What you eat directly influences which bacteria dominates in your gut. The types of bacteria that can feed best on the foods you choose to eat will grow better (read more on this fascinating topic here). This explains why some cultures handle different types of food better than others. In fact, scientists found that the gut bacteria in healthy Japanese people are higher in bacteria that can digest the types of carbohydrates in seaweed (source).
Potential Risks from Eating Seaweed
There are a couple of potential concerns to be aware of when consuming seaweed:
Too Much Iodine Can Cause Thyroid Problems
Iodine is a very important mineral for thyroid functions, and seaweed is a great source of iodine. While the thyroid can adjust to higher intakes of iodine, it is possible to develop thyroid problems from too much iodine. This may be especially true if you are susceptible to thyroid issues.
A Japanese study found that women who regularly consumed 15 – 30 grams of kombu had elevated TSH, and reduced free T3 and T4. When these women stopped consuming seaweed, then their TSH and thyroid hormone levels returned to normal. Therefore, the authors of this study recommended not to exceed 3 mg of iodine (a serving of seaweed typically contains 20 – 50 mg).
Asian cuisines typically serve seaweed along with foods that contain goitrogens that inhibit iodine absorption by the thyroid. These include the common Asian staples such as tofu, soy milk, and cruciferous vegetables. This might explain why most Japanese and other Asian people can consume seaweed without any problem (source).
Those with existing thyroid disease (or those predisposed to it) should monitor total iodine intake. This is especially important for those who live in countries that fortify foods and table salts with iodine. Generally, consumption of seaweed on occasion (2 – 3 times a week) as a condiment (1 – 2 tablespoons) generally will not exceed the 3 mg limit of iodine.
To be safe, monitor thyroid hormone levels with your doctor as you introduce seaweed into your diet to see if eating seaweed will possibly cause a thyroid problem for you.
Toxic Heavy Metals
While rich in beneficial minerals, seaweed also can contain toxic metals. This likely depends on the type of the seaweed, where it is harvested from, and the variation of toxin levels in the water. Several reports detail the heavy metal content of seaweed:
Hijiki, regardless of brand, contains arsenic that is above the safety limit (source).
A Spanish study extensively compared various types of seaweed imported from Japan, China, Korea, and Chile that are sold in Spain. They concluded that most seaweed products are safe with respect to WHO guidelines. However, some species such as Hijiki and H. fusiforme may be high in arsenic (source).
Heavy metals levels in seaweed can really vary from batch to batch. The best way to know for sure is to purchase your seaweed from companies that regularly third-party lab test their products for heavy metal levels. One company I like that does this is Maine Coast. They publish their test results on their website here.
Remember that heavy metal exposure also comes through other sources like the environment and foods like fish and seafood. Everyone’s ability to remove these heavy metals from their bodies differs. If you are concerned about heavy metal levels, it might be wise to avoid seaweed and seafood altogether.