Seaweed: strong trials, run on extracts

Seaweed is one of the few foods here with meta-analyses behind it, and almost none of that work was done on seaweed as anyone eats it. A meta-analysis of 11 trials in adults with overweight or obesity found that at least 8 weeks of seaweed supplementation lowered BMI by a standardized mean difference of 0.40 (95% CI -0.65 to -0.16). That is a figure in standard deviations rather than kilograms, and the preparations were mostly brown seaweed, refined fractions and extracts, with no dose given in the abstract. A second meta-analysis, of randomized trials of brown seaweed and its extracts, moved total cholesterol down by 3.0 mg/dL (95% CI -5.8 to -0.23) and LDL down by 6.5 mg/dL (95% CI -12.9 to -0.15). The upper edge of that LDL interval nearly touches zero, and HDL and triglycerides did not move at all. Across 23 randomized trials in healthy adults, people with prediabetes and people with type 2 diabetes, from single meals to 180 days, brown seaweed improved postprandial glucose, HbA1c and HOMA-IR while fasting glucose and insulin stayed put. That abstract gives direction and significance without effect sizes, and the subgroup that did better took 1000 mg or more of a seaweed preparation, which is a comparison between subgroups rather than a dose that was tested head to head. A capsule of extract is not a bowl of wakame.

The numbers, per 3.5 oz (100 g)

As people eat it, per 3.5 oz (100 g)

How much is actually in it

• The USDA reference entry for raw kelp gives 66 micrograms of vitamin K, 180 micrograms of folate and 121 mg of magnesium per 3.5 oz (100 g), and folate across ten seaweed records runs from 9 to 580 micrograms. — USDA composition records for seaweed, ten entries, reference entry Seaweed, kelp, raw (n = 10 records) Evidence E source
  Vitamin K 66 ug, folate 180 ug, magnesium 121 mg per 3.5 oz (100 g); folate range 9 to 580 ug across records
• A US agency profile states that sea plants such as kelp concentrate iodine in their tissues, which is why seaweed carries iodine at all. — General US population, sources of dietary iodine Evidence E source
  Kelp named as an iodine source because sea plants concentrate iodine from sea water

What a real portion looks like

• One cup of raw seaweed weighs 2.8 oz (80 g) in the USDA FNDDS portion table, and the unspecified quantity default is 10 g. — US dietary survey portion sizes for Seaweed, raw Evidence E source
  1 cup = 80 g, quantity not specified = 10 g

What your body absorbs

• In a randomized crossover trial in 20 healthy adults, a minimally processed seaweed meal delivered no measurable gain in pooled plasma essential amino acids over 4 hours compared with a control meal. — 20 healthy adults, single-blinded randomized crossover, blood sampled over 4 hours and urine over 24 hours (n = 20) Evidence B source
  Pooled plasma essential amino acids not different between meals (P > 0.6)

What cooking and storage change

• Boiling in a little water and draining keeps 65 percent of folate and 95 percent of magnesium by the USDA retention table for Vegetables and Vegetable Products, the group seaweed falls into. — USDA retention factors for the group Vegetables and Vegetable Products, method boiled in little water and drained Evidence E source
  Folate 65 percent retained, magnesium 95 percent, vitamin C 60 percent, thiamin 85 percent

What it does to your health

• In a meta-analysis of 11 trials, seaweed supplementation for at least 8 weeks lowered BMI by a standardized mean difference of 0.40 in adults with overweight or obesity. — Adults with overweight and obesity in 11 interventional studies, 10 parallel and 1 crossover, at least 8 weeks (n = 11 trials) Evidence A source
  BMI SMD -0.40 (95% CI -0.65 to -0.16 kg/m2), fat mass SMD -1.48 (95% CI -2.66% to -0.30%)
• A meta-analysis of randomized trials found brown seaweed intake lowered total cholesterol by 3.0 mg/dL and LDL cholesterol by 6.5 mg/dL, with no change in HDL or triglycerides. — Participants of randomized trials of brown seaweed and its extracts Evidence A source
  Total cholesterol MD -3.001 (95% CI -5.770 to -0.232), LDL MD -6.519 (95% CI -12.884 to -0.154)
• A meta-analysis of 23 randomized trials found brown seaweed improved postprandial glucose, HbA1c and HOMA-IR, while fasting glucose and insulin stayed unchanged, and doses of 1000 mg or more worked better. — Healthy adults, people with prediabetes and people with type 2 diabetes in 23 randomized controlled trials, from acute interventions to 180 days (n = 23 trials) Evidence A source
  Postprandial glucose, HbA1c and HOMA-IR significantly improved; fasting glucose and insulin not significant

Safety, limits and interactions

• In the same trial of 20 healthy adults, one minimally processed seaweed meal raised serum iodine, urinary iodine and urinary arsenic and increased distension and belching. — 20 healthy adults, single-blinded randomized crossover, one seaweed meal, urine collected over 24 hours (n = 20) Evidence B source
  Serum iodine P = 0.02, urinary iodine P < 0.001, urinary arsenic P = 0.007, distension P = 0.03, belching P < 0.001
• In a randomized dinner study in 9 euthyroid adults, a sushi meal with freshly harvested seaweed salad raised urinary iodine excretion by 385 percent and serum TSH by 150 percent, both back to baseline within 3 days. — Nine euthyroid adults randomized to three sushi dinners, spot urine and blood daily for one week (n = 9) Evidence B source
  Urinary iodine excretion +385% peaking at 393 ug/g at 6 hours, serum TSH +150% on day 2, baseline by day 3
• Among 32 marine-sourced supplements, inorganic arsenic was detectable only in the seaweed samples and reached 8900 ng per g in one product. — 32 marine-sourced dietary supplements analyzed within the New Hampshire Birth Cohort Study (n = 32 supplements) Evidence C source
  Total arsenic above 1000 ng/g in seaweed, krill and calanus products, inorganic arsenic 8900 ng/g in one seaweed product

What people believe that the data does not support

• The EU has authorized no health claim for seaweed, and a submitted wording about seaweed fibers and the colon was refused with the status non-authorized. — EU labeling of foods containing sodium alginate and ulva Evidence E source
  Status Non-authorized, zero authorized claims for seaweed in the register snapshot

Who this works differently for

• Three breastfed infants developed transient neonatal hypothyroidism after their mothers ate seaweed soup daily, an intake often above 2000 micrograms of iodine a day. — Three infants of Korean descent whose mothers ate seaweed soup during pregnancy and postpartum (n = 3) Evidence D source
  Transient neonatal hypothyroidism, one case treated as congenital for 6 years before the diet history was taken
• In Japanese preschool children, 1.76 g of roasted nori a day for 10 weeks lowered diastolic blood pressure by 6.77 mm Hg against 0.05 mm Hg in controls. — 81 Japanese children aged 4 to 5 years, 55 in the intervention group and 26 controls, 10 weeks (n = 81) Evidence B source
  Diastolic blood pressure change -6.77 mm Hg vs -0.05 mm Hg (P = 0.031), systolic -8.29 vs +0.50 mm Hg (P = 0.051)

What is still unknown

• A scoping review found only five human research publications on carrageenan, the additive made from seaweed, against 69 in mammals and 33 in vitro, and called for randomized trials at realistic dietary exposure. — Publications on intestinal adverse effects of carrageenan and processed Eucheuma seaweed, humans, experimental mammals and in vitro (n = 262 publications, 196 on carrageenan) Evidence D source
  5 human, 69 mammal and 33 in vitro research publications on carrageenan, evidence gaps mapped

The bottom line

The reason to be careful with seaweed is iodine, and it moves fast. In 9 euthyroid adults randomized to three sushi dinners, a meal with freshly harvested seaweed salad raised urinary iodine excretion by 385 percent and serum TSH by 150 percent, both back to baseline within 3 days. Nine people and one dinner say nothing about eating it every day. Three breastfed infants developed transient neonatal hypothyroidism after their mothers ate seaweed soup daily, at an iodine intake estimated rather than measured, above 2000 micrograms a day. One of those infants was treated as a case of congenital hypothyroidism for six years before anyone asked about the diet. If you are pregnant, nursing, or being treated for a thyroid condition, seaweed as a daily habit is a question for the person treating you. The nori trial in preschool children, further down, measured blood pressure and not iodine, arsenic or thyroid function, so it is no evidence that a daily seaweed habit is safe for a child. Arsenic travels with the iodine. In a crossover trial in 20 healthy adults, one minimally processed seaweed meal raised serum iodine, urinary iodine and urinary arsenic and increased distension and belching, all reported as significance without absolute concentrations, from a single meal. Among 32 marine- sourced supplements, inorganic arsenic was detectable only in the seaweed products and reached 8900 ng per g in one of them, which is a finding about supplements rather than about the sheets in your kitchen. Nothing on this page sets a ceiling: the US agency profile names kelp as a plant that concentrates iodine from sea water, and it gives neither a concentration of iodine in seaweed nor an upper limit of intake, so there is no number here to hold a daily habit against. That absence is a gap in this page, not a sign that any amount is safe. Seaweed also gets sold as a protein food. In that same trial of 20 healthy adults, the seaweed meal produced no measurable gain in pooled plasma essential amino acids over 4 hours against a control meal (P > 0.6), and the authors call their own sample too small to settle the question. In 81 Japanese children aged 4 and 5, 1.76 g of roasted nori a day for 10 weeks lowered diastolic blood pressure by 6.77 mm Hg against 0.05 mm Hg in the controls. The groups were unequal in size, the report does not describe how the children were allocated, and the girls showed no effect. For everything else the numbers are small. One cup of raw seaweed is 80 g in the USDA survey table, and the default when a quantity is not stated is 10 g. Raw kelp carries 66 micrograms of vitamin K, 180 micrograms of folate and 121 mg of magnesium per 3.5 oz (100 g), and folate runs from 9 to 580 micrograms across the ten records, which are ten different species rather than ten samples of one. We hold no card on how that vitamin K bears on blood-thinning medicines, so if you take one, ask your doctor before making seaweed a regular food. Boiling in a little water and draining keeps 65 percent of folate and 95 percent of magnesium, by a USDA retention factor set for vegetables as a whole group and not for seaweed. The EU register of health claims authorizes nothing at all for seaweed, and a submitted wording about seaweed fibers and the colon sits there as non-authorized, which records the state of the evidence at the time it was assessed. Carrageenan, the additive made from seaweed, is a different product from the plant. A scoping review counted five human research publications on it against 69 in mammals and 33 in cells.

Sources

1. swd-daily-r3-10 · composition dataset
   Seaweed, kelp, raw (FDC 168457): vitamin K 66 ug, folate 180 ug, magnesium 121 mg per 100 g; folate across 10 seaweed records 9 to 580 ug
   USDA FoodData Central, SR Legacy, 2019, FDC 168457 · checked 2026-09-25
2. swd-daily-r3-13 · agency profile
   Iodine is in ocean fish, shellfish, and certain plants that grow in the ocean (kelp). This is because there is iodine in sea water, and some sea plants and animals concentrate iodine in their tissues.
   Toxicological Profile for Iodine, Centers for Disease Control and Prevention (US), 2004 · checked 2026-09-25
3. swd-daily-r3-12 · portion dataset
   Seaweed, raw, FNDDS food code 75127500, fdc_id 2709805: 1 cup 80 g; Quantity not specified 10 g
   USDA FNDDS 2021-2023, food code 75127500, fdc_id 2709805 · checked 2026-09-25
4. swd-daily-r3-04 · randomized crossover trial
   In a single-blinded, randomized crossover trial, 20 healthy adults consumed SWM or a control meal (CON). ... Pooled plasma essential amino acids showed no statistically significant difference between meals (P > 0.6); however, the pilot sample size may have limited the ability to detect meaningful differences.
   Clin Nutr ESPEN, 2026 · checked 2026-09-25
5. swd-daily-r3-11 · retention dataset
   Retn_Code 3004, VEG,GREENS,BOILD,LITTLE WATER DRAIN, group Vegetables and Vegetable Products: folate_total_ug 65%, magnesium_mg 95%, vitamin_c_mg 60%, thiamin_mg 85%
   USDA Table of Nutrient Retention Factors, Release 6, 2007, Retn_Code 3004 · checked 2026-09-25
6. swd-daily-r3-01 · meta-analysis of RCTs
   Meta-analysis showed a significant effect, favoring the intervention group for BMI (body mass index) (standardized mean difference [SMD]: -0.40; 95% CI: -0.65 to -0.16 kg/m2; P = 0.0013) and percentage of fat mass (SMD: -1.48; 95% CI: -2.66% to -0.30%, P = 0.0138).
   Nutr Rev, 2025 · checked 2026-09-25
7. swd-daily-r3-02 · meta-analysis of RCTs
   In this meta-analysis, we found that brown seaweed intake significantly decreased the levels of total cholesterol (mean difference (MD): -3.001; 95% CI: -5.770, -0.232) and low-density lipoprotein (LDL) cholesterol (MD: -6.519; 95% CI: -12.884, -0.154); nevertheless, the statistically significant association of brown seaweed intake with high-density lipoprotein (HDL) cholesterol and triglycerides were not observed in our study (MD: 0.889; 95% CI: -0.558, 2.335 and MD: 8.515; 95% CI: -19.354, 36.383).
   Mar Drugs, 2023 · checked 2026-09-25
8. swd-daily-r3-03 · meta-analysis of RCTs
   In this meta-analysis, postprandial blood glucose, glycated hemoglobin (HbA1c), and Homeostatic Model Assessment of Insulin Resistance (HOMA-IR) levels demonstrated significant improvements in the seaweed group compared to the control group. Conversely, fasting blood glucose and insulin levels did not show significant associations with seaweed consumption. Subgroup analysis revealed that a high dose (1000 mg or more) was more beneficial than a low dose, and seaweeds such as Laminaria digitata, Undaria pinnatifida, Acophyllum nodosum, and Fucus vesiculosus were found to be more effective at improving blood glucose levels than control treatments.
   Nutrients, 2023 · checked 2026-09-25
9. swd-daily-r3-05 · randomized crossover trial
   Increases in serum iodine (P = 0.02), urinary iodine (P < 0.001), and urinary arsenic (P = 0.007) were observed following the SWM. Gastrointestinal symptoms, including distension and belching, were greater after SWM (treatment-time interaction P = 0.03 and < 0.001, respectively).
   Clin Nutr ESPEN, 2026 · checked 2026-09-25
10. swd-daily-r3-06 · randomized crossover dinner study
    Pre-meal urinary iodine excretion (UIE) was 75 µg/g. UIE rose (p < 0.001) by 385%, 59% and 43% for groups A, B, and C, peaked in the 6-h spot urine sample at 393, 120, and 109 µg/g, and was down to pre-meal values by day 2. Serum TSH rose (p = 0.012) 150% on day 2 and was down to pre-meal values by day 3.
    J Endocrinol Invest, 2020 · checked 2026-09-25
11. swd-daily-r3-08 · cohort with product analysis
    A study of 32 marine-sourced supplements found higher total arsenic concentrations (>1000 ng g-1) in supplements made from seaweed, krill and calanus oil, and in fish and fish liver products marketed as "unprocessed". Inorganic arsenic was only detectable in the seaweed samples, and was elevated (8900 ng g-1) in one product.
    Chemosphere, 2022 · checked 2026-09-25
12. swd-daily-r3-09 · regulatory decision
    POL-HC-7631 Name of Food product: Product-specific claim: sodium alginate and ulva Description of food in terms of food legislation categories: food not covered by specific food legislation Was food on Irish market before 1st July 2007: No Exact wording of claim as it appears on product: Seaweed fibres support body detoxification Examples of any alternative wording that may be used in relation to claim: Seaweed fibres rid toxins from the body/Seaweed fibres aid colonic health/Seaweed fibres protect the colon/ Seaweed fibres can detox the body/Seaweed fibres help maintain a healthy colon/Seaweed fibres promote healthy conditions in the colon Is claim a picture: No Non-authorised
    EU Register on nutrition and health claims, claim POL-HC-7631, snapshot 2026-09-21 · checked 2026-09-25
13. swd-daily-r3-07 · case series
    Seaweed soup (miyeok-guk) is traditionally consumed daily by Korean women during pregnancy and postpartum, often resulting in maternal iodine intake exceeding 2000 µg/day, which can induce neonatal hypothyroidism (NH) in breastfed infants. ... We present 3 infants of Korean descent with NH associated with maternal seaweed soup consumption.
    JCEM Case Rep, 2026 · checked 2026-09-25
14. swd-daily-r3-14 · controlled dietary intervention
    Changes in SBP were -8.29 mm Hg in the intervention group and +0.50 mm Hg in the control group (P for difference in change = 0.051). Changes in DBP were -6.77 mm Hg in the intervention group and -0.05 mm Hg in the control group (P = 0.031). In girls, no difference in blood pressure changes was found between the intervention and control groups.
    J Epidemiol, 2021 · checked 2026-09-25
15. swd-daily-r3-15 · scoping review
    For CGN, five, 69 and 33 research publications on humans, experimental mammals and in vitro experiments were found, further separated as degraded or native (non-degraded) CGN. ... For both additives, randomized controlled trials of sufficient power and with realistic dietary exposure levels of single additives, performed in persons of all ages, including potentially vulnerable groups, are needed.
    Crit Rev Toxicol, 2023 · checked 2026-09-25