Seaweed Under the Microscope: Omega-3 Benefits Meet Iodine Concerns

The researchers analysed dried dulse, nori, wakame, arame, kombu and spirulina purchased online in March 2025, examining their mineral content, potentially toxic elements and fatty acid composition.

Seaweed brings more than flavour to sushi, soups and snacks, with minerals and fatty acids that have attracted interest from researchers and food producers. A study titled 'Commercial Edible Seaweeds: Characterization by Mineral Content and Fatty Acid Composition,' published in Foods, examined six commercial algae products and found striking differences in their nutritional makeup and concentrations of potentially harmful elements. Researchers from the University of Messina, including Vincenzo Nava, Angela Giorgia Potortì and Ambrogina Albergamo, found promising nutrient profiles alongside iodine and contaminant concerns that deserve closer attention.

Six Products Reveal Different Nutritional Profiles

The researchers analysed dried dulse, nori, wakame, arame, kombu and spirulina purchased online in March 2025, examining their mineral content, potentially toxic elements and fatty acid composition. Dulse and nori belong to the red algae group, while wakame, arame and kombu are brown algae; spirulina is a cyanobacterium commonly sold as a microalgal food or supplement. Each product underwent three laboratory analyses, providing repeated measurements of the same sample.

Brown algae generally contained substantial amounts of calcium, magnesium and sodium, with kombu recording the highest calcium, potassium and sodium concentrations among the products tested. Arame had the most magnesium, while spirulina stood out for phosphorus and iron, reaching approximately 555 milligrams of iron per kilogram of dried product. Dulse also contained considerable iron, and nori recorded the highest zinc and copper concentrations.

Eating five grams of seaweed a week provided only a small share of most mineral needs. Eating it more often could provide more minerals, but the study calculated how much people would eat, not how much their bodies would actually absorb.

Kombu's Iodine Content Raises a Clear Question

Kombu produced the study's most striking iodine result, containing approximately 3,126 milligrams per kilogram of dried seaweed, above the 2,000-milligram reference value cited from the French seaweed organisation CEVA. The other five products remained below that concentration benchmark, with iodine levels ranging from about 11 milligrams per kilogram in spirulina to 208 milligrams in wakame.

At the measured concentration, a five-gram portion of dried kombu would contain roughly 15.6 milligrams of iodine before preparation. The researchers' weekly-consumption model estimated an average daily contribution equivalent to around 1,459% of the iodine recommended daily allowance, illustrating how a small quantity of this particular product could deliver a large dose. A recommended daily allowance describes nutritional requirements and should not be confused with a toxicity threshold.

The authors identify excess iodine as a particular concern for people susceptible to thyroid problems, while stressing the need for clearer product information and exposure assessments. Their experiments did not measure iodine remaining after household preparation, leaving an important gap between the composition of the dried product and what someone might consume in a finished meal.

Contaminant Findings Need Careful Interpretation

Cadmium concentrations exceeded the CEVA reference value of 0.5 milligrams per kilogram in four products: dulse, wakame, kombu and nori. Nori had the highest concentration at 1.06 milligrams per kilogram, while spirulina had the lowest at 0.22 milligrams. Lead and mercury remained below the comparison limits used in the paper, and nickel concentrations were below the seaweed limits cited by the researchers.

Arsenic requires a more cautious reading because the laboratory measured total arsenic, which combines different chemical forms with different toxicities. Concentrations ranged from 4.92 milligrams per kilogram in spirulina to 45.63 milligrams in kombu, all numerically above the three-milligram reference value discussed in the paper. That reference applies to inorganic arsenic, so comparing it with total arsenic cannot establish whether the products exceeded the relevant benchmark.

The researchers explain that measuring arsenic forms is essential for a meaningful safety assessment, since seaweed can contain substantial amounts of organic arsenic compounds. Their arsenic exposure estimates should be understood in light of this unresolved distinction, rather than treated as proof of harm from eating the tested products. More detailed testing is needed to understand the possible risks, especially in products with high total arsenic levels.

Nori's Omega-3 Promise Comes With Limits to the Evidence

Nori and dulse had the strongest omega-3 profiles among the products examined, with EPA accounting for approximately 33.65% and 27.21% of their analysed fatty acid profiles, respectively. Nori also had the highest proportion of polyunsaturated fatty acids, at about 44.66%, and favourable calculated indices used to assess fat composition. These laboratory indicators suggest potential for food development; the study did not test whether eating the products improved cardiovascular health.

Brown algae showed more varied fat profiles, including larger contributions from the omega-6 fatty acid arachidonic acid, while arame had the highest proportion of monounsaturated fats. Spirulina contained substantial saturated and omega-6 fatty acids, including gamma-linolenic acid, and the researchers reported a higher DHA proportion than in the other samples. These percentages describe the extracted fat fraction, not the whole food, and should not be interpreted as equivalent amounts of omega-3 in a serving of seaweed.

The study tested only one product of each algae type, all bought at the same time. Researchers tested each sample three times to check their measurements, but this cannot show how products differ between brands, batches, seasons or growing locations. They tested only dried products, so they could not tell how soaking or cooking might change the nutrients and other substances they contained.

The authors say future studies should test more products, identify the different forms of arsenic and examine seaweed after preparation. They also want clearer labels explaining how much to eat and how to prepare it. Seaweed can provide useful nutrients, but its benefits and possible risks depend on the product and how much someone eats.