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What Are the Most Common Kelp Side Effects?

By Eric Stolze
Updated Jun 04, 2024
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Kelp, or Fucus vesiculosus, is a brown-colored seaweed that grows along coastal areas of the northern Pacific and northern Atlantic oceans as well as the Baltic Sea and North Sea. This natural sea plant also has several other names including bladderwrack, black tang and rockweed. Natural kelp supplements and products are used by many people to improve their health and self-treat several medical conditions. These kelp supplements are typically prepared with extracts of Fucus vesiculosus and other related types of seaweed and may cause side effects in some people. The most common side effects may be linked to kelp’s iodine content and generally include increased or decreased thyroid hormone levels in the blood as well as severe skin lesions.

Unwanted kelp side effects may vary from person to person, and many individuals consume kelp products without experiencing any side effects at all. The presence of iodine can cause stomach irritation, excessive saliva production or an unusual brass-like taste in the mouth. Some kelp products may be contaminated with heavy metals that can cause side effects related to these toxins. People who consume kelp contaminated by arsenic can experience nerve problems or kidney damage in some instances. Long-term use may cause a laxative effect in some people from alginic acid, a component of the seaweed.

Some medical professionals may recommend that pregnant women and breastfeeding mothers refrain from using kelp products due to their high iodine content and potential heavy metal contamination. Iodine in kelp can interact with thyroid medications such as levothyroxine and may alter the functioning of a patient’s thyroid in some cases. People who take kelp along with blood-thinning drugs such as warfarin or anti-platelet drugs such as clopidogrel may experience an increased risk of bleeding. Individuals who use laxative products together with kelp may notice an increased laxative effect in some cases.

The kelp plant is a common traditional food in Vietnam and some other Asian cultures. Raw kelp may be served as a vegetable or prepared in a salad, although it can be pickled or cooked in a vegetable soup. Many people have used kelp as a cosmetic and as a traditional medicine for several ailments such as coughs, stomachaches and hemorrhoids. Scientific evidence is generally inconclusive regarding the effectiveness of kelp for most medicinal uses.

In some case, kelp may fight bacterial or fungal infections due to its antibacterial and antifungal properties. Fucoidans are components of kelp that may have antioxidant effects in the human body. Fucans, another kelp component, and fucoidans may both have anticoagulant or blood-thinning effects in some people. In certain instances, kelp may lower levels of blood sugar in patients with diabetes.

Kelp may also have cancer-fighting properties that limit the growth of some cancer cells. Many products have high levels of iodine that can improve goiters in some people. A goiter is an enlarged thyroid gland that may react to changes in a person’s thyroid hormone levels and increased iodine consumption. Some weight-loss products include kelp, but the effectiveness of this seaweed for losing weight has not generally been studied by scientific researchers.

What Are the Different Types of Seaweed?

There are many different types of seaweed, and kelp is just one of them. There are three classes of seaweed: green algae, red algae, and brown algae. Kelp is a type of brown algae. 

The three classes of seaweed are not as closely related to one another as they may seem. Red algae and brown algae belong to different biological kingdoms altogether. Each of these three varieties has a unique array of nutrients.

Nutritional Value of Kelp vs. Other Seaweeds

All seaweeds are generally low in calories due to their high soluble and insoluble fiber content. However, despite this low-calorie count, seaweeds are typically rich in nutrients. Seaweed has about ten times more nutrients than soil-grown plants. Thus, eating seaweed somewhat frequently can help prevent nutrient deficiencies in the body.

Brown seaweed has the highest iodine content of the three. Red seaweed has the second-highest content, while green seaweed has the least. It tends to contain the most protein, although all seaweeds are considered high in protein due to their balanced amino acid content. Amino acids are the molecules that comprise proteins.

Aside from its high iodine content, kelp and other brown seaweeds can provide many vital nutrients, including iron, magnesium, and fiber. Due to the presence of these nutrients, seaweed can have specific positive effects on the body. 

For example, iron helps the body produce red blood cells. Magnesium is vital for muscles and nerves and also helps produce energy. Finally, fiber plays an essential role in healthy digestion.

How Does Kelp Affect the Thyroid?

Because of its high iodine content, kelp can affect the thyroid in a few different ways. The thyroid is a small gland located in the neck. It’s responsible for producing important thyroid hormones, which are then transported through the blood to the rest of the body. 

Iodine is a necessary mineral for the thyroid to produce these thyroid hormones. Therefore, an iodine deficiency in the body can lead to hypothyroidism: an underactive thyroid.

However, patients with an underactive thyroid should avoid taking kelp as a treatment or at least speak to a medical professional before doing so. Aside from the possible interactions between kelp and thyroid medications, taking kelp may also aggravate an existing thyroid disorder due to its iodine content. 

Taking a large quantity of iodine can worsen hypothyroidism if the individual’s body does not adjust to this high iodine dose. Iodine content in kelp products can also vary widely, making it difficult for individuals to regulate their iodine intake.

In the long run, taking an excessive quantity of iodine can lead to hyperthyroidism: an overactive thyroid. So, patients should be careful with how much kelp they use to avoid these adverse effects on the thyroid.

Does Kelp Affect Mental Health?

Brown seaweeds, including kelp, contain many antioxidants. Out of the three classes of seaweed, brown seaweeds have the highest antioxidant content. 

Antioxidants may prevent or slow oxidative stress. Oxidative stress is an imbalance that occurs as a result of antioxidant deficiency. It can cause cell damage and diseases such as cancer and cardiovascular disease. 

Oxidative stress is also linked to brain disorders such as Alzheimer’s and schizophrenia. Antioxidants are an excellent defense to prevent oxidative stress and cell damage. Therefore, antioxidants are thought to boost immunity, decrease inflammation, and fight the diseases mentioned above.

Some studies also show a link between a lack of antioxidants and mental disorders such as anxiety and depression. This link aligns with research showing the importance of a good diet while treating mental health. So, individuals who include more foods that are high in antioxidants in their diet may notice an improvement in levels of anxiety and depression.

In some cases, however, high doses of antioxidants can be harmful to the body. Antioxidant supplements can also interact with certain medications. So, just as you should take both iodine and antioxidants in moderation, the same goes for kelp: it can have many positive side effects on your body with the right amount, but too much of it can cause harmful side effects. 

Make sure to consult a medical professional before taking any supplements or introducing kelp into your diet for medicinal purposes.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon1005949 — On Dec 27, 2021

I was taking a kelp powder for its nutritional benefits and a thyroid that seems to go hypo and hyper. After a month of increasing anxiety/racing brain, I finally figured out it was the kelp. I’m so bummed. I wish I could take it. Racing brain went away immediately when I stopped.

By anon940545 — On Mar 19, 2014

I have been taking two kelp tablets daily for about three weeks. My metabolism seems to have been boosted and I feel more energetic also I have reduced my laxatives to one tablet daily as the kelp has a laxative effect on me so for me that is a positive.

By anon352608 — On Oct 24, 2013

I have been using a kelp supplement daily -- one tablet. I am feeling great and very energetic. I had a sluggish thyroid, but after using kelp and with a diet change, my thyroid is getting back to normal. I completely stopped eating cruciferous vegetables like cabbage, broccoli, peas and cauliflower. I engage in normal exercise. We have to change our lifestyle in order to keep our thyroids healthy. Good luck and take care. --Mujahid

By anon337465 — On Jun 05, 2013

I've been using kelp for years. I have lost 9 stone in weight. I was a big, big lady. I have had no side effects. at all. In fact, I never felt better. That was until the darn menopause took over.

By anon290099 — On Sep 07, 2012

I started taking Kelp tablets once a day for thyroid support. Dosage 650 mcg. After taking for about two weeks, I am having stomach upset, cramps and loose bowel. Otherwise, I am well, no flu or cold, so I have to suspect my symptoms are related to the kelp.

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