There are three major active constituents in bladderwrack: iodine, alginic acid, and fucoidan.
The amount of iodine in bladderwrack is highly variable, 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. 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 studies but has not been demonstrated to be effective in humans.
Alginic acid has also been shown to inhibit HIV in the test tube. However, this effect has not been studied in humans. Alginic acid may help lower LDL (“bad”) cholesterol levels, according to animal studies. No human trials have studied this effect of bladderwrack. It is widely used in food and pharmaceuticals as a thickener and gelling agent.
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, lowering blood glucose levels, anti-inflammatory activity, possible anticoagulant effects, and antibacterial and anti-HIV activity. 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. To date, no human clinical trials have been done with fucoidan or bladderwrack to support their use for any of these conditions.