The category of seaweed, or ocean vegetables, includes edible seaweed or algae such as dulse, kelp (including arame, kombu, and wakame), nori, and sea lettuce (Ulva lactuca and various Monostroma). Generally speaking, seaweed is a good dietary source of calcium, iodine, and fiber.
Some varieties are also good sources of vitamin A, vitamin B12, vitamin C, vitamin E, vitamin K, manganese, magnesium, selenium, copper, or zinc. Various seaweeds have been shown to have anti-inflammatory, antioxidant, anticoagulant, and antibiotic properties.
Seaweed can be a rich source of carotenoids. For example, brown seaweeds contain beta-carotene, fucoxanthin, and violaxanthin. Some algae are a source of astaxanthin. In addition, seaweed contains some unique compounds, such as various fucoidans (found in brown seaweeds), stearidonic acid (brown seaweeds), phlorotannins (kelp), mycosporine-like amino acids and phenolic acid (dulse), all of which have been shown to have anticancer properties in the laboratory. Seaweed is also a good dietary source of the lignan enterolactone. Various extracts of seaweed have been shown to inhibit the growth of colon and breast cancer cells in the laboratory.

Breast cancer-related effects of consuming seaweed

The evidence that seaweed consumption could reduce the risk of breast cancer is based primarily on studies of brown seaweeds:
  • Consumption of brown seaweeds has been shown to favorably alter estrogen metabolism in women. Extract of common kelp has been shown to inhibit the binding of estradiol to estrogen receptors and progesterone to the progesterone receptor. Bladderwrack (another type of edible kelp) has been found to exert anti-estrogenic effects in pre-menopausal women
  • Specific unique components (fucoidan, fucoxanthin, alginic acid, laminarin) of brown seaweeds have been shown to inhibit cancer proliferation, including triple negative (ER-/PR-/HER2-) cell proliferation. However, red seaweeds, which lack these compounds, also inhibit proliferation, suggesting that other compounds found in seaweed may also be important
  • Seaweed in the Asian diet may enhance intestinal conversion of phytoestrogens, in particular the production of equol, which could explain some of the breast cancer protective effects of dietary seaweed and soy
  • The brown seaweed wakame (a type of kelp) has been shown to contain the omega-3 fats eicosapentaenoic acid and stearidonic acid, which may favorably increase the omega-3/omega-6 polyunsaturated fatty acid ratio (which in turn is thought to reduce breast cancer risk). Some other types of seaweed have been found to contain docosahexaenoic acid, another marine omega-3 fat
  • Wakame was also found to reduce breast cancer proliferation in rats in one Japanese study
  • Mekabu (wakame root) extract has been shown to induce cell death in human hormone receptor negative (ER-/PR-) breast cancer cells and to suppress mammary carcinogenesis in rats when administered in daily drinking water, without toxicity
  • Various types of red, brown and green algae have been found to be good sources of melatonin. Melatonin protects against breast cancer in several ways, including by reducing aromatase activity within the breast, thereby decreasing estrogen production
  • The relatively high iodine content of seaweed may contribute to reduction in risk of breast cancer
  • The relatively high levels of calcium found in seaweed are also thought to contribute to seaweed's chemopreventive effects, in combination with its other components.
Postmenopausal women with breast cancer and a high intake of enterolactone have been found to be less likely to die from their breast cancer than those with a low intake. Enterolactone has also been found to increase the sensitivity of breast cancer cells to radiation, thereby potentially enhancing the treatment effects of radiotherapy.
We suggest consuming seaweed as food rather than fucoidan, kelp or other seaweed supplements since (1) as outlined above, various components of seaweed in addition to fucoidan or iodine may be responsible for seaweed's apparent anti-carcinogenic effects; (2) the compounds, minerals and fiber found in seaweed may act synergistically to reduce cancer risk; (3) safe and effective levels of fucoidan have not been established; (4) supplementation may result in increased blood thinning, depending on the formulation.

Additional comments

Most seaweed is dried after being harvested and must be rehydrated before use. Nori is a common seaweed ingredient in sushi. Currently, in the U.S., seaweeds are used mainly as a source of gums, gelling compounds and thickeners (such as agar) for use in foods such as ice cream, and some industrial applications.
Seaweed readily incorporates minerals from the surrounding water; minerals can account for more than one-third of seaweed dry matter. While this can make various seaweeds a rich source of beneficial micronutrients, it can also make them a dietary source of heavy metals, including arsenic. One analysis of dried seaweed samples purchased in London and over the Internet found that some contained unacceptable levels of arsenic. On the other hand, one study found that wakame is effective in preventing the absorption and reabsorption of dioxin from the gastrointestinal tract and therefore might be useful in treatment of humans exposed to dioxin.
Several types of freshwater microalgae, including spirulina and other types of blue-green algae, have been marketed as supplements in the U.S. since the early 1980s. Blue-green algae contains phycocyanin and alpha-linolenic acid, both of which have been shown to have chemopreventive activities. However, such freshwater algae can also contain concentrated amounts of heavy metals, as well as toxic microcystins, depending on where it is grown.
Whether or not they contain heavy metals, many microalgaes are not edible. In fact, they may be harmful if consumed, and there have been instances of microalgae supplements sold that turned out to be toxic. Therefore, consumers of such supplements should assure themselves of the reputability of the manufacturer.
Since seaweeds have varying amounts of iodine and other nutrients, it can be difficult to know how much is in any given supplement. People with thyroid disorders may find that their conditions are made worse by eating kelp or taking seaweed supplements. High intake of iodine may result in elevated blood pressure and blood sugar. Some seaweed also contains large amounts of sodium, which can also worsen high blood pressure for those susceptible.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning seaweed and its components. For a more complete list, including less recent studies, please click on seaweed.