How does processing affect the bioavailability of iodine from seaweed?

Seaweeds, or macroalgae, are a diverse group of marine plant-like organisms that are usually divided into three groups: red (Rhodophyta), green (Chlorophyta), and brown algae (Phaeophyceae). Since seaweeds absorb carbon dioxide and nutrients from the ocean as they grow, they can be cultivated with a very low carbon footprint.

European seaweed cultivation has increased over the past decade, with brown algae dominating production, particularly sugar kelp (Saccharina latissima) and winged kelp (Alaria esculenta).

Macroalgae, especially brown algae, contain very high amounts of iodine; some species and products can contain several thousand times as much of the trace element as other types of food.

Studies show that processing strategies may reduce the iodine content of brown algae significantly; however, content may still be high, even after processing.

Assessing bioavailability

Iodine intakes vary greatly across the globe: in Asia, where seaweed is a staple of many diets, intakes are very high, whereas in Western countries, deficiency is more common. Both are associated with negative health effects. But questions remain as to how much of the iodine is absorbed by the human body, and how its bioavailability is affected by cooking.

Scientists from the Norwegian Institute of Marine Research (IMR) and food research institute Nofima carried out a meta-analysis, reviewing studies of iodine in brown macroalgae in relation to bioavailability, bioaccessibility, processing, and speciation. They found that the results varied greatly, with bioavailability ranging from 31 to 90%. In species containing a lot of iodine, such as sugar kelp and kombu, bioavailability was medium to high. Tangle had high bioavailability.

“This has been a matter of debate for a long time. We know there is a great deal of variation in the proportion of iodine from brown algae that is absorbed by our bodies. It has been measured at anywhere between 2 and 90%, depending on the method used and the species of alga,” said Inger Aakre, from the IMR. “In other words, we cannot assume that bioavailability is low in species with high iodine content; on the contrary, our study suggests that the reverse is true.”

She added: “There has been a lot of discussion about both bioavailability and the processing of iodine in brown algae. That is why it is useful to look at and summarise the knowledge that already exists.”

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To blanch, boil or fry: Techniques for reducing iodine

The loss of iodine from brown algae has been reported after several types of processing, including washing, blanching, boiling, soaking, drying, and fermentation. It can be removed via processing in water even at low temperatures, including washing, soaking, and rehydration of dried algae.

“If you boil or blanch kelp, 10 to 94% of the iodine is eliminated. The amount of water used affects how much iodine is released by sugar kelp,” said researcher Marthe Jordbrekk Blikra.

Asked about how other methods of preparation affect iodine content, Blikra told Vitafoods Insights:

“Frying and drying can remove some iodine. We also have some results on the use of novel technology – namely, pulsed electric field (PEF) processing – which are under publication now. So far, we have managed to remove roughly 40% of the iodine from sugar kelp by PEF. Seawater blanching is also very promising (around 90%).”

These findings may be used in food safety evaluations, the authors wrote in the journal Comprehensive Reviews in Food Science and Food Safety.

Knowledge gaps pose challenge to regulation

Iodine exists in a variety of forms, and there may be differences between how inorganic iodine and organic iodine are absorbed by the body. Different species of seaweed may contain different chemical species of iodine, which may also affect how its absorption – but the research on this is lacking.

The European Food Safety Authority (EFSA) has called for more data on the consumption of seaweed and its iodine and heavy metal content.

Blikra said: “The results are inconsistent, so we need more studies to find out why they vary so much. It will be important to find processing methods that enable producers to sell safe, predictable products.”

Asked where research in the area is headed next, she said: “We’re continuing to look at new and improved processing methods for reducing the iodine content in alga – for example, in the SusKelpFood project. We are also looking into utilisation of other species with a lower content of iodine initially, and hoping to look into the effect on processing on bioavailability in future projects.”