum. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Official or reference statement Limitations: A summary page, not a study. Funding and conflicts: Public body or reference database. Read: full-text |
| Ichikawa 2010 Ingestion and excretion of arsenic compounds present in edible brown algae, Hijikia fusiforme, by mice Food Chem Toxicol · PMID 19808076 · DOI |
Animal research Mouse |
Mice | Cooked or dried hijiki given by mouth Whole hijiki Single and repeated doses (amounts not in the abstract) Up to repeated dosing |
In mice, 66 to 92% of the arsenic was excreted within 3 days of a first dose. With repeated doses, arsenic was found in all tissues but only about 5% of the dose remained. 66 to 92% excreted within 3 days (Not stated in the abstract) Methods, limits and fundingOutcomes: Arsenic excretion and tissue levels Limitations: Mice, not people. Not a safety guarantee for human eaters. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Lee 2025 Analysis and Risk Assessment of Total Iodine Content in Edible Seaweeds in South Korea Foods · PMID 40870777 · DOI |
Laboratory analysis Hizikia fusiforme (hijiki) |
348 348 seaweed samples from 12 Korean coastal regions, 2020 to 2024, including 87 hijiki (53 wild, 34 farmed) |
Seaweeds measured for iodine (ICP-MS) Whole seaweed, dry and wet weight |
Hijiki iodine averaged 526.6 mg/kg dry weight (range 159 to 1,548) and 60.5 mg/kg wet weight (full text). Sea tangle was highest at 2,432 mg/kg. For people who actually ate hijiki, the hazard index was above 1 (4.02 under the Korean limit, full text). Hijiki mean 526.6 mg/kg dry weight; consumer hazard index 4.02 (Korean limit) (Not stated in the abstract) Methods, limits and fundingOutcomes: Iodine content and a hazard index against Korean, EFSA and JECFA limits Limitations: Korean samples and Korean eating estimates. The hijiki figures come from the full text; the abstract names hijiki but gives numbers only for sea tangle. Funding and conflicts: The authors declare no conflicts of interest. Read: full-text |
| Lin H et al. 2025 Sargassum fusiforme Fucoidan Ameliorates Obesity-Associated Metabolic Dysfunction via a Tauroursodeoxycholic Acid-Mediated TGR5-cAMP-PKA Signaling Pathway J Agric Food Chem · PMID 40758868 · DOI |
Animal and cell Mouse |
Obese (ob/ob) mice and fat cells in the lab | Fucoidan (a sugar-like extract) from Sargassum fusiforme Seaweed extract · vs Untreated mice |
In obese mice the extract raised blood TUDCA 6.3-fold and the authors linked this to better insulin resistance and less fat tissue inflammation. It had no direct effect on fat cells in the lab. 6.3-fold rise in serum TUDCA (Not stated in the abstract) Methods, limits and fundingOutcomes: Insulin resistance, fat tissue inflammation, bile acid levels Limitations: Mice and cells only. An extract, not the food. Funding and conflicts: Two authors are affiliated with Hangzhou Nutrition Biotechnology Co. Ltd. Read: abstract |
| Mise N et al. 2019 Hijiki seaweed consumption elevates levels of inorganic arsenic intake in Japanese children and pregnant women Food Addit Contam Part A · PMID 30632947 · DOI |
Cross sectional duplicate diet Hijiki in the diet |
205 104 children and 101 pregnant women in two Japanese cities |
3-day duplicate diet (a copy of everything eaten, measured in the lab) Whole diet including hijiki 3 days |
Estimated inorganic arsenic intake was 1.74 microg/kg body weight a week in pregnant women and 8.46 in children. Both total and inorganic arsenic intake were higher in frequent hijiki eaters, in pregnant women and in children. Rice was the usual main source in Japan. Higher intake in frequent hijiki eaters (numbers not in the abstract) (Not stated in the abstract) Methods, limits and fundingOutcomes: Total and inorganic arsenic intake Limitations: Three days of eating in two cities; no health outcomes were measured. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Nakamura 2008 Cancer risk to Japanese population from the consumption of inorganic arsenic in cooked hijiki J Agric Food Chem · PMID 18324769 · DOI |
Risk model Hijiki, cooked |
14 14 Japanese households that supplied cooked hijiki portions |
Cooked hijiki (boiled and fried with vegetables, fried bean curd) measured for inorganic arsenic Whole cooked hijiki dish Mean estimated 6.5 g a day of cooked hijiki (range 1.1 to 14), from serving size and questionnaire frequency |
Inorganic arsenic 0.4 to 2.8 mg/kg wet weight in the cooked dish; intake 0.0005 to 0.023 mg a day. Using a US EPA slope factor, the mean modelled skin cancer risk was 2.4 x 10^-4, above the authors' acceptable level of 10^-5. Mean modelled skin cancer risk 2.4 x 10^-4 (range 1.6 x 10^-6 to 7.0 x 10^-4) (Not stated in the abstract) Methods, limits and fundingOutcomes: Inorganic arsenic in the dish; modelled skin cancer risk Limitations: A calculation from measured arsenic, not a count of cancers. Only 14 households. Risk models rest on assumptions. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Oguri 2014 Inorganic arsenic in the Japanese diet: daily intake and source Arch Environ Contam Toxicol · PMID 23949604 · DOI |
Laboratory analysis Hijiki and rice |
159 159 food items bought in Shizuoka city, Japan, mixed into 19 food composites |
Food composites measured for arsenic forms Whole foods |
Daily inorganic arsenic intake was about 24 microg per person on a content basis. Cereals supplied the most (13 microg), then algae (5.7 microg). Rice and hijiki supplied virtually all of the inorganic arsenic from cereals and algae. Algae about 5.7 microg per person per day, almost all from hijiki (Not stated in the abstract) Methods, limits and fundingOutcomes: Daily inorganic arsenic intake and which foods supply it Limitations: One city, one year of purchases; a model of average intake, not of people who eat hijiki often. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Park GY et al. 2019 Reduction of total, organic, and inorganic arsenic content in Hizikia fusiforme (Hijiki) Food Sci Biotechnol · PMID 30956875 · DOI |
Laboratory experiment Hizikia fusiforme |
Laboratory samples of whole hijiki plant | Heating and salt-water soaking Whole plant, stalk and leaves Heating at 90 C for 5 minutes; soaking in 2% salt solution Minutes · vs Untreated hijiki |
Only arsenate (inorganic) and arsenobetaine (organic) were found among six arsenic compounds. Heating at 90 C for 5 minutes cut arsenic by about 33 to 80%. Salt solution lowered it more, and the two together lowered it by a further 5 to 20%. About 33 to 80% lower after 5 minutes at 90 C (Not stated in the abstract) Methods, limits and fundingOutcomes: Total, organic and inorganic arsenic Limitations: Laboratory test, not people. A cut in arsenic does not mean it is gone; agency advice was not changed by this. Funding and conflicts: The authors declare no conflict of interest. Read: abstract |
| Rose M et al. 2007 Arsenic in seaweed--forms, concentration and dietary exposure Food Chem Toxicol · PMID 17336439 · DOI |
Laboratory analysis Sargassum fusiforme (hijiki), as sold |
31 31 dried seaweed samples of five varieties bought in London shops and online |
Dried seaweeds measured for total and inorganic arsenic Whole dried seaweed, before and after soaking as advised |
Total arsenic 18 to 124 mg/kg. Inorganic arsenic was found only in the nine hijiki samples (67 to 96 mg/kg). The other seaweeds had less than 0.3 mg/kg, the limit of detection. Hijiki inorganic arsenic 67 to 96 mg/kg (dried, as bought) (Not stated in the abstract) Methods, limits and fundingOutcomes: Total and inorganic arsenic Limitations: One UK survey of 31 samples. Retail products at that time, not all hijiki today. Funding and conflicts: Government laboratory (Defra Central Science Laboratory); no conflict stated in the abstract. Read: abstract |
| Teas 2004 Variability of iodine content in common commercially available edible seaweeds Thyroid · PMID 15588380 · DOI |
Laboratory analysis 12 seaweed species |
12 seaweed species from US shops and harvesters in Canada, Tasmania and Namibia | Seaweeds measured for iodine Whole seaweed |
Iodine ranged from 16 microg/g in nori to over 8,165 microg/g in one kelp granule product. Iodine is water-soluble in cooking, so the iodine in a prepared dish is hard to estimate. Some Asian seaweed dishes may exceed the tolerable upper intake of 1,100 microg a day. 16 to over 8,165 microg/g across species (Not stated in the abstract) Methods, limits and fundingOutcomes: Iodine per gram Limitations: Hijiki is not named in the abstract. Funding and conflicts: Not stated in the abstract. Read: abstract |
| Tokyo Metropolitan Government / MHLW 2004 Japanese Ministry of Health position on hijiki (2004), reported by the Tokyo Metropolitan Government · Source |
Regulator statement n/a |
Not applicable | Not applicable |
Not read. The page redirected and then returned 404 on 2026-10-26. Not applicable (Not applicable) Methods, limits and fundingOutcomes: Official or reference statement Limitations: A summary page, not a study. Funding and conflicts: Public body or reference database. Read: not-read |
What we do not know yet
- No human trial of hijiki as food for any health outcome was found in a PubMed check on 2026-10-26.
- No study counted illness or cancer in hijiki eaters; the cancer figure is a model.
- The Japanese ministry's 2004 position and the UK FSA's own current page could not be read.
- No hijiki-specific advice for pregnancy, breastfeeding or children from an official body, and no data on seaweed allergy or medicine interactions, were found.
- No reliable nutrient table for hijiki was found, so none is shown.
- History, farming and how much home soaking removes 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 2
Urinary iodine
Lowered 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
Shows seaweed was adding a lot of iodine. Hijiki is not named in the abstract.
Safety implication
Very high urinary iodine can mean intake above the upper level.
| Study | Result | People | Product |
|---|---|---|---|
| 1 | Median fell from 270 to 87 microg/L after six weeks without seaweed | 49 habitual seaweed eaters in Norway (mixed seaweeds) | Stopping seaweed for 6 weeks |
Thyroid-stimulating hormone (TSH)
Lowered in trials
A hormone from the brain that tells the thyroid how hard to work. A blood test measures it.
Low certaintyResults and what it means
Why it was measured
It is the first blood test doctors use to check the thyroid.
Statistical vs. clinical meaning
A blood-test change, not a disease. It suggests seaweed iodine nudged TSH upward.
Safety implication
People with thyroid disease should ask their clinician about seaweed.
| Study | Result | People | Product |
|---|---|---|---|
| 1 | Fell significantly after stopping seaweed, most in those with the highest earlier iodine intake | 49 habitual seaweed eaters in Norway (mixed seaweeds) | Stopping seaweed for 6 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 3
- Inorganic arsenic comes first. Hijiki holds much more inorganic arsenic than other seaweeds: 67 to 96 mg per kg in dried UK samples, against under 0.3 mg per kg in four other seaweeds. The Canadian Food Inspection Agency, Hong Kong's Centre for Food Safety and Food Standards Australia New Zealand have each advised people to avoid hijiki, and the UK Food Standards Agency has too (as reported). Hong Kong and the UK add that occasional eating is unlikely to have done harm. The CFIA says no illnesses have been linked to hijiki so far. Laboratory measurement and regulator advice 17 2 3 8 7
- Pregnancy and young children. In a Japanese 3-day food study of 104 children and 101 pregnant women, frequent hijiki eaters had higher total and inorganic arsenic intake. Unborn babies and children are thought to be more sensitive to arsenic. Ask your doctor or midwife about hijiki in pregnancy. The CFIA page does not mention pregnancy or children. Observational 13 2
- Iodine and the thyroid. Hijiki is a high-iodine seaweed (Korean average 527 mg per kg dry weight, below sea tangle but far above nori). Too much iodine can disturb the thyroid. In Norwegian habitual seaweed eaters, TSH fell after six weeks without seaweed. If you have thyroid disease or take thyroid medicine, ask your clinician. Laboratory measurement and observational 11 1 18
Side effects 2
| Effect | How often | Severity and recovery | Evidence |
|---|---|---|---|
| Long-term higher inorganic arsenic intake (linked to skin, bladder and lung cancers in people who take in inorganic arsenic over many years) EFSA's link is for inorganic arsenic from the whole diet and water. For cooked hijiki the cancer risk is a modelled calculation (Nakamura 2008), not an observed result. | Not found for hijiki eaters. The CFIA says no illnesses have been linked to hijiki so far. | can be serious over many years Not described in the sources read. | Epidemiology of inorganic arsenic and regulatory 6 14 2 |
| Thyroid disturbance from too much iodine Teas 2004 says some Asian seaweed dishes may exceed the tolerable upper intake of 1,100 microg a day. TSH fell when 49 seaweed eaters stopped seaweed. | Not found for hijiki as food. | can be serious over time Not described in the sources read. | Laboratory measurement and observational 11 1 18 |
Liver
No report of liver injury from eating hijiki was found in the sources read. The 2007 UK arsenic abstract states that inorganic arsenic can cause liver cancer. 17
How often. No frequency was found.
Monitoring. No agency monitoring advice was found.
This is a safety connection. It is not evidence that Hijiki 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 hijiki changes how thyroid medicine works was not found in any source read. | No source on hijiki with thyroid medicine was read. Given the iodine facts, ask a clinician. | None found Not enough evidence 1 | If you have thyroid disease or take thyroid medicine, ask your clinician before adding hijiki. |
| Arsenic from other foods such as rice food | Intakes add up. In Japan rice and hijiki supplied virtually all the inorganic arsenic from cereals and algae. | FSANZ says people with high arsenic exposure from other foods who eat hijiki regularly may face greater risk. | Regulator statement and survey Low certainty 8 15 | Stated by FSANZ and the Japanese survey; not a prescription interaction. |
| Blood thinners (warfarin) and other medicines prescription | No hijiki-specific source was read, and no vitamin K value for hijiki was found. | Not known. | None found Not enough evidence | Ask your pharmacist if you take warfarin or other medicines and eat seaweed often. |
Who should check first 4
| Situation | Advice | Evidence |
|---|---|---|
| Pregnancy or breastfeeding | The agencies read do not give pregnancy-specific hijiki rules, but a Japanese study found higher arsenic intake in pregnant women who ate hijiki often. Ask your midwife or doctor. | Observational 13 |
| Young children | A Japanese study found higher arsenic intake in children who ate hijiki often. No child-specific agency advice was read. Ask a clinician. | Observational 13 |
| Thyroid disease, or taking thyroid medicine | Ask your clinician before adding hijiki. It is a high-iodine seaweed. | Expert guidance 11 1 |
| A macrobiotic diet with large amounts of seaweed | The CFIA says people on a macrobiotic diet that includes large amounts of seaweed may be at greater risk from hijiki. | Regulator statement 2 |
Overdose, product quality and regulators
Summary. The safety points for hijiki as food are inorganic arsenic and iodine. No other toxicity was found in the sources read.
Animal. In mice, 66 to 92% of arsenic from cooked hijiki was excreted within 3 days and about 5% of repeated doses remained (one study).
High dose or long term signals. EFSA: long-term inorganic arsenic intake from diet or water is linked to cancers of the skin, bladder and lung. Iodine: see above.
Overdose response. Not found.
Genotoxicity. EFSA calls inorganic arsenic a genotoxic carcinogen (arsenic in general, not hijiki tested alone).
Hijiki takes up arsenic from sea water.
Inorganic arsenic. UK dried hijiki had 67 to 96 mg/kg inorganic arsenic. In 14 Japanese households cooked hijiki had 0.4 to 2.8 mg/kg wet weight. The sample, the soaking and the cooking all change the figure. 17 14
Soaking and boiling. In a lab test, heating at 90 C for 5 minutes cut arsenic by about 33 to 80% and a salt-water soak cut it further. The agencies' advice to avoid hijiki still stands. 16
Other metals. EFSA's 2023 seaweed assessment found cadmium exposure from seaweed in the range of whole-diet estimates and inorganic arsenic and lead at 10 to 30% of whole-diet estimates for adults who eat seaweed. Hijiki is not broken out in the abstract. 5
| Body | Action | Date |
|---|---|---|
| UK Food Standards Agency (as reported) (United Kingdom) | Advises people to stop eating hijiki; occasional eaters are unlikely to have raised their cancer risk significantly; arame, kombu, nori or wakame can be chosen instead. source | 2010-08-05 |
| Canadian Food Inspection Agency (with Health Canada) (Canada) | Advises avoiding hijiki; a small amount could exceed the tolerable daily intake of inorganic arsenic; no known illnesses linked so far. source | 2019-12-09 |
| Centre for Food Safety, Hong Kong (Hong Kong) | Avoid eating hijiki or choosing it as an ingredient; read labels; occasional small amounts unlikely to cause harm. source | 2011-02 |
| Food Standards Australia New Zealand (Australia and New Zealand) | 2004 advice to avoid hijiki; 2013 survey reviewed it and found one hijiki composite above the 1 mg/kg maximum level. source | 2013-01 |
| European Food Safety Authority (European Union) | Inorganic arsenic from the diet raises a health concern (margins of exposure low). Not specific to hijiki. source | 2024-01-18 |
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 hijiki?
Hijiki is a thin, black brown seaweed, Sargassum fusiforme, sold dried and eaten after soaking and cooking, mostly in Japan.
Why do governments advise avoiding hijiki?
Has anyone become ill from eating hijiki?
The Canadian Food Inspection Agency says no known illnesses have been linked to hijiki so far. The concern is long-term exposure to inorganic arsenic, not a known poisoning.
Is eating hijiki once in a while a problem?
Does soaking or boiling remove the arsenic?
In a lab test, heating at 90 C for 5 minutes cut arsenic by about 33 to 80% and a salt-water soak cut it further. It was a lab result, and the agencies still advise avoiding hijiki.
Is hijiki safe in pregnancy?
No hijiki-specific rule from an official body was found. A Japanese study found higher arsenic intake in pregnant women who ate hijiki often. Ask your doctor or midwife.
Is hijiki safe for children?
In a Japanese study, children who ate hijiki often had higher arsenic intake. No child-specific agency rule was found. Ask a clinician.
Is hijiki high in iodine?
Is hijiki good for the thyroid?
Does hijiki help with weight or blood sugar?
Does hijiki cause cancer?
Are other seaweeds high in arsenic too?
In the UK survey, inorganic arsenic was found only in hijiki; the four other seaweeds tested had under 0.3 mg per kg. Other seaweeds have their own iodine and metal questions.
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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 hijiki summary and re-checked it: PubMed abstracts were re-read, the Korean iodine full text was read for the hijiki numbers, the species name was checked in GBIF, and the Canadian, Hong Kong and Australia and New Zealand agency pages were read. The UK advice was read through a news report. 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 most of the abstracts used.
Sources 19
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 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)
- Canadian Food Inspection Agency. Inorganic arsenic and hijiki seaweed consumption (fact sheet). . 2019. (full-text)
- Centre for Food Safety, Hong Kong. Risk assessment on hijiki and inorganic arsenic. . 2011. (full-text)
- Cherry P, O'Hara C, Magee PJ, McSorley EM, Allsopp PJ. Risks and benefits of consuming edible seaweeds. Nutr Rev. 2019. PMID 30840077. DOI 10.1093/nutrit/nuy066. (abstract)
- EFSA (Dujardin B et al.). 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)
- EFSA CONTAM Panel (Schrenk D et al.). Update of the risk assessment of inorganic arsenic in food. EFSA J. 2024. PMID 38239496. DOI 10.2903/j.efsa.2024.8488. (abstract)
- Food Safety News. Food Standards Agency issues seaweed warning (news report of UK FSA advice). . 2010. (full-text)
- Food Standards Australia New Zealand. Survey of inorganic arsenic in seaweed. . 2013. (full-text)
- GBIF. GBIF Backbone: Sargassum fusiforme (Harvey) Setchell, 1931 (usageKey 3196917). . 2026. (full-text)
- Ichikawa S, Nozawa S, Hanaoka K, Kaise T. Ingestion and excretion of arsenic compounds present in edible brown algae, Hijikia fusiforme, by mice. Food Chem Toxicol. 2010. PMID 19808076. DOI 10.1016/j.fct.2009.09.037. (abstract)
- Lee Y, Park HJ, Jo M, Ha KS, Mok JS. Analysis and Risk Assessment of Total Iodine Content in Edible Seaweeds in South Korea. Foods. 2025. PMID 40870777. DOI 10.3390/foods14162865. (full-text)
- Lin H et al.. Sargassum fusiforme Fucoidan Ameliorates Obesity-Associated Metabolic Dysfunction via a Tauroursodeoxycholic Acid-Mediated TGR5-cAMP-PKA Signaling Pathway. J Agric Food Chem. 2025. PMID 40758868. DOI 10.1021/acs.jafc.5c06044. (abstract)
- Mise N et al.. Hijiki seaweed consumption elevates levels of inorganic arsenic intake in Japanese children and pregnant women. Food Addit Contam Part A. 2019. PMID 30632947. DOI 10.1080/19440049.2018.1562228. (abstract)
- Nakamura Y, Narukawa T, Yoshinaga J. Cancer risk to Japanese population from the consumption of inorganic arsenic in cooked hijiki. J Agric Food Chem. 2008. PMID 18324769. DOI 10.1021/jf0731797. (abstract)
- Oguri T, Yoshinaga J, Tao H, Nakazato T. Inorganic arsenic in the Japanese diet: daily intake and source. Arch Environ Contam Toxicol. 2014. PMID 23949604. DOI 10.1007/s00244-013-9947-8. (abstract)
- Park GY et al.. Reduction of total, organic, and inorganic arsenic content in Hizikia fusiforme (Hijiki). Food Sci Biotechnol. 2019. PMID 30956875. DOI 10.1007/s10068-018-0501-3. (abstract)
- Rose M et al.. Arsenic in seaweed--forms, concentration and dietary exposure. Food Chem Toxicol. 2007. PMID 17336439. DOI 10.1016/j.fct.2007.01.007. (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)
- Tokyo Metropolitan Government / MHLW. Japanese Ministry of Health position on hijiki (2004), reported by the Tokyo Metropolitan Government. . 2004. (not-read)
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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