Iodine is an essential micronutrient. Dietary iodine is required for thyroid hormone production (which in turn orchestrates numerous vital bodily functions).
Iodine deficiency is common worldwide, but vegan diets are consistently associated with the lowest iodine status compared to other dietary groups1. Animal-free foods are naturally low in iodine, but seaweed is an exception to this rule. This raises the question of whether vegans should increase their consumption of seaweed to improve their iodine status. The answer, I believe, is no. I'll explain why below.
Iodine recommendations differ between countries. But the Department of Health (UK) advises adults to aim for 140mcg2 and the National Institutes of Health (US) recommends 150mcg for adults3. However, the NIH endorses higher amounts for pregnant (220mcg) and lactating mothers (290mcg).
Seaweed can contain excessive amounts of iodine
Credible dietary organisations have estimated the upper tolerable limit for iodine to be between 600mcg4 & 1100mcg5 per day. However, one serving of seaweed can far exceed these amounts.
One study examined seaweed products available in the UK and found that many products, according to the products' packaging, surpassed the European upper tolerable limit per serving. In fact, several products claimed to contain more than 10,000mcg of iodine6. Moreover, a team of researchers tested seaweed products available in Norway and discovered that many products contained similarly high levels of iodine per portion. Worryingly, they found that one particular sample of oarweed (a type of brown seaweed) contained a whopping 62,400mcg of iodine per portion (over 600 times the European upper tolerable limit!).
While vegans on the whole don't get enough iodine, it seems that vegans that consume seaweed regularly have a tendency to obtain too much12.
Seaweed may contain insufficient amounts of iodine
Consumers cannot be sure that a seaweed product contains sufficient iodine. There is no legal requirement to display the iodine content of food products and more importantly, those that do give an indication of their iodine content may be terribly imprecise. For example, a kelp supplement that claimed to contain 200mcg per dose, was found to contain only 40mcg (furthermore, the opposite can also be true: a product may contain much more than declared on the packaging)7.
Iodine concentrations in seaweed varies wildly
Iodine concentrations in seaweed are extremely variable. For example, one 5g serving of dulse could contain as much as ≈3000mcg6, or as little as 25mcg9. Most types of seaweed carry a risk of containing far too much; but depending on the specific sample they may not contain enough to meet the RDA of 140mcg/150mcg.
According to my research, nori (the type of seaweed used to make sushi rolls) is an exception in that it's very unlikely to contain hazardous quantities of iodine 7, 10, 11. Eating sushi (in reasonable amounts) is unlikely to result in an intake of iodine above 600mcg per day. However, while the iodine content per portion of nori is found to be consistently safe, due to variability, it may not contain enough to meet our iodine needs.
How do vegans ensure they get the right amount of iodine?
Due to the issues outlined above, relying on seaweed (somewhat paradoxically) increases one's risk of both getting too little and too much iodine. Both scenarios have the potential to cause thyroid problems (e.g., hyperthyroidism, hypothyroidism, autoimmunological diseases)8.
Arguably, eating seaweed (other than nori) in small amounts occasionally is unlikely to be dangerous. But vegans should consume more reliable animal-free sources of iodine on a daily basis. These fall into three categories: iodised salt, supplements & fortified foods.
Iodised salt
Some countries have adopted a policy of universal salt iodisation. In other countries, consumers have to seek out iodised salt. One thing to be aware of: these products can contain low amounts of iodine, necessitating eating high amounts of salt (which I don't recommend) in order to meet the RDA.
Supplements
Ensure that supplements contain potassium iodide. Seaweed-based supplement are unreliable for reasons explained above. Many multivitamins for vegans contain potassium ioidate/iodide.
Fortified foods
There are only a limited number of fortified products on the market at present, but I expect this to change in the near future. Look for plant milks that are iodine-fortified.
Summary
- Seaweeds are not reliable sources of iodine, because they contain wildly variable amounts.
- Seaweed products contain potentially dangerous amounts of iodine, meaning that it's potentially dangerous to consume them on a daily basis.
- Nori is an exception: it's safe to consume on a daily basis.
- Potassium iodide supplements, fortified foods and iodised salt are reliable animal-free sources of iodine.
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References
1. Eveleigh, E., Coneyworth, L. and Welham, S., 2023. Systematic review and meta-analysis of iodine nutrition in modern vegan and vegetarian diets. British Journal of Nutrition, pp.1-43.
2. https://www.nhs.uk/conditions/vitamins-and-minerals/iodine/
3. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/
4. Scientific Committee on Food, 2006. Tolerable upper intake levels for vitamins and minerals. European Food Safety Authority.
5. SACN, 2014. SACN Statement on Iodine and Health.
6 Bouga, M. and Combet, E., 2015. Emergence of seaweed and seaweed-containing foods in the UK: focus on labeling, iodine content, toxicity and nutrition. Foods, 4(2), pp.240-253.
7. Aakre, I., Solli, D.D., Markhus, M.W., Mæhre, H.K., Dahl, L., Henjum, S., Alexander, J., Korneliussen, P.A., Madsen, L. and Kjellevold, M., 2021. Commercially available kelp and seaweed products–valuable iodine source or risk of excess intake?. Food & Nutrition Research, 65.
8. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/
9. Mouritsen, O.G., Dawczynski, C., Duelund, L., Jahreis, G., Vetter, W. and Schröder, M., 2013. On the human consumption of the red seaweed dulse (Palmaria palmata (L.) Weber & Mohr). Journal of Applied Phycology, 25, pp.1777-1791.
10. Yeh, T.S., Hung, N.H. and Lin, T.C., 2014. Analysis of iodine content in seaweed by GC-ECD and estimation of iodine intake. journal of food and drug analysis, 22(2), pp.189-196.
11. Teas, J., Pino, S., Critchley, A. and Braverman, L.E., 2004. Variability of iodine content in common commercially available edible seaweeds. Thyroid, 14(10), pp.836-841.
12. Eveleigh, E.R., Coneyworth, L.J., Avery, A. and Welham, S.J., 2020. Vegans, vegetarians, and omnivores: how does dietary choice influence iodine intake? A systematic review. Nutrients, 12(6), p.1606.
