Iodine in seaweed

Iodine in seaweed

On a recent trip to the wild pristine Westcoast of Vancouver Island to learn about local seaweeds, many of the participants expressed concern about iodine in seaweeds. Iodine is a mineral that our bodies cannot produce.  Our bodies use it to make thyroid hormone.  Thyroid hormone regulates the rate at which our body uses energy, and affects growth and development (1).  In 1920’s, all table salt in Canada became iodized.  This practically eliminated all the cases of goitre and iodine deficiency in the population (2).  On the other hand, there have been Canadian cases of excess consumption of iodine from seaweeds.  High iodine intake can cause thyroid gland inflammation and thyroid cancer (1). Let’s explore seaweed iodine content and how much is safe to consume.

Iodine in seaweed upper tolerable intake limit is 1100 micrograms/day

Iodine in seaweeds is significantly more than in other food sources such as dairy products, fish, meat and vegetable (1). The Canadian guideline for safe upper tolerable limits of iodine intake is 1100 micrograms/d for an adult (1).  The recommended daily iodine intake (to avoid iodine deficiency) is only 110 micrograms. Children need less, and pregnant women need more (3). By the way, 1 tsp. of table salt contains 380 mcg of iodine. Designer salts such as Himalayan rock salt, kosher, pickling and sea salt contain no iodine.

Kelps contain the most seaweed iodine

Kelps contain the most seaweed iodine. 1 g of dried kombu contains 2353 mcg of iodine and 1 g of dried macro kelp contains 2115 mcg of iodine (4). However, 99% of the iodine in kelp is lost when boiled or blanched for 15 min. (5). The soup stock (dashi) would contain iodine. Research reports that iodine content in store prepared kelp dashi broth per 250 ml could be as high as 165 to 7750 mcg (8).

Cooking kelp with goitrogens will lower seaweed iodine absorption

iodine in seaweeds, goitrogens and kelp
Cruciferous vegetables and other goitrogens lower absorption of iodine in seaweed

Furthermore, if you cook your seaweed with cruciferous vegetables such as broccoli, cabbage, cauliflower, bok choy, and brussel sprouts, and other goitrogens such as soybeans and cassava, then iodine absorption is even less (7).  A 250 ml bowl of miso soup could contain high amounts of iodine from the dashi broth but the soy beans in the miso would prevent the iodine absorption in our bodies.

Beware of iodine in purchased kelp powder…read the label or make your own

Please note, all my kelp powder recipes require you to boil the kelp, dry it in the oven, then grind to powder. They are also less salty then the non-boiled kelp powders. Read the labels on kelp powders that you buy. Some advise to only use 0.5 g per day or 1/5th of a teaspoon whereas I often recommend to use 1 tbsp per dish.  

Beware of iodine content in wakame seaweed salad

iodine in seaweed, wakame salad
Wakame salad has high iodine content, beware!

Dried wakame has 171 micrograms iodine per 1 g of seaweed (4). One cup (8oz or 80g) of wakame salad has 13 g of dried wakame which would be 2223 mcg of iodine. That is 2 times the upper tolerable daily limit! This is worth considering because wakame is not boiled to prepare this dish.

Dulce has low iodine content

Dulce contains 72 mcg of iodine per 1 g of dulce (6). 1 tbsp. of dulce powder weighs 5 g; therefore, the upper tolerable limit for iodine would be 15 g or 3 tbsp. per day. That’s a lot of dulce, considering you need only 1-3 tbsp per dish whether it be a dulce loaf or sprinkled on 4 servings of roasted vegetables.

Nori and sea lettuce have the lowest seaweed iodine content

Nori and sea lettuce have the least amount of iodine, only 16 mcg per 1 g in nori (5) and 27 mcg per 1g sea lettuce (5). You could safely eat 7 g of nori per day which equivalates to 1 1/2 packs of 5 g nori snacks or 2-3 sheets of nori sushi sheets. In fact, the upper tolerable limit of iodine from nori would be 20 sheets which is a lot of sushi!

Tips to remember are that the upper tolerable limit of daily iodine intake is 1100 micrograms per day. Also, kelp has the highest iodine content but loses 99% of it when cooked. Dulce has low iodine content. Sea lettuce and nori have the least amount of iodine; you can eat both safely without risk of thyroid toxicity

References

  1. “Food sources of iodine”. Dieticians of Canada. Feb. 28, 2014. Retrieved on 2018/05/27 from https://www.dietitians.ca/Your-Health/Nutrition-A-Z/Minerals/Food-Sources-of-Iodine.aspx
  2. Baute, N. “Health Canada weighs in on Table salt.” The star. Oct. 27, 2010. Retrieved on 2018/05/27 from https://www.thestar.com/life/health_wellness/2010/10/27/health_canada_weighs_in_on_table_salt.html
  3. “Iodine”. National Institute for health. US dept. of Health and Human services. Retrieved on 2018/05/27 from https://ods.od.nih.gov/factsheets/Iodine-Consumer/
  4. JL Smith, G Summers & R Wong (2010) Nutrient and heavy metal content of edible seaweeds in New Zealand, New Zealand Journal of Crop and Horticultural Science, 38:1, 19-28, DOI: 10.1080/01140671003619290
  5. Patel, K. “How can I safely eat seaweed?”. Examiner. Mar. 17, 2017. Retrieved on 2018/05/27 from https://examine.com/nutrition/how-can-i-safely-consume-seaweed/
  6. Garvin, C., “Is kelp of dulse higher in iodine”. Livestrong. Oct. 3, 2017. Retrieved on 2018/05/27 from https://www.livestrong.com/article/492175-is-kelp-or-dulse-higher-in-iodine/
  7. Wikipedia contributors. (2018, March 12). Goitrogen. In Wikipedia, The Free Encyclopedia. Retrieved 20:53, May 27, 2018, from https://en.wikipedia.org/w/index.php?title=Goitrogen&oldid=830048842
  8. Zava TT, Zava DT. Assessment of Japanese iodine intake based on seaweed consumption in Japan: A literature-based analysis. Thyroid Research. 2011;4:14. doi:10.1186/1756-6614-4-14.