Bladderwrack

Bladderwrack is a type of brown algae (seaweed) that grows on the northern Atlantic and Pacific coasts of the United States and on the northern Atlantic coast and Baltic coast of Europe. The main stem of bladderwrack, known as the thallus, is used medicinally. The thallus has tough, air-filled pods or bladders to help the algae float—thus the name bladderwrack. Although bladderwrack is sometimes called kelp, that name is not specific to this species and should be avoided.

Bladderwrack’s mucilaginous thallus has long been used to soothe irritated and inflamed tissues in the body.¹ It was also historically used as a bulk-forming laxative.² People living near oceans or seas have a historically low rate of hypothyroidism, due, in part, to ingestion of iodine-rich food, such as seafood and seaweeds like bladderwrack. It has also been used to counter obesity, possibly due to its reputation for stimulating the thyroid gland. Clinical research in this area has failed to confirm that seaweeds like bladderwrack help with weight loss,³ though more specific research is warranted.

There are three major active constituents in bladderwrack: iodine, alginic acid, and fucoidan.

The amount of iodine in bladderwrack is highly variable,⁴ probably as a result of different amounts of iodine in the water where it grows. A reasonable portion of bladderwrack may contain the U.S. adult recommended dietary allowance (RDA) of iodine (150 mcg). The RDA amount of iodine is believed to be necessary for maintenance of normal thyroid function in adults (infants and children need proportionally less). Thus, in people with insufficient iodine in their diet, bladderwrack may serve as a supplemental source of iodine. Either hypothyroidism or goiter due to insufficient intake of iodine may possibly improve with bladderwrack supplementation, though human studies have not confirmed this.

Alginic acid is a type of dietary fiber that can be used to help relieve constipation and diarrhea. However, human studies have not been done on how effective bladderwrack is for either of these conditions. An over-the-counter antacid, Gaviscon®, containing magnesium carbonate and sodium alginate (the sodium salt of alginic acid), has been shown to effectively relieve the symptoms of heartburn compared to other antacids in a double-blind study.⁵ However, bladderwrack has not been studied for use in people with heartburn. Bladderwrack might also help indigestion, though again clinical trials have not been conducted. Calcium alginate (the calcium salt of alginic acid) has shown promise as an agent to speed wound healing in animal studies⁶ but has not been demonstrated to be effective in humans.

Alginic acid has also been shown to inhibit HIV in the test tube.⁷ However, this effect has not been studied in humans. Alginic acid may help lower LDL (“bad”) cholesterol levels, according to animal studies.⁸ No human trials have studied this effect of bladderwrack. It is widely used in food and pharmaceuticals as a thickener and gelling agent.⁹

Fucoidan is another type of dietary fiber in bladderwrack that contains numerous sulfur groups. According to test tube and animal studies, this appears to give fucoidan several properties, such as lowering LDL cholesterol levels,¹⁰ lowering blood glucose levels,¹¹ anti-inflammatory activity,¹² possible anticoagulant effects,¹³ and antibacterial¹⁴ and anti-HIV activity.¹⁵ Though it has not been definitively proven, fucoidan is thought to prevent bacteria and viruses from binding to human cells, a necessary step in starting an infection, as opposed to killing the microbes directly.^{16, 17} To date, no human clinical trials have been done with fucoidan or bladderwrack to support their use for any of these conditions.

For short-term use (a few days) to relieve constipation, powdered bladderwrack can be taken in the amount of 1 teaspoon three times per day along with at least 8 oz of water each time.¹⁸ For thyroid problems, gastritis, or heartburn, 5 to 10 grams of dried bladderwrack in capsules three times per day has been recommended. Alternately, bladderwrack may be eaten whole or made into a tea using 1 teaspoon per cup of hot water, allowing each cup to sit at least 10 minutes before drinking. Three cups per day of tea can be drunk. No more than 150 mcg iodine should be consumed from all sources, including bladderwrack, per day.¹⁹ However, most bladderwrack products do not give any indication of their iodine content. Therefore, anyone considering taking bladderwrack should first consult a physician trained in nutrition and herbal medicine.

Bladderwrack is generally safe, though there are three potential problems with its consumption: acne, thyroid dysfunction, and heavy-metal contamination. Iodine in any form—including from bladderwrack and other seaweeds—can cause or aggravate acne in some people.²⁰ Excessive iodine ingestion can cause either hypothyroidism or hyperthyroidism and should be avoided.^{21, 22} Bladderwrack and other seaweeds that grow in heavy-metal-contaminated waters may contain high levels of these toxins (particularly arsenic and lead), leading to nerve damage,²³ kidney damage,²⁴ or other problems. Only bladderwrack known to have been harvested from clean water or labeled to indicate the absence of heavy metals or other contaminants should be consumed. The safety of using bladderwrack during pregnancy and breast-feeding is unknown. People who are allergic to iodine may need to avoid bladderwrack.