What is it? Overview Usage Side Effects and Warnings

What is Kelp?

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).

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 , 1 herpes simplex , 2 and HIV . 3 Similarly weak evidence hints that kelp possesses cancer preventive effects, 4 5 6 7 8 and may lower blood pressure . 9 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?...

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. 10 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. 11 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. 12 Seawater contains highly diluted arsenic, but...