Kelp refers to several species of large, brown algae that can grow to enormous sizes far out in the depths of the ocean. Kelp is a type of seaweed, but not all seaweed is kelp: "seaweed" loosely describes any type of vegetation growing in the ocean, including many other types of algae and plants.
Kelp is a regular part of a normal human diet in many parts of the world, such as Japan, Alaska, and Hawaii. It is also incorporated into some vitamin and mineral supplements because of its nutrient value. Kelp is a good source of folic acid (a B vitamin), as well as many other vitamins and minerals—especially iodine; but iodine is also a potential source of side effects (see Safety Issues below).
Supplements containing kelp can be purchased at most pharmacies and health food stores. Kelp used in food preparation is available at groceries that stock specialties for Asian cooking.
There is no appropriate "therapeutic" dosage of kelp, as it is not yet known whether kelp is truly therapeutic for any conditions. However, because of its high iodine content, it is important not to overdo your use of kelp. The iodine content in 17 different kelp supplements studied by one group of researchers varied from 45 to 57,000 mcg per tablet or capsule.1 The recommended daily intake for iodine is 150 mcg per day for people over the age of four, and taking a great deal more than this can cause thyroid problems (see Safety Issues).
Kelp is used primarily as a nutrient-rich food supplement.
The results of highly preliminary test tube and animal studies have suggested other potential uses for kelp. For example, there is some evidence that elements in kelp might help to prevent infection with several kinds of viruses, including influenza,2 herpes simplex,3 and HIV.4 Similarly weak evidence hints that kelp possesses cancer preventive effects,5-10 and may lower blood pressure.11 However, far more research, including double-blind, placebo-controlled studies, would be necessary to know whether kelp is actually helpful for any of these health problems. (For information on why this type of study is essential, see Why Does This Database Depend on Double-Blind Studies?)
Additionally, kelp has been marketed as a weight-loss product, but there are no meaningful scientific studies to indicate that it is effective for this purpose.
Another common claim regarding kelp is that, because of its high iodine content, it can help all kinds of thyroid problems. This claim, however, is misleading and even dangerous. It is true that if you are deficient in iodine, kelp is probably good for you, but iodine deficiency is rare, and taking extra iodine when you don’t need it can cause dysfunction of the thyroid (see Safety Issues).
Taking excessive kelp can overload the body with iodine and cause either hypothyroidism or hyperthyroidism —conditions in which the thyroid gland either produces too little or too much thyroid hormone.12-18,22 This is a potentially dangerous side effect and is definitely cause for caution. If your thyroid gland is already functioning incorrectly, you should avoid high doses of kelp except on a physician's advice.
Additionally, published reports describe two cases of acne apparently caused or worsened by taking large doses of kelp.19 This effect is also believed to be due to the large amounts of iodine in the supplement.
Finally, some kelp supplements have been found to contain levels of arsenic high enough to be toxic.20,21, 23 Seawater contains highly diluted arsenic, but kelp (like other ocean life) can concentrate arsenic in its tissues, and there are reports of two people with symptoms of arsenic poisoning who had been consuming kelp.
1. Norman JA, Pickford CJ, Sanders TW, et al. Human intake of arsenic and iodine from seaweed-based food supplements and health foods available in the UK. Food Addit Contam. 1988;5:103-109.
2. Kathan RH. Kelp extracts as antiviral substances. Ann N Y Acad Sci. 1965;130:390-397.
3. Carlucci MJ, Ciancia M, Matulewicz MC, et al. Antiherpetic activity and mode of action of natural carrageenans of diverse structural types. Antiviral Res. 1999;43:93-102.
4. Lee JB, Hayashi K, Hayashi T, et al. Antiviral activities against HSV-1, HCMV, and HIV-1 of rhamnan sulfate from Monostroma latissimum. Planta Med. 1999;65:439-441.
5. Chida K, Yamamoto I. Antitumor activity of a crude fucoidan fraction prepared from the roots of kelp ( Laminaria species). Kitasato Arch Exp Med. 1987;60:33-39.
6. Maruyama H, Watanabe K, Yamamoto I. Effect of dietary kelp on lipid peroxidation and glutathione peroxidase activity in livers of rats given breast carcinogen DMBA. Nutr Cancer. 1991;15:221-228.
7. Teas J. The dietary intake of Laminaria, a brown seaweed, and breast cancer prevention. Nutr Cancer. 1983;4:217-222.
8. Funahashi H, Imai T, Tanaka Y, et al. Wakame seaweed suppresses the proliferation of 7,12-dimethylbenz(a)-anthracene-induced mammary tumors in rats. Jpn J Cancer Res. 1999;90:922-927.
9. Shan BE, Yoshida Y, Kuroda E, et al. Immunomodulating activity of seaweed extract on human lymphocytes in vitro. Int J Immunopharmacol. 1999;21:59-70.
10. Ohno Y, Yoshida O, Oishi K, et al. Dietary beta-carotene and cancer of the prostate: a case-control study in Kyoto, Japan. Cancer Res. 1988;48:1331-1336.
11. Chiu KW, Fung AY. The cardiovascular effects of green beans ( Phaseolus aureus), common rue ( Ruta graveolens), and kelp ( Laminaria japonica) in rats. Gen Pharmac. 1997;29:859-862.
12. Konno N, Makita H, Yuri K, et al. Association between dietary iodine intake and prevalence of subclinical hypothyroidism in the coastal regions of Japan. J Clin Endocrinol Metab. 1994;78:393-397.
13. Eliason C. Transient hyperthyroidism in a patient taking dietary supplements containing kelp. J Am Board Fam Pract. 1998;11:478-480.
14. Shilo S, Hirsch HJ. Iodine-induced hyperthyroidism in a patient with a normal thyroid gland. Postgrad Med J. 1986;62:661-662.
15. Okamura K, Inoue K, Omae T. A case of Hashimoto's thyroiditis with thyroid immunological abnormality manifested after habitual ingestion of seaweed. Acta Endocrinol (Copenh). 1978;88:703-712.
16. Tajiri J, Higashi K, Morita M, et al. Studies of hypothyroidism in patients with high iodine intake. J Clin Endocrinol Metab. 1986;63:412-417.
17. Yamaguchi K, Fukushima H, Uzawa H, et al. A case of iodide myxedema observed for three years under a low iodide diet—especially on the restoration of the escape mechanism from the Wolff-Chaikoff effect. Nippon Naibunpi Gakkai Zasshi. 1984;60:79-88.
18. Ishizuki Y, Yamauchi K, Miura Y. Transient thyrotoxicosis induced by Japanese kombu. Nippon Naibunpi Gakkai Zasshi. 1989;65:91-98.
19. Harrell BL, Rudolph AH. Kelp diet: a cause of acneiform eruption [letter]. Arch Dermatol. 1976;112:560.
20. Pye KG, Kelsey SM, House IM, et al. Severe dyserythropoiesis and autoimmune thrombocytopenia associated with ingestion of kelp supplements. Lancet. 1992;339:1540.
21. Walkiw G, Douglas DE. Health food supplements prepared from kelp—a source of elevated urinary arsenic. Clin Toxicol. 1975;8:325-331.
22. Clark MD CD, Bassett MD B, Burge MD MR. Effects of kelp supplementation on thyroid function in euthyroid subjects. Endocr Pract. 2003;9:363-9.
23. Amster E, Tiwary A, Schenker MB. Case report: potential arsenic toxicosis secondary to herbal kelp supplement. Environ Health Perspect. 2007;115:606-608.
Last reviewed December 2015 by EBSCO CAM Review Board
Last Updated: 12/15/2015
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