Kelp

Also indexed as: Ascophyllum nodosum, Laminaria digitata

What is it?

Kelp is a sea vegetable that is a concentrated source of minerals, including iodine, potassium, magnesium, calcium, and iron. Kelp as a source of iodine assists in making thyroid hormones, which are necessary for maintaining normal metabolism in all cells of the body.

Where is it found?

Kelp can be one of several brown-colored seaweed species called Laminaria.

Kelp has been used in connection with the following condition (refer to the individual health concern for complete information):

Rating	Health Concerns
	Iodine deficiency
Reliable and relatively consistent scientific data showing a substantial health benefit. Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit. For an herb, supported by traditional use but minimal or no scientific evidence. For a supplement, little scientific support and/or minimal health benefit.

Who is likely to be deficient?

People who avoid sea vegetables, as well as dairy, seafood, processed food, and the salt shaker, can become deficient in iodine. Iodine deficiency can cause low thyroid function, goiter, and cretinism. Although iodine deficiency is now uncommon in Western societies, the U.S. population has shown a trend of significantly decreasing iodine intake.

How much is usually taken?

Since the introduction of iodized salt, additional sources of iodine, such as kelp, are not necessary for most people. However, kelp can be consumed as a source of other minerals. A report from Great Britain indicated that the average kelp-based supplement contained 1,000 mcg of iodine (the adult RDA in the United States is 150 mcg per day). It has been suggested that intakes above 2,000 mcg per day should be regarded as excessive or potentially harmful.¹

Are there any side effects or interactions?

There have been several case reports of high intakes of kelp providing too much iodine and interfering with normal thyroid function.² ³ ⁴ ⁵ ⁶ People with thyroid disease should check with a doctor before taking supplements that contain kelp.

At the time of writing, there were no well-known drug interactions with kelp.

References

1. Wolff J. Iodide goiter and the pharmacologic effects of excess iodide. Am J Med 1969;47:101–24.

2. Shilo S, Hirsch HJ. Iodine-induced hyperthyroidism in a patient with a normal thyroid gland. Postgrad Med J 1986;62:661–2.

3. Ishizuki Y, Yamauchi K, Miura Y. [Transient thyrotoxicosis induced by Japanese kombu]. Nippon Naibunpi Gakkai Zasshi 1989;65:91–8 [in Japanese].

4. Hartman AA. [Hyperthyroidism during administration of kelp tablets (letter)]. Ned Tijdschr Geneeskd 1990;134:1373 [in Dutch].

5. de Smet PA, Stricker BH, Wilderink F, Wiersinga WM. [Hyperthyroidism during treatment with kelp tablets]. Ned Tijdschr Geneeskd 1990;134:1058–9 [in Dutch].

6. Eliason BC. Transient hyperthyroidism in a patient taking dietary supplements containing kelp. J Am Board Fam Pract 1998;11:478–80.

The information presented in this website is for informational purposes only and was created by a team of U.S. registered dietitians and food experts. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements, making dietary changes, or before making any changes in prescribed medications.