Product Reviews
Kelp Supplements Review

Reviewed and edited by Tod Cooperman, M.D. Tod Cooperman, M.D.
Initial Posting: 4/22/17 Last Update: 1/27/18
Kelp Supplements Reviewed by
Sections: Jump to a section by clicking on its name.
  • Do I need it? 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 else can kelp do? 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? 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 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 What to Consider When Buying and Using).
  • Cautions: 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).

What It Is:
The term kelp refers to several genera and species of brown algae that grow in the ocean. Common kelp species found in supplements include Laminaria hyperborea, Laminaria digitata, Laminaria setchelli, Macrocytis integrifolia, Macrocystis pyrifera. These species grow in the northern Atlantic and Pacific oceans. Another brown alga found in supplements is Ascophyllum nodosum; it is commonly known as rockweed, but sometimes loosely referred to as kelp (i.e. "Norwegian kelp" or "knotted kelp"). It grows on the northwest coast of Europe, and the northeast coast of the U.S.

Kelp, and other edible seaweeds such as kombu (Laminaria japonica) and nori (Porphyra yezoensis) are commonly consumed in countries such as China, Korea and Japan as "sea vegetables" and are a staple of traditional Asian cuisine.

Kelp contains a number of vitamins, such as folate, vitamin A, E & K, and minerals such as calcium, magnesium and potassium. However, it is best known as a rich source of iodine. Kelp accumulates iodine from seawater, and is more efficient in doing so than any other living organism. Laminaria digitata is known to accumulate the highest amounts of iodine (Küpper Proc Natl Acad Sci USA 2008). However, the same species was also found to have the highest levels of arsenic (a toxin) in a study of seaweed species harvested in New England (Taylor, Chemosphere, 2016).

What It Does:
Preventing/treating iodine deficiency
The iodine in kelp can effectively prevent and treat iodine deficiency — although you may need a higher dose of iodine from kelp than from a potassium iodide supplement. For example, a nine-year-old boy with a condition which required a restrictive diet (including no dairy) was diagnosed with hypothyroidism caused by iodine deficiency. After online research, his mother began to supplement him with a kelp supplement providing 400 mcg of iodine per capsule (number of capsules not reported). After three weeks, his thyroid hormone levels returned to normal, and remained normal after 7 months of continued supplementation (Brooks, J Pediatr Endocrinol Metab 2014). In a small study in Britain among healthy young women (ages 22 to 34) who were not pregnant and had low self-reported dietary intakes of iodine, daily supplementation for two weeks with one 500 mg capsule of Ascophyllum nodosum, which provided the equivalent of 356 mcg of iodine, significantly increased average urinary iodine excretion (a measure of iodine status) from 78 (mildly insufficient) to 145 mcg/L (adequate) (Combet, Br J Nutr 2014). However, the study found that iodine from the seaweed was not absorbed as well as an equivalent amount of iodine from a potassium iodide supplement: 33% of the iodine from seaweed was bioavailable vs. 59% of the iodine from a potassium iodide supplement. This study suggests that one absorbs only about half as much iodine from kelp as from a potassium iodide supplement.

Iodine is essential for healthy functioning of the thyroid gland which, among other things, controls the body's metabolism, and, during pregnancy and infancy, influences bone and brain development. In the U.S., manufacturers voluntarily add iodine to table salt. Most people get enough iodine from their diets, however women who are pregnant require more iodine and people who don't consume table salt may be at risk for deficiency (see ConsumerTips: What to Consider When Using).

Iodine deficiency causes hypothyroidism (underactive thyroid), which can cause impaired mental function and a range of other symptoms, as well as enlargement of the thyroid. This enlargement of the thyroid, called goiter, is often the first sign of deficiency. Deficiency may also increase the risk of the follicular form of thyroid cancer.

Mild to moderate iodine deficiency during pregnancy is associated with an increased risk of attention deficit hyperactivity disorder (ADHD) in offspring, while more severe deficiency can cause neurodevelopmental deficits and growth retardation in the fetus, miscarriage and stillbirth. Cretinism, a condition characterized by stunted growth, mental retardation, deaf mutism, and other abnormalities, can also occur.

Iodine deficiency In infants and children can cause lower than average intelligence and other neurodevelopmental issues.

It's possible that kelp supplementation may be helpful as an alternative source of iodine for people with iodine deficiency who also maintain a low-salt diet; however, there do not appear to be any clinical studies investigating kelp supplements for this use.

Energy and Weight Loss
Kelp supplements are sometimes promoted for energy and weight loss, due in part, to the fact that iodine is needed for healthy thyroid function. Hypothyroidism (underactive thyroid) can cause fatigue and weight gain. However, there is no evidence that kelp supplementation increases energy or promotes weight loss in people who don't have underactive thyroids.

Additionally, a gel-like substance that can be produced from kelp plant cell walls, called alginate, has shown some promise in reducing appetite and lowering blood sugar after eating (Paxman, Appetite 2008; Hoad, J Nutr 2004); however, alginate is a specially prepared, gel-forming extract. It's not known if consuming the dried kelp in regular kelp supplements (such as the ones in this review) has any effect on appetite, blood sugar or weight. One small study among healthy young men in Britain who consumed bread enriched with a branded Ascophyllum nodosum ingredient (Seagreens) found the enriched bread reduced energy intake at a subsequent meal (4 hours later) by 16.4% compared to consuming bread which was not enriched; however, there were no significant reductions in hunger, fullness, or blood sugar levels (Hall, Appetite 2012).

Quality Concerns and What CL Tested For:
Kelp supplements can be a safe source of iodine but can potentially be contaminated with arsenic (Amster, Environ Health Perspect 2007). However, neither the FDA nor any other federal or state agency routinely tests kelp supplements for quality prior to sale. Consequently, purchased and tested kelp supplements to determine if they contained listed amounts of iodine — typically the only nutrient listed on the Supplement Facts panels of kelp supplements -- as well as arsenic and other heavy metals (lead, cadmium, and mercury). Standard tablets and caplets were also tested to be sure that they would disintegrate ("break apart") properly (see Testing Methods and Passing Score).

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