What Is Brown Seaweed?

This vegetable of the sea may help manage diabetes and obesity

Brown seaweed dried herb, capsules, and powder

Verywell / Anastasia Tretiak

Besides being a staple of Asian cuisine, two types of brown seaweed, Fucus vesiculosus (also known as bladderwrack) and Laminaria japonica, have also been used in traditional medicine systems to treat various health conditions, including thyroid disease. Seaweed contains iodine, a trace mineral needed for proper thyroid function.

Both types of brown seaweed also contain fucoidan, a substance that, according to a report by the Memorial Sloan Kettering Cancer Center, may have a variety of effects—boosting the immune system, reducing inflammation, preventing cancer, lowering blood pressure, and preventing blood clots and infection. They're also sources of fucoxanthin, an antioxidant that gives brown algae its characteristic color, and a substance that's earned particular attention as a potential anti-obesity supplement.

To date, however, there's insufficient evidence to support any benefits of brown seaweed on human health. Even its traditional use in treating thyroid disease is not advised, as there are no studies of efficacy, dosing, or safety to support its use, nor standardization of iodine content.

What Is Brown Seaweed Used For?

Preliminary research on brown seaweed extract is promising and suggests that it may offer these health effects, though more information is clearly needed.

Hormone Modulation

In a case report published in 2004, researchers found that dietary intake of bladderwrack produced anti-estrogenic effects in three pre-menopausal women. According to the study's authors, these findings suggest that bladderwrack may help reduce the risk of estrogen-related cancers—likely because brown seaweed lowers levels of cholesterol, which is a precursor to the synthesis of sex hormones. However, the authors caution that further research is needed before any conclusions about bladderwrack's cancer-fighting effects can be drawn.

A study in 2005 found that a diet containing brown seaweed lowered levels of the potent sex hormone estradiol in rats, which suggests that it might decrease the risk of estrogen-dependent diseases, such as breast cancer in humans.

However, researchers caution against a run on seaweed because of these early results, saying that the study "points to the need for more studies."

Weight Loss

Inhibition of lipases, a pancreatic enzyme that causes the breakdown of fats, is one of the main therapeutic targets of anti-obesity drugs.

In one 2016 study, researchers assessing the anti-pancreatic lipase activity of preparations from three brown seaweeds, including Fucus vesiculosus, found that they showed significant inhibition of lipase activity.

A 2018 study found that supplementing the diet of rats with Laminaria japonica for 16 weeks altered the gut’s microbe content. More specifically, they found healthy probiotics were increased, and families of gut bacteria associated with controlling weight were boosted. According to researchers, "the overall results support multiple prebiotic effects of seaweed L. japonica on rats as determined by bodyweight reduction, enhanced immune response, and desirable changes in intestinal microbiota composition, suggesting the great potential of L. japonica as an effective prebiotic for the promotion of host metabolism and reduction of obesity in humans."


Edible seaweed is low in calories and rich in dietary fiber, unsaturated fatty acids, and vitamins, making it suitable for managing diabetes, according to the authors of a 2015 review of the potential bioactive compounds in seaweed for diabetes management.

Indeed, an animal study from 2010 confirmed that an active component of Laminaria japonica, which is widely used in Chinese medicine as a treatment for diabetes, has significant activity in the prevention of the digestion of carbohydrates. This might make it easier for your body to stabilize your blood sugar levels. The researchers indicated that it could be developed as an agent for type 2 diabetes therapy.

Cognitive Improvement

A 2018 study provided the first evidence for modulation of cognition with seaweed extract. In the study, researchers tested a combination supplement of Ascophyllum nodosum and Fucus vesiculosus on 60 healthy adults. After a large carbohydrate-heavy lunch, half of the volunteers were given the supplement and the other half were given a placebo. Compared with their results on a pre-lunch cognitive test, the group receiving the supplement performed better at the cognitive tasks than the placebo group.

These findings are likely explained by the fact that components in brown seaweeds inhibit the key enzymes involved in the breakdown and absorption of carbohydrates, allowing for the steady blood sugar levels that allow the brain to function consistently well. Researchers call for future research to examine the effects on cognition in parallel with blood glucose and insulin responses.

Skin Care and Arthritis

Research suggests that the Fucus vesiculosus extract may possess anti-aging and anti-inflammatory activities too.

Possible Side Effects

Fresh seaweed is rich in dietary fiber, minerals and vitamins, and low in fat, and though eating it is considered to be safe for most people, consuming it regularly or in high amounts may cause side effects. Although brown seaweed is also available in supplement form, there is a lack of scientific evidence to support the use of brown seaweed supplements.

It's also important to note that supplements are not always tested for safety and are largely unregulated, therefore the content of some products may differ from what is specified on the product label. 

Iodine and Thyroid Disease

Studies have found that levels of iodine vary widely among bladderwrack products. Because of this, if you use bladderwrack as a regular supplement, there's the risk you may receive an overdose of iodine and develop hyperthyroidism.

Overconsumption of iodine may disrupt thyroid health. There are case reports of seaweed, especially bladderwrack, causing both hypothyroidism and hyperthyroidism, presumably due to its iodine content. Seaweed may also interfere with thyroid replacement therapies. Because there are no studies of efficacy, dosing, or safety to support its use and no standardization of iodine content, it is not advised to use seaweed without the supervision of a doctor.

Heavy Metal Toxicity

Another concern about brown seaweed is what's called its "high bioaccumulative capacity." The concentration of heavy metals like arsenic, particularly in the Fucus species, which is one of the most bioabsorbent, can reach toxic levels. The National Institutes of Health warn that, due to possible contamination with heavy metals, consumption of bladderwrack "should always be considered potentially unsafe."

Unpredictable Effects

Other concerns about brown seaweed include that it theoretically might slow blood clotting, which may cause excessive bleeding during and after surgery. Stop taking it at least two weeks before surgery.

If you have diabetes and take medication to lower your blood sugar, adding brown seaweed might make your blood sugar drop too low. Be sure to monitor your blood sugar carefully.

Be aware, too, that preliminary research suggests that brown seaweed might hinder fertility.

Also note that the safety of supplements in pregnant women, nursing mothers, children, and those with medical conditions or who are taking medications has not been established.

Given the potential health risks associated with brown seaweed, it's important to consult your health-care provider before using brown seaweed supplements or consuming brown seaweed on a regular basis.

Brown seaweed powder
Verywell / Anastasia Tretiak

Dosage and Preparation

At this time, there's not enough scientific information to determine an appropriate range of doses for supplements of brown seaweed. Be sure to follow relevant directions on product labels and consult your pharmacist or physician or other healthcare professional before using.

What to Look For

As with any other supplement, always choose one from a reputable manufacturer. If you decide to give brown seaweed supplements a try, find a brand tested and approved by a recognized certifying body such as U.S. Pharmacopeia (USP), NSF International, or ConsumerLab. Doing so can help you get the highest quality and safety possible.

Was this page helpful?
8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Memorial Sloan Kettering Cancer Center. Fucoidan. July 29, 2020.

  2. Skibola CF. The effect of Fucus vesiculosus, an edible brown seaweed, upon menstrual cycle length and hormonal status in three pre-menopausal women: a case report. BMC Complement Altern Med. 2004 Aug 4;4:10. doi: 10.1186/1472-6882-4-10. PMID: 15294021; PMCID: PMC514561.

  3. Skibola CF, Curry JD, VandeVoort C, Conley A, Smith MT. Brown kelp modulates endocrine hormones in female sprague-dawley rats and in human luteinized granulosa cells. J Nutr. 2005 Feb;135(2):296-300. doi: 10.1093/jn/135.2.296. PMID: 15671230.

  4. Chater PI, Wilcox M, Cherry P, Herford A, Mustar S, Wheater H, Brownlee I, Seal C, Pearson J. Inhibitory activity of extracts of Hebridean brown seaweeds on lipase activity. J Appl Phycol. 2016;28:1303-1313. doi: 10.1007/s10811-015-0619-0. Epub 2015 May 26. PMID: 27057089; PMCID: PMC4789227.

  5. Kim JY, Kwon YM, Kim IS, Kim JA, Yu DY, Adhikari B, Lee SS, Choi IS, Cho KK. Effects of the brown seaweed Laminaria japonica supplementation on serum concentrations of IgG, triglycerides, and cholesterol, and intestinal microbiota composition in rats. Front Nutr. 2018 Apr 12;5:23. doi: 10.3389/fnut.2018.00023. PMID: 29707542; PMCID: PMC5906548.

  6. Sharifuddin Y, Chin YX, Lim PE, Phang SM. Potential bioactive compounds from seaweed for diabetes management. Mar Drugs. 2015 Aug 21;13(8):5447-91. doi: 10.3390/md13085447. PMID: 26308010; PMCID: PMC4557030.

  7. Bu T, Liu M, Zheng L, Guo Y, Lin X. α-Glucosidase inhibition and the in vivo hypoglycemic effect of butyl-isobutyl-phthalate derived from the Laminaria japonica rhizoid. Phytother Res. 2010 Nov;24(11):1588-91. doi: 10.1002/ptr.3139. PMID: 21031613.

  8. Haskell-Ramsay CF, Jackson PA, Dodd FL, Forster JS, Bérubé J, Levinton C, Kennedy DO. Acute post-prandial cognitive effects of brown seaweed extract in humans. Nutrients. 2018 Jan 13;10(1):85. doi: 10.3390/nu10010085. PMID: 29342865; PMCID: PMC5793313.

Additional Reading