Summary
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Do I need kelp?
Kelp supplements may contain small amounts of a variety of vitamins and minerals, but they are primarily a source of iodine — easily providing the daily requirement in a small pill or serving of powder. (See What It Is) -
Why take a kelp supplement for iodine?
People are generally not deficient in iodine unless they follow a restrictive diet, avoiding iodized table salt (be aware that specialty salts are not iodized), dairy, bread, and/or seafood. However, pregnant and nursing women should supplement with iodine. You don't have to take a kelp supplement to get iodine. Taking a supplement containing potassium iodide (common in multivitamins) is just as good and may be a safer choice since you're more likely to get the listed amount of iodine, there is less likelihood of contamination with arsenic or other heavy metals, and iodine from kelp appears to be absorbed only half as well as that from a potassium iodide supplement. Nevertheless, if you prefer a natural source of iodine, a carefully chosen kelp supplement can be fine. -
What are the other health benefits of kelp?
Kelp supplements have been promoted for increasing energy and for weight loss. This has not been clinically proven, but such effects are possible when iodine in kelp treats underlying hypothyroidism due to iodine deficiency (See What It Does) -
How's the quality of kelp supplements?
You need to be very careful. ConsumerLab's tests in this Review found that half of the iodine supplements contained approximately twice as much iodine as listed on their labels, which is a safety concern. In addition, one of these was contaminated with arsenic, a toxin. It is not clear if any particular kelp plant species or harvesting location is better than another (See What CL Found). 
Best choice of kelp supplements?
Among the kelp supplements which contained their listed amounts of iodine and did not exceed contamination levels for arsenic or other heavy metals, CL chose one (costing just 1 cent per dose) as its Top Pick.-
How much kelp to take?
Adults generally need 150 mcg of iodine per day, but this increases with pregnancy and lactation. To get 150 mcg of iodine you need about 30 mg to 150 mg of kelp powder, or less if using a kelp extract (which is more concentrated in iodine) (See ConsumerTips™). -
Safety and side effects of kelp supplements:
Since many kelp supplements contain too much iodine and may be contaminated, you must choose a product carefully. Women who are pregnant or nursing should get iodine from a supplement containing potassium iodide rather than from a kelp supplement. (See Concerns and Cautions).
