Benefits
Natural Iodine Source
Brown seaweeds concentrate iodine from seawater, and seaweed has long been part of East Asian diets. In a small trial in UK women with low iodine intake, half a gram of Ascophyllum seaweed daily for 2 weeks raised urinary iodine, though seaweed iodine was absorbed less well than potassium iodide (about 33 versus 59 percent of the dose).
Iodine status in people with low intake
In women with low iodine intake, 0.5 g of seaweed daily for 2 weeks raised median urinary iodine from 78 to 140 µg/L, but TSH also rose, with two women above the normal range. A review of iodine in pregnancy advises a standard iodine supplement of about 150 µg/day and says kelp and seaweed products should be avoided because their iodine content varies too much.
Iodine for thyroid hormone production
The thyroid needs iodine to make its hormones, so seaweed can add iodine to the diet. In an 8-week trial in overweight Japanese adults, iodine-reduced kelp providing about 1 mg of iodine daily did not raise thyroid hormone levels. Too much iodine, which is easy to reach with some kelp products, can itself disturb thyroid function.
Whole-Food Marine Nutrition
Kelp contains iodine plus minerals (calcium, magnesium, iron, potassium), trace elements, fucoxanthin (carotenoid), fucoidan (bioactive polysaccharide), and dietary fiber. Whole-food matrix provides additional nutrition beyond iodine alone.
Culinary Use
Kombu (Laminaria japonica) is fundamental to Japanese cuisine (dashi broth). Wakame in miso soup. Nori for sushi. In these cuisines seaweed is a regular iodine source, though amounts vary widely.
Mechanism of action
Iodine Concentration in Seaweeds
Brown seaweeds concentrate iodine from seawater. In a Norwegian product survey, mean iodine levels were about 7,800 µg per gram in oarweed, 4,469 in sugar kelp and 2,276 in kombu. Content also varies with species, harvest location and processing, and declared iodine was inaccurate for several products.
Natural vs Pharmaceutical Iodine
Seaweed iodine was absorbed less completely than potassium iodide in one human study (median about 33 versus 59 percent of the dose), so the labeled iodine in a seaweed product does not translate one to one into absorbed iodine.
Heavy Metal Bioaccumulation
Seaweeds can take up arsenic and other elements from seawater. In a US analysis of 32 marine-sourced supplements, inorganic arsenic was detected only in the seaweed products and was high (8,900 ng/g) in one of them. Choosing products with third-party heavy metal testing reduces this risk.
Fucoidan and Fucoxanthin
Brown seaweeds also contain fucoidan, a sulfated polysaccharide that is a strong anticoagulant in the test tube but had little effect on clotting when swallowed in a small human study, and fucoxanthin, a carotenoid studied for weight management with mixed results.
Clinical trials
Bioavailability test followed by a 2-week supplementation study of encapsulated Ascophyllum nodosum seaweed (0.5 g daily, about 356 µg iodine). (Combet et al. 2014, Br J Nutr)
Healthy non-pregnant UK women of childbearing age with low dairy and seafood intake (22 in the absorption test, 42 in the 2-week study).
Median urinary iodine rose from 78 to 140 µg/L. Seaweed iodine was about 33 percent bioavailable versus 59 percent for potassium iodide. TSH rose from 1.5 to 2.1 mIU/L, with two women above the normal range but normal free thyroxine.
Randomized, double-blind, placebo-controlled trial of 6 g/day iodine-reduced Laminaria japonica powder (about 1.03 mg iodine) or kelp-free tablets for 8 weeks. (Aoe et al. 2021, Mar Drugs)
50 overweight Japanese adults (BMI 25 to 30).
Serum thyroid hormone levels did not rise. Body fat percentage fell versus placebo in men only, with no significant change in women. One author worked for a kelp processing company.
Laboratory analysis (ICP-MS) of 96 seaweed foods and supplements bought in Norway. (Aakre et al. 2021, Food Nutr Res)
43 whole seaweed products, 39 seaweed-containing foods and 14 dietary supplements.
Supplements provided 5 to 5,600 µg iodine per daily dose. For 54 products, one portion or dose would exceed the tolerable upper intake level, and labelling of species and iodine content was inadequate or inaccurate for several products.
Analysis of total arsenic, inorganic arsenic and arsenolipids in 32 marine-sourced dietary supplements. (Taylor et al. 2022, Chemosphere)
Commercial supplements made from seaweed, krill, calanus, fish and fish liver.
Inorganic arsenic was detectable only in the seaweed products and reached 8,900 ng/g in one product; seaweed supplements were among those with total arsenic above 1,000 ng/g.