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Although seaweed is considered a very healthy food, there may be some potential dangers of consuming too much.
Excess Iodine
Seaweed can contain a very large and potentially dangerous amount of iodine. Interestingly, Japanese people’s high iodine intake is considered to be one reason why they are among the healthiest people in the world.
However, the daily average intake of iodine in Japan is estimated to be 1,000–3,000 mcg (667–2,000% of the RDI). This poses a risk to those who consume seaweed every day, as 1,100 mcg of iodine is the tolerable upper limit (TUL) for adults. Fortunately, in Asian cultures seaweed is commonly eaten with foods that can inhibit the uptake of iodine by the thyroid gland. These foods are known as goitrogens and found in foods like broccoli, cabbage, and bok choy.
Additionally, it’s important to note that seaweed is water-soluble, which means cooking and processing it can affect its iodine content. For example, when kelp is boiled for 15 minutes, it can lose up to 90% of its iodine content. While a few case reports have associated iodine-containing kelp consumption and thyroid dysfunction, thyroid function returned to normal once consumption stopped.
Nevertheless, high amounts of seaweed can affect thyroid function, and symptoms of too much iodine are often the same as symptoms of not enough iodine. If you think you are consuming too much iodine and experience symptoms like swelling around your neck region or weight fluctuations, reduce your intake of iodine-rich foods and talk to your doctor.
Heavy Metal Load
Seaweed can absorb and store minerals in concentrated amounts. This poses a health risk, as seaweed can also contain large amounts of toxic heavy metals such as cadmium, mercury and lead. That said, the heavy metal content in seaweed is usually below the maximum concentration allowances in most countries.
A recent study analyzed the concentration of 20 metals in 8 different seaweeds from Asia and Europe. It found that the levels of cadmium, aluminum and lead in 4 grams of each seaweed did not pose any serious health risks. Nonetheless, if you consume seaweed regularly, there is the potential for heavy metals to accumulate in your body over time. If possible, buy organic seaweed, as it’s less likely to contain significant amounts of heavy metals. *