How much Iodine is in Seaweed?

by Monica Wilde
January 2015 

A common question is "how much iodine is there in seaweed?" to which there is no straightforward answer. Monica Wilde, our herbal researcher looks different varieties and analyses how much iodine is in some of her foraged seaweed dishes. The results surprised her and may put off anyone invited to dinner!

The content of iodine in a gram of seaweed is not fixed. It varies hugely from species to species. Also iodine levels can vary even between plants of the same species. Iodine levels can fluctuate depending on ocean water temperature, time of year, depth that the seaweed is growing, habitat and distance from the equator. Other factors come into play after harvest, such as whether seaweed has started to dry out, where and how it is stored.  For example, our Napiers Seagreens Organic Hebridean Kelp has a mean average of 700 mcg/g (or 350 mcg/capsule) but in the study table below the mean value was 646 mcg.

Luckily for us humans, our bodies have evolved to be highly adaptable and tolerant of variation in our diet. Seaweeds are not pharmaceutical chemicals where it is critical for us to be exact - they are foods and food supplements. We know that the absolute minimum amount of iodine that our bodies need to function is 140 mcg a day (250 mcg in pregnancy). We know less about the optimum amount - personally I need 700 mcg/day to function well and I really feel the difference if I forget for a while. That is equivalent to a full teaspoon (equiv. 2 capsules).

From an upper limit point of view, WHO recommend 1100 mcg iodine. This is based on studies with potassium iodine. Potassium iodide is absorbed and excreted at different rates in the body (Combet et al, 2014) that chelated (seaweed) iodine. As only 37% of seaweed iodine is absorbed, you can consume roughly three times as much to get the same absorption rate as you would in a potassium iodide supplement. Even so, this is a tiny amount when you think of a gram in dietary form.

I regularly cook with fingered tangle, Laminaria digitata, a type of kelp pictured here on the right. This small bundle weight 15 grams and is the amount that I would cook in a saucepan of soup to feed 6 people. According to the table below, it probably contains anywhere from 1997 to 8165 mcg/g iodine. As Scotland (where I picked it) is halfway between the latitudes of Maine and Iceland, I am going to use a mid-way point just for discussion. So assuming my bundle has about 5081 micrograms per gram x 15 grams = a whopping 76215 micrograms of iodine in my soup! So each of the 6 guests is served 12702 mcg iodine assuming that none is lost in the cooking. At an absorption rate of 37% (Combet et al, 2014), within three hours of the meal their bodies contain 4670 mcg iodine - that's 4 times over the WHO recommended safe upper limit. I have regularly made this soup for years and no one has experienced any ill effects.

Take another example, sushi rolls. According to Food Standards Australia New Zealand, one sushi roll can contain as much as 92 mcg iodine per roll. However, according to the table below (Teas et al, 2004), 1 sheet of sushi paper contains just 16 mcg. A sheet weighs 2.8 g and you get about 14 small (2.5 cm wide) rolls out of one sheet. This makes the seaweed content only 3.2 mcg per cut roll (not including the fish). So to reach 150 mcg/day you would need to eat anywhere from 2 rolls to 14 rolls to meet your iodine requirements from 1 food source. So iodine can vary widely even in tested foods. 

Read more about the amounts of iodine in common foods.

References

Food Standards Australia New Zealand, Sept 2012. Accessed online at http://www.foodstandards.gov.au/consumer/nutrition/iodinefood/Pages/default.asp

Teas, J., Pino, S., Critchley, A., & Braverman, L.E. (2004). Variability of iodine content in common commercially available edible seaweeds. Thyroid 14(10).