Seaweed

Seaweed may bring to mind the slippery green, red, and brown plants that curl around your toes when walking in the ocean, or mounds of dried tangles washed up on shore. It may be less recognized as an aquatic food on your dinner plate. But certain types of “seaweed” (the common name for countless species of marine plants and algae) have been consumed globally for centuries and are a staple of East Asian and Pacific cuisines. They are rich in polysaccharides, a type of carbohydrate, that is used by the food industry as a thickener and emulsifier. These polysaccharides also act like dietary fibers and promote gut health as a prebiotic food source for beneficial gut bacteria. Seaweed has been labeled a functional food or nutraceutical because of its disease-preventive components, like polyphenols, carotenoids, and omega-3 fatty acids. Because of this, seaweed or its components are sometimes supplemented to foods to improve their nutritional and antioxidant profile. An example would be adding seaweed polysaccharides to noodles, flour, or biscuits. [1] Yet the evidence from human studies substantiating health claims about seaweed, particularly as a nutraceutical supplement, is still lacking. [2]

As the common name suggests, much of our seaweed, called macroalgae, are marine algae harvested from the ocean. Algae and other aquatic plants (such as water spinach, or Ipomoea aquatica) also grow in rivers, lakes, and other bodies of water. However not all of species of algae are edible, and some may even be toxic.

This page primarily focuses on marine algae, of which there are several thousand types classified into three main groups:

Brown algae, or Phaeophyceae
- Kelp – kombu, wakame
- Fucus – hijiki
Red algae, Rhodophyceae – nori or purple laver, dulse, carrageen
Green algae, or Chlorophyceae – sea grapes, sea lettuce, chlorella

Source Of

Seaweed is low in calories due to its high content of fiber and water, but potentially rich in minerals absorbed from seawater. It also contains an amino acid called glutamic acid, which is converted into glutamate that imparts a rich umami flavor when added to recipes. However, it is important to note the wide variation in nutrient content depending on the species of seaweed, and the location and time of harvest. It may contain:

Fiber, soluble and insoluble
Polysaccharides
Polyunsaturated essential fatty acids (DHA and EPA)
Iodine (Laminaria and other brown seaweed)
Potassium
Iron (Sargassum brown seaweed)
Magnesium (Laminaria japonica brown seaweed)
Sugar alcohol (mannitol, sorbitol)
Phytochemicals (polyphenols, carotenoids)
Sodium varies widely: 100 grams or about 1/4 cup raw provides 9 mg in agar, 48 mg in laver, 233 mg in kelp, 872 mg in wakame

Does seaweed contain vitamin B12?

Nori (purple laver), the dried edible seaweed used to make sushi rolls, is sometimes promoted as a plant source of vitamin B12. [3] It does contain small amounts of active vitamin B12, but the amount varies among types of seaweed, with some containing none. Vitamin content may also vary depending on the region from which it is harvested. Therefore, it is not considered a reliable food source of this vitamin.

Seaweed and Health

Seaweed is not a major source of dietary protein, especially because it tends to be eaten in small quantities, but also the digestibility of the protein in the gut may be low. Interestingly, even among seaweeds that contain less protein, it is a high-quality protein containing all nine essential amino acids. Seaweed is very low in fat but contains small amounts of polyunsaturated fats, particularly omega-3 fatty acids. [4] Seaweed is a good source of various fibers, including polysaccharides, agar, alginate, carrageenan, and cellulose. These fiber types have long been used by the food industry to help improve the texture and stability of processed food products due to their thickening and emulsifying properties. In the human digestive system, they bind to water and reduce speed of digestion, which in the case of most fibers, can help prevent constipation, control blood sugars, and promote satiety and weight loss. [5]

Epidemiological research on associations of dietary seaweed and cancer, mainly from Japan, has not shown any clear associations. [6] A large prospective study of Japanese men and women showed that diets including seaweed were associated with a decreased risk of cardiovascular disease and deaths from any cause. [7]

Research is still in its early stages on the health-promoting properties of seaweed and seaweed components in supplement form, with mostly laboratory and animal studies and small randomized trials. However, larger human trials are needed because it is unclear how well these seaweed components are absorbed and digested in the body.

Proposed benefits include

Weight control. Some research suggests alginate, a fiber in brown seaweed, suppresses hunger and reduces calorie intake. [8,9] Alginate and beta glucan in seaweed may prevent blood sugar surges and control appetite by slowing digestion. [2] The fermentation of seaweed fibers by bacteria in the colon into short-chain fatty acids is also associated with weight regulation.
Prebiotic. Laboratory and animal studies have shown the prebiotic effects of polyphenols and fibers like polysaccharides (fucoidans, alginate, carrageenans) in seaweed. [2,10,11] These fibers are fermented by bacteria to produce short-chain fatty acids that support gut health, protect against pathogens, and promote immunity, as well as support the growth of healthful gut bacterial strains. [11]
Anticancer. Fucoidan is a polysaccharide found in brown seaweed that is sold in supplement form, labeled by the Food and Drug Administration as GRAS (Generally Recognized As Safe). Laboratory studies show that it can slow blood clotting (acting as a blood thinner), lower cholesterol, and may prevent the growth of cancer cells and have immune-boosting effects, but human studies are needed. [1,2]
Antioxidant. Plant chemicals called polyphenols are bound to the cell walls of seaweed, which have been shown in some studies to have anticancer and antioxidant effects by protecting the cells from damage. Animal studies have shown that these seaweed polyphenols may also affect the breakdown and absorption of glucose and fats, causing a reduction in total cholesterol and blood glucose. [2]

Algal oil: An alternative to fish oil

Omega-3 fatty acids are a type of polyunsaturated fat found in some fish and plant foods. They are essential to the body and found in many cell membranes, including the brain and eyes. DHA and EPA are two main forms of omega-3s. Although research shows that food sources of omega-3s are more strongly associated with health benefits than supplements, fish oil supplements are quite popular. A plant-based and environmentally-friendly alternative to fish oil is algal (or algae) oil that also contains omega-3s, mostly DHA. Algal oil is extracted from seaweed and other types of algae. Oily fish like salmon obtain DHA and EPA by eating algae. Some research has shown that the health benefit of algal oil supplements may be comparable to fish oil supplements. [12,13] The unsaturated fats in algal oil supplements are susceptible to oxidation and spoilage, so they should be stored in a cool dark area and used by the expiration date.

Potential adverse effects

Seaweed is sometimes labeled a “superfood,” rich in plant chemicals and minerals, increasing the chance that people may want to consume it daily or even several times a day. Processed seaweed products like seaweed chips are growing in popularity as a low-calorie alternative to potato chips. But there are safety concerns when overeating seaweed, mainly due to a high concentration of trace minerals and heavy metals in certain types:

Iodine. Seaweed, especially kombu, is one of the richest food sources of iodine, a trace element that the body needs in small amounts. The Recommended Dietary Allowance (RDA) for iodine in adults is 150 micrograms daily, and the tolerable upper limit is 1,100 micrograms. One serving of dried nori, about 10 grams of dry weight, contains 232 micrograms of iodine (155% of the RDA). A serving of processed seaweed chips (about 5 grams) contains about 80 micrograms of iodine. In comparison, the average intake of seaweed in Japan is about 10 grams a day. [6]
- Excessive intakes can sometimes lower the production of thyroid hormones and lead to a condition called hypothyroidism. However, because iodine content varies so widely among seaweeds, it is difficult to provide accurate dietary guidance on seaweed intake and thyroid health. Studies have found that regular intake of seaweed (several servings a week) can increase blood levels of thyroid stimulating hormone, which is a risk factor for hypothyroidism. [14] Generally, high iodine intakes do not cause health problems in most healthy people, but certain groups may be more sensitive to excessive iodine and should limit their intake of seaweed products. These include people with existing thyroid disorders, either hypothyroidism or hyperthyroidism (excess production of thyroid hormones), school-aged children, and infants. [15]
Heavy metals. Both fresh and dried seaweed may contain heavy metals such as arsenic, mercury, cadmium, or lead, depending on the habitat from which they were sourced. [16] Seaweed absorbs these metals when growing in contaminated areas, such as from industry or poor sewage systems. Consuming these seaweeds in small or infrequent amounts carries low risk to human health, but a regular or high intake may increase risk.
- Some seaweeds contain inorganic arsenic, which is considered toxic and poses health risks; it is associated with an increased risk of cardiovascular disease, diabetes, and neurodegenerative disorders, and an increased incidence of lung, skin, and bladder cancers. [2,6] However, the type of arsenic in most seaweeds is called arsenosugars, which is not the inorganic type though its health effects are unclear. A few seaweed types contain high amounts of inorganic arsenic like Laminaria digitate (a tough inedible brown species) and hijiki. Hijiki is commonly sold in Japanese restaurants and supermarkets as an ingredient in soups, salads, and appetizers. There is no documented evidence of health problems from arsenic intake in hijiki; however, health organizations from some countries including the United Kingdom and Australia have recommended avoiding hijiki. [17,18] It is also included on California’s Proposition 65 list of foods that contain chemicals found to cause cancer or birth defects. If hijiki is a concern to you, check the ingredients list on food labels of seaweed products (or contact the manufacturer if the type is not specified) and ask restaurants if hijiki is used in their seaweed dishes. Low-arsenic seaweed types that are safe to eat include wakame, kombu, and nori.
- Seaweed supplements may also contain these heavy metals, as they are not reviewed by the U.S. Food and Drug Administration for safety or effectiveness before they are sold to consumers. The type and location of harvest of seaweed extracts used in the supplements is not required information to be disclosed by supplement companies, nor is it known if these industries conduct thorough analysis of their nutrient and metal concentrations. [5]
Other effects. Other potential side effects when eating seaweed include a laxative effect or irritation of the digestive tract in sensitive individuals. This is more commonly seen when people eat seaweed in large amounts or very frequently.

Is carrageenan safe to eat?

Carrageenan is a fiber in the cell walls of seaweed that has gelling properties, which makes it a common additive to foods to thicken and stabilize texture. It is used in protein foods like milk, pudding, and other dairy foods as well as deli and processed meats. It is also used as a vegan alternative to gelatin. Carrageenan has been shown in some animal and laboratory studies to lower cholesterol and act as a blood thinner, as well as have antiviral and anticancer activity. [1] It has been approved by the Food and Drug Administration (FDA) as a food additive. However, certain types of processed carrageenan have been shown in some animal and cell studies to cause inflammation and digestive problems such as bloating and even colon cancer. [1] This type is called poligeenan or degraded carrageenan, in which it is processed with acids. It is not approved by the FDA, is believed to be proinflammatory, and is classified as a 2B substance (possible human carcinogen) by the International Agency for Research in Cancer. [19] To contrast, the type of carrageenan used in food products is processed with alkaline substances and is not associated with these negative side effects. However, some researchers believe that even this type may not be safe, as it is digested with stomach acid and therefore may be degraded similarly to poligeenan. There is much debate, but overall the research in humans has been limited with conflicting results so that the long-term safety of carrageenan is unclear. [20] People with digestive issues such as inflammatory bowel disease who may be more sensitive to potential side effects from carrageenan may keep an eye out for carrageenan as an additive on food labels. Those who regularly take blood-thinning medications such as aspirin, ibuprofen, heparin, or warfarin might also choose to avoid eating carrageenan, as it may increase the risk of excess bleeding.

Purchase

Various types of seaweed are sold dried in Asian food markets and online. Nori (used in sushi) is the most recognized type in the U.S. and is available in many supermarkets.

Fresh seaweed from oceans, rivers, and lakes can be polluted with heavy metals or other contaminants in the water that are not necessarily removed when washing or cooking the seaweed. Therefore, if you are looking to harvest your own fresh seaweed, it is advisable to research the water source and learn about any local water quality testing guidelines.

Storage

Keep dried seaweed in an airtight container in a dark dry spot. It can last for years and may still taste good after the “best buy” date, but exposure to light, humidity, and oxygen can reduce nutrient content, texture, and flavor. Do not store dried seaweed in the refrigerator because of the higher humidity. It can last many months in a freezer if packed in an airtight container.

Make

Dried seaweed can be softened by soaking in warm water for a few minutes. Seaweed does not need to be cooked before eating. It can be added to salads or hot dishes like soups and casseroles. Many seaweed types are a source of umami flavor from its high glutamate content. It is often added to soups (such as dashi broths), stews, and sauces to create a richer, deeper flavor. The following are common types and uses:

Nori – Roasted, dried, and pressed into sheets. Eaten dry, it is used to wrap sushi rolls, added to stir-frys, or crumbled into soups. It is also popular as seaweed chips or snacks.

Hijiki – Dried into small twigs, it is added to stir-fries and stews. It may also be rehydrated in water and added to salads or eaten on its own. Hijiki may contain high levels of inorganic arsenic, so low to moderate consumption is advised.

Wakame – This type is rehydrated into a bright green color, often used in seaweed salads and miso soups.

Dulse – Has a distinctive reddish color. Sold as dried leaves, flakes, or a powder. It can be crisped by toasting in a pan or frying in oil, creating a taste that mimics bacon. It rehydrates quickly in water and may be added to soups or noodle dishes. The powdered form is used as a seasoning for soups, meats, and fish.

Kelp – Dried and sold as flakes, this can be sprinkled into cold or hot dishes as a seasoning. Kelp is also sold raw as low-carbohydrate “noodles.”

Kombu – A type of kelp known for its salty umami flavors that is used to make dashi, an Asian soup base. It is sold dried or pickled. Kombu, like most kelp, may contain very high amounts of iodine, so should be eaten in moderation.

Arame – The long brown strands are rehydrated and eaten as is or added to salads and stir-fried meals.

Did You Know?

Most edible seaweed today is obtained through seaweed farming, which uses saltwater tanks or farming plots in the sea, rather than harvesting wild from the ocean.

References

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Last reviewed August 2023

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