What Are Side Effects of Eating Seaweed?

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When consumed in moderation, seaweed snacks are a good source of iodine and other nutrients.
Image Credit: Mizina/iStock/GettyImages

Seaweed snacks are growing in popularity because they are salty, tasty and marketed as healthy foods. There is even an emerging market for seaweed pills — containing various sea vegetables and marine algae — to treat iodine deficiency among other claims.


Like many things, too much seaweed is not always a good thing. Despite your intentions on consuming seaweed for the health benefits, you should also be aware of the possible side effects. Knowing the seaweed benefits and side effects will help to consume the right amount of this popular sea vegetable.

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When consumed in moderation, seaweed snacks are a good source of iodine and other nutrients. When overconsumed, the side effects may include thyroid problems, thyroid medication interaction, digestive discomfort and potential exposure to radiation and heavy metals.

Seaweed Nutrition Facts

There are many different types of seaweed. Some include nori, kelp, dulse, kombu, spirulina and chlorella. Nori sheets are commonly used in sushi, while spirulina and chlorella are often consumed in supplement form. Kelp and dulse can be sprinkled onto food for their salty umami flavor.

According to the USDA, one nori sheet contains the following nutrition facts:

  • 10 calories
  • 0 grams of fat
  • 1 gram of protein
  • 1 gram of carbohydrates
  • 1 gram of fiber
  • 6 percent daily value (DV) of vitamin A
  • 4 percent DV of vitamin C

Nori sheet nutrition does not stop there. It is also a powerhouse of B vitamins, manganese, iron and omega-3 fatty acids. Since it is low in calories, fat and carbohydrates, it can be incorporated into a variety of diets.


One mineral found in seaweed that is especially noteworthy is iodine. The National Institutes of Health considers seaweed to be one of the best dietary sources of iodine, as one nori sheet provides at least 11 percent DV of iodine. This is both an advantage and disadvantage of seaweed, as excessive iodine intake has several side effects.

Health Benefits of Eating Seaweed

In addition to being an excellent source of many vitamins and minerals, seaweed snacks have several health benefits. Their fiber content encourages proper digestion. Kombu is especially helpful to the gut, and it can be incorporated into soups like miso soup.


A September 2014 study published in Marine Drugs found that seaweed has anticancer properties and may slow the progression of colon and breast cancer in humans. Certain compounds found in seaweed are proven to inhibit the growth of cancer cells. Seaweed also encourages the apoptosis or death of cancer cells.


Consuming adequate amounts of seaweed can also prevent iodine deficiency. In the past, iodine was one of the most common nutritional deficiencies in the world, according to the Centers for Disease Control and Prevention. Now, iodine deficiency is less common, though not obsolete, in America due to increased consumption of iodized salt and dietary sources of iodine like fish and seaweed.


Read more: 6 Benefits of Kelp That Will Keep Your Body Healthy

Side Effects of Seaweed

The seaweed benefits and side effects go hand-in-hand. A benefit to one person may be a side effect to another.

The high-fiber content in seaweed can aid digestion, but it can also cause digestive discomfort. Each gram of fiber adds up, and several servings of seaweed per day can easily push you over the recommended daily allowance of fiber. Too much fiber can cause bloating, gas and constipation.


People with health issues related to the thyroid should be especially cautious of overconsuming seaweed because of its high iodine content. According to a March 2014 study published in Nature Reviews Endocrinology, excess iodine consumption does not have major consequences in the average person. However, people with specific risk factors related to thyroid diseases — such as hypothyroidism and hyperthyroidism — may find that too much iodine can affect their thyroid function and thyroid medications.


One side effect of consuming seaweed is related to the environment rather than the actual food. Most of the world's seaweed is grown in China, but Korea and Japan are also major producers of seaweed. There is concern that seaweed grown on Japanese coasts is contaminated by radioactivity resulting from the Fukushima nuclear accident in 2011. A January 2014 study published in the Journal of Plant Research found contaminated samples of algae. However, researchers do not advise limiting your seaweed intake due to potential radioactive exposure.



Another side effect related to the environment is heavy metal exposure. According to a February 2018 study published in Scientific Reports, red seaweed contains significantly higher levels of copper, nickel and other metals compared to brown seaweed. Though researchers found heavy metals like lead and mercury, they report the risk level is low. However, they recommend the routine surveillance of metals in seaweed.

When it comes to seaweed and seafood, checking the source of your products will help prevent contamination. The health risks are low, but getting to know where your food comes from is part of being an informed, health-conscious consumer.

Read more: Hazards of Eating Nori Seaweed

Should You Eat Seaweed Snacks?

When weighing the seaweed benefits and side effects, you may wonder whether you should consume seaweed products, such as seaweed pills and snacks.

Though the risks are low, certain people should avoid the high iodine content in seaweed. People with pre-existing thyroid diseases or increased risk of thyroid diseases should not overconsume seaweed snacks. However, people at risk of iodine deficiency, such as populations who live far away from natural iodine sources and those who don't use iodized table salt, may consume seaweed snacks.

People who are wary of radioactive and heavy metal exposure should also avoid seaweed grown in Japan and China.

Read more: Things to Consider When Snacking on Dried Seaweed

Incorporate Seaweed into Your Diet

Iodine deficiency was once a major nutritional crisis. Like iodized salt, seaweed prevents this crisis from returning. Seaweed is also a good source of many vitamins and minerals. It is naturally salty and can be used as a seasoning on food to cut back on table salt.

However, excessive seaweed intake is not recommended for everyone. Like most things, seaweed should be enjoyed in moderation. Incorporate nori sheets into your diet by making at-home sushi rolls, or sprinkle dulse flakes on your food for umami flavor.

Seaweed snacks may be more beneficial than seaweed pills because supplements are not regulated by the FDA, and seaweed snacks are closer to the whole sea vegetable.