Sea kelp is a variety of seaweed grown underwater in salty coastal environments. While there are many purported benefits of eating kelp, there are some sea kelp side effects to be aware of, including its high iodine and arsenic content.
What is Sea Kelp?
According to the Yale School of Forestry and Environmental Studies, some forms of kelp are brown in color and can grow up to 100 feet in length. Others like Ecklonia radiata are native to the waters surrounding western Australia.
Giant canopies of kelp exist in shallow water, and provide shelter and nutrition to a variety of marine life, explains the National Ocean Service. Just like plants on land, sea kelp requires light from the sun to carry out photosynthesis and prefers growing in bodies of water that are rich in nutrients.
Read more: Does Sea Salt Have Iodine in It?
Certain sea kelp, like sugar kelp, (Saccharina latissima) thrive in cold waters, off the coast of Connecticut and Maine. These are being grown and harvested as a new sea vegetable. According to the Connecticut Department of Agriculture, seaweeds like sea kelp are able to extract nutrients, both inorganic and organic, from seawater, making themselves naturally nutrient-rich.
Sea Kelp Nutrition
Sea kelp contains relatively few calories. A 2-tablespoon serving of raw sea kelp has just 4 calories, 0.17 grams of protein, 0.96 grams of carbohydrates and less than 0.1 grams of both fat and sugar.
Sea kelp is high in minerals like sodium, which, according to U.S. National Library of Medicine, is responsible for maintaining the body's fluid balance. A 2-tablespoon serving contains 23 milligrams of sodium, or 1 percent of the recommended daily dose, as well as 17 milligrams of calcium and 12 milligrams of magnesium. Minerals like iron, zinc phosphorus and potassium are also available in sea kelp, in smaller amounts.
Read more: Watch Out for the Sodium in Seaweed Salad
According to the USDA, sea kelp contains trace amounts of vitamins like vitamin B5, vitamin C and vitamin E, as well as vitamin K, which is an essential vitamin for the body's blood clotting process. A 2-tablespoon serving offers 6 percent recommended daily dose of vitamin K and 1 percent of the recommended daily dose of vitamins B5 and E.
Sea Kelp Side Effects
Kelp, which is sometimes consumed in the form of kelp supplements, can have an adverse effect on people with hypothyroidism due to its high iodine content. The American Thyroid Association states that the recommended daily allowance for healthy adults is 150 milligrams of iodine and between 220-290 milligrams for women who are either pregnant or lactating.
Read more: Side Effects of Iodine Supplements
Kelp supplements often contain several thousand times the daily recommended limit of iodine, which can sometimes do more harm than good. According to the Mayo Clinic, too much iodine may worsen the symptoms of hypothyroidism in patients with abnormal thyroid glands, especially if a lack of iodine is not the primary cause of the under-active thyroid gland.
Other kelp side effects include sea kelp's high arsenic content. According to an August 2017 study published in Analytical Methods: Advancing Methods and Applications, high levels of arsenic are toxic and carcinogenic in nature.
However, only approximately one percent of the arsenic in sea kelp is the toxic inorganic form, while close to 95 percent of the arsenic present is arsenosugar, a harmless organic compound. Authors of the February 2018 study published in Analytical and Bioanalytical Chemistry explain that one of the dietary kelp side effects is toxicosis due to its levels of arsenic, resulting in diarrhea, nausea and overall body weakness.
A June 2016 study published in Environmental Science and Pollution Research International found that water with high levels of mercury can be remediated by kelp's mercury absorption abilities. While this may be a good thing for reducing heavy metal contamination in seawater, it is important to keep in mind the potential kelp side effects of heavy metal ingestion when consuming kelp supplements.
- Yale School of Forestry and Environmental Studies: "As Oceans Warm, the World’s Kelp Forests Begin to Disappear”
- National Ocean Service: "What Is a Kelp Forest?"
- Connecticut Department of Agriculture: “The State of Kelp"
- USDA FoodData Central: "Seaweed, Kelp, Raw"
- U.S. National Library of Medicine: “Sodium”
- MyFoodData.com: “Nutrition Facts for Kelp Seaweed"
- Mayo Clinic: "Can Iodine Supplements Help Regulate Thyroid Function in a Person With Hypothyroidism?"
- American Thyroid Association: "American Thyroid Association (ATA) Issues Statement on the Potential Risks of Excess Iodine Ingestion and Exposure"
- Analytical Methods: Advancing Methods and Applications: "SI Traceable Determination of Arsenic Species in Kelp (Thallus laminariae)"
- Analytical and Bioanalytical Chemistry: “Development of a Kelp Powder (Thallus laminariae) Standard Reference Material"
- Environmental Science and Pollution Research International: "Comparative Study on Metal Biosorption by Two Macroalgae in Saline Waters: Single and Ternary Systems”