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Sea Vegetables

Other common name(s): seaweed, sea veg or sea vegg, sealogica, seaweed, algae, red algae, green algae, brown algae, kelp, kombu, bladderwrack, wakami, nori, dulse, and others

Scientific/medical name(s): Fucus vesiculosus, Laminaria digitata, Macrocystis pyrifera, Porphyra tenera, and others


Seaweed is a type of algae that grows in or near the sea. Certain types of seaweed and other algae have been eaten as food for thousands of years. Several companies sell seaweed and other algae as food supplements, either individually or in combinations, and call them sea vegetables. They have some special uses in medicine and as food additives.


Certain types of algae and seaweed have been used as food throughout history. They have some special uses in medicine and as food additives. Generally, edible seaweeds are safe for people who are not allergic.

Despite claims that sea vegetables are super-rich in nutrients that can prevent cancer and help numerous diseases, there is no reliable clinical evidence that this is true. Most seaweed does contain iodine, which is also available in iodized salt. However, amounts of iodine in seaweed vary widely, and getting too much of iodine can cause thyroid and skin problems for some people. Early laboratory and animal studies of seaweed extracts suggest that certain compounds may one day be used in medicine.

How is it promoted for use?

Proponents claim that sea vegetables contain nutrients that "regenerate" the body. They maintain that sea vegetables are concentrated, containing several times more nutrients than land vegetables. Marketers often claim that seaweed will regulate body weight, and that taking seaweed supplements will reduce food cravings. In Infomercials set up to look like interviews have claimed that "degenerative diseases," including tuberculosis, fibromyalgia, cancer, asthma, and diabetes are due to nutritional deficiencies. They claim that these problems can be prevented or helped by nutrients in sea vegetables, which they say offer nutrients that cannot be found elsewhere. For example, some of these proponents say that the iodine added to salt cannot be used by the body, while organic iodine from seaweed is easily absorbed.

What does it involve?

Most sea vegetables are dried after being harvested. If they are to be used as supplements, they are ground up and sold as powders, tablets, or capsules. The marketers of some blends recommend 3 or more capsules per day. Other sellers suggest different amounts of the dried plant in capsule form, or suggest blending the dried powders into soups or other foods. Supplements are sold in health food stores, by phone, and over the Internet. Some are even recommended for pets as both vitamin supplements and immune system boosters.

Dried sea vegetables are also sold in Asian and specialty markets to be prepared as food. Most of these plants must be rehydrated before use. Some kinds of seaweed, such as nori, are common ingredients in sushi.

What is the history behind it?

Algae such as kelp (called kombu in Japan), nori, and wakami have been used in Asian cooking for thousands of years. Many types of seaweed are known in the United States by their Japanese names. Seaweed has been used medicinally in China for centuries to treat liver problems, swelling, phlegm, cysts, and enlarged thyroid glands. (see Traditional Chinese Medicine).

In the 18th century, kelp was discovered as a source of iodine in the diet and used to treat enlarged thyroid. Called goiter, this condition is often caused by not getting enough iodine in food. Iodine was later added to salt to prevent iodine deficiency in the United States, and the rate of goiter declined dramatically.

Several types of algae have been marketed for use as supplements since the early 1980s. At that time, spirulina and other types of blue-green algae that grow in ponds and lakes became popular. Since then, an ever-increasing variety of seaweeds from ocean sources have been sold as dietary supplements in the United States. (see Chlorella).

In conventional medicine, stems from kelp were at one time used to enlarge the cervix for medical procedures, although there are now man-made devices that can be used. Purified compounds from seaweed are approved for use in many types of food. Carrageenan gum, a seaweed extract, is used in foodstuffs to create gels, stabilize mixtures, and thicken liquids. It is a common ingredient in ice cream, jelly, and infant formula. Agar, which is made from red algae, is used to add texture and thickness to foods. It is also a purely vegetable gelatin, unlike the more commonly used type that is made from animal protein. Agar has many uses, from clarifying wine to growing bacteria in the laboratory.

What is the evidence?

Observational studies of people who eat seafood and algae regularly tend to show that they have less breast cancer than those whose diet relies more on meat. Women in Japan, for example, have a lower risk of breast cancer than women in the United States. However, women who move from Japan to the United States are at the same risk as U.S. women within a few generations. This suggests that the difference is not genetic. Even though there are many factors that may be responsible for the differences in risk—such as other foods, exposures, or lifestyle—scientists have focused on foods, such as omega-3 fatty acids in fish, as one possible explanation. The differences between typical Asian and North American diets and lifestyles are so numerous, however, that it is unlikely that seaweed is the only or even the main factor responsible for differences in cancer risk. For example, soy foods and green tea are also being studied to see if they influence cancer risk. There are many other possible explanations yet to be explored.

In laboratory studies and some animal studies, compounds from several types of algae have slowed the growth of cancer cells and caused cancer cells to die, often by a process of natural cell death called apoptosis. However, clinical studies in humans have not been done using seaweed supplements. It is also important to note that extracted compounds are not the same as whole algae, and study results are not likely to show the same effects.

Dried seaweed contains large amounts of some nutrients by weight because the water is removed. The standard nutritional analysis is for 3 ½ ounces (100 grams), which in some cases, would be more than 6 cups of dried seaweed. Actual serving sizes are very small when using seaweed supplements, and nutrient amounts are not guaranteed.

The American Cancer Society's nutrition guidelines recommend eating a balanced diet that includes 5 or more servings a day of vegetables and fruit, choosing whole grains over processed and refined foods, and limiting red meats and animal fats in order to help reduce cancer risk. Eating whole seaweed rather than supplements is a way to include a larger variety of plant-based foods. It is best to choose foods from many kinds of fruits, vegetables and other plant sources such as nuts, seeds, whole grain cereals, and beans. (For more information, see American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Prevention).

Are there any possible problems or complications?

This product is sold as a dietary supplement in the United States. Unlike drugs (which must be tested before being allowed to be sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is written on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

Since most seaweeds have varying amounts of iodine and other nutrients, it is difficult to know how much is in any one supplement. Edible seaweeds are generally safe for those who are not allergic. In some people, however, large amounts of iodine can cause goiter (enlargement of or growths on the thyroid) or other serious health problems. In people with known thyroid disorders, their conditions may be made worse by eating kelp or taking seaweed supplements. Eating a lot of seaweed or getting too much iodine from other sources can cause skin outbreaks that look like acne.

Some seaweed also contains large amounts of sodium, which may worsen high blood pressure or heart failure. Depending on where it is grown, some algae can contain concentrated amounts of heavy metals, including arsenic. These contaminated supplements have caused serious toxic effects in the past. Many types of algae are toxic or even fatal when eaten, so it is important to get supplements from trustworthy sources. Early or mild symptoms of toxicity may include nausea, diarrhea, weakness, numbness and tingling.

A recent analysis of dried seaweed samples purchased in London and over the Internet found that some contained worrisome levels of arsenic.

Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

Additional resources

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-ACS-2345).

Dietary Supplements: What Is Safe?

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management

Complementary and Alternative Methods and Cancer

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer


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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 11/01/2008
Last Revised: 11/01/2008