Scientific Name(s): Ascophyllum, Chlorophceae (green algae), Chondrus crispus (Irish moss; red algae), Chondrus, Ecklonia, Fucus, Gelidium, Gracilaria, Hijikia fusiformis (hijiki; brown algae), Laminaria japonica (makonbu; brown algae), Laminaria (kombu; brown algae), Palmaria palmate (dulse; red algae), Phaeophyceae (red algae), Porphyra (nori, red algae), Pterocladia, Rhodophyceae (red algae), Saccharina (kombu; brown algae), Ulva fasciata (limu palahalaha; green algae), Undaria pinnatifida (wakame; brown algae)
Common Name(s): Algae, Brown algae, Brown seaweed, Dulse/dillisk, Egg wrack, Green algae, Hijiki, Irish moss/carrageenin, Kelp, Kombu/konbu, Makonbu, Nori, Red algae, Red seaweed, Sea grass, Sea lettuce, Sea spaghetti, Seaweed, Wakame
Medically reviewed by Drugs.com. Last updated on Aug 23, 2021.
Clinical trials are generally lacking to support definitive therapeutic recommendations for seaweeds. However, seaweeds are an important nutritional source of minerals and elements and many are low in sodium. Applications may exist for use in cardiovascular conditions due to potential cholesterol reduction and appetite suppression. Alginates extracted from seaweed have been used in wound dressings.
Clinical trials have used an oral dosage range of 4 to 12 g seaweed daily for up to 2 months.
Contraindications have not been identified.
Information regarding safety and efficacy in pregnancy and lactation is lacking.
Patients taking warfarin and consuming a large quantity of food containing seaweed may experience a change in international normalized ratio (INR) because of seaweed's high vitamin K content.
Contact dermatitis, goiter, and, occasionally, GI effects may occur.
Excessive intake of dried seaweed may result in increased serum thyroid-stimulating hormone (TSH). There have been case reports of carotenodermia (yellowing of the skin) with excessive seaweed consumption.
Seaweeds are marine algae, saltwater-dwelling, simple plants, including red (rhodophyta), brown (phaeophyta), and green (chlorophyta) algae. Most algae have root-like structures called holdfasts that anchor the plant to rocks and other substrates.1 While blue-green species of algae or aquatic plants, such as spirulina (cyanophyta), are actually photosynthetic bacteria. For information on Spirulina, see the individual monograph.
The Japanese term and common name for seaweed, kombu/konbu, should not be confused with kombucha. See Kombucha monograph.
For centuries, seaweed has inspired botanical, industrial, and pharmaceutical interest. Because of the high nutrient content, seaweed has been used as food throughout Asia.
Traditional Chinese medicine used hot water extracts of several types of seaweed in the treatment of cancer. Additionally, the Japanese and Chinese cultures used seaweed to treat goiter and other glandular problems as long ago as 300 BC.
The Romans used seaweed in the treatment of wounds, burns, and rashes.2 The Celts noted that seaweed contracted as it dried and then expanded with moisture. In Scotland during the 18th century, physicians used dried seaweed stem to successfully drain abdominal wall abscesses. They also inserted seaweed into the cervix in an attempt to treat dysmenorrhea. Many reports outline the use of seaweed to induce abortion. Seaweed was employed intravaginally for vaginal atresia and was used urethrally and rectally for strictures.2, 3, 4
Analyses of various seaweeds has identified high carbohydrate levels, as well as minerals, vitamins, and trace elements (eg, calcium, magnesium, iodine, selenium), making them an important nutritional resource, especially when compared with terrestrial sources of trace elements and minerals. The major polysaccharides of seaweed are either structural (contributing to the fiber content) or storage components, including alginates, agars, and carrageenans. The metal salts of alginic acid readily dissolve in cold water to yield viscous solutions. Agars are polysaccharides derived from red seaweed and consist of alternating D- and L-galactopyranose units. Carrageenan, in contrast to agar, is built up from D-galactopyranose units only.1, 5 Beta-glucan has also been extracted from brown seaweed.38
Uses and Pharmacology
Due to the widespread consumption of seaweed as a food source, data from animal studies are largely irrelevant. However, despite the many purported clinical applications for seaweed preparations, only a few, small clinical trials have been conducted by a limited number of researchers.
A clinical study evaluated the effect of undaria seaweed among patients with active and latent herpes simplex infections. Lesion healing rates were observed to be increased in those with active infections and, in vitro, undaria inhibited the virus.6 A review of in vitro studies has been published, describing the effect of sulfated seaweed polysaccharides on herpes simplex and other encapsulated viruses. The polysaccharide may form a complex, blocking the interaction of the virus and cell membranes.7, 8
A nasal spray containing 0.12% iota-carrageenan from red seaweed administered for 7 days to children (mean age, 5 years) with acute rhinitis significantly reduced viral load, time to clearance of disease, and lowered the incidence of secondary infections with other respiratory viruses. The primary end point (total symptom score) for this randomized, double-blind, placebo-controlled study, however, was not found to be significantly different between treatment and placebo groups.31
Numerous components of seaweeds have demonstrated varying degrees of antioxidant capability and carcinogen inhibition in vitro and in vivo, such as sulfated polysaccharides (including fucoidan), phenolic compounds (including phlorotannins and bromophenols), and the carotenoid fucoxanthin. Wakame, mekabu, and kelp were found to inhibit dimethylbenz(a)anthracene (DMBA)-induced breast, liver, and skin cancer in rats. Diets supplemented with powdered seaweeds, including Laminaria and Porphyra spp, significantly reduced the incidence of mammary and intestinal tumors in rats.32
Evidence for a preventive effect of seaweed on breast cancer is derived largely from epidemiological studies. Japanese case-control studies found seaweed to be a protective factor against colon cancer (n = 100), breast cancer (n = 362), and esophageal cancer.32 Data comparing women in Japan with the United States suggest the incidence of breast cancer in both pre- and postmenopausal women is lower in Japan and has been attributed in part to higher consumption of seaweed.9, 10 More specifically, the importance of co-factor foods or food interactions was described in a double-blind, randomized, placebo-controlled crossover trial in 30 healthy postmenopausal American women when the addition of 5 g/day seaweed powder (Alaria esculenta) to a diet containing 100 g/day soy powder, halved the increase in insulin-like growth factor 1 (IGF-1) caused by the soy. High levels of IGF-1 have been associated with increased risk of breast cancer in postmenopausal women.34
Effects on serum and urinary estrogen levels have been demonstrated experimentally in small clinical trials, with a dose-dependent relationship demonstrated with seaweed supplementation. This may possibly be due to modulation of colonic bacteria, resulting in increased urinary excretion of 2-hydroxyestrogen.10, 11 Clinical trials are lacking.
Increased life expectancy is associated with the Japanese Okinawa diet consisting of low salt intake and consumption of soy beans, fish, and seaweed.12 Decreases in serum cholesterol and blood pressure are associated with daily consumption of seaweed either as powder or fiber.5
In a double-blind, crossover study, 62 middle-aged patients with mild hypertension were given a potassium-releasing seaweed preparation. Mean blood pressure was reduced from 112 to 101 mm Hg after ingestion of 12 g/day seaweed.13
In a small study among men and women with metabolic syndrome, decreases in waist circumference (in women only) and blood pressure were demonstrated with daily consumption of 4 to 6 g dried undaria seaweed over 2 months.5
In a crossover study, a seaweed-derived sodium alginate preparation decreased the mean energy intake, leading researchers to consider the contribution of energy intake to obesity.14 Because dietary fiber consumption has been associated with decreased energy intake and weight reduction, a four-way double-blind, placebo-controlled, crossover trial (n = 19) evaluated the effects of a preload low- and high-volume 3% alginate extract beverage (from Laminaria hyperborea and Lessonia trabeculata) on satiety, energy intake, and gastric emptying rate in healthy Danish subjects. Satiety and fullness were significantly increased, while hunger and prospective food intake were significantly decreased with the high-volume alginate treatment versus high-volume control. A significant reduction in energy intake was documented for the low-volume, but not the high-volume, treatment. High-volume alginate preload also resulted in a significant (40%) reduction in glucose response, whereas neither treatment resulted in changes to blood pressure or heart rate.35 In another small trial (n = 10), a reduction in energy intake of 16.4% versus control was significantly lower following consumption of Ascophyllum nodosum 4% enriched wholemeal bread in overweight males.37
A small (n = 23) randomized, double-blind, crossover study reported positive changes in post-load plasma insulin concentrations (7.9% increase), suggesting improvement in insulin sensitivity subsequent to an acute 500 g dose of brown seaweed capsules (A. nodosum and Fucus vesiculosus; at least 10% polyphenols with alginates removed) prior to ingestion of a 50 g carbohydrate test meal. No significant effect was noted on glucose response.36
See individual 33
The polysaccharides alginate and chitin derived from algae are used in dressings and as topical agents for wound management as well as to treat esophageal reflux. They are being evaluated further for use in drug delivery and tissue regeneration systems.15, 16
Kainate, an excitatory amino acid, has been extracted from seaweed. Researchers studying the effect of epilepsy show that animal models suggest this chemical generates seizures in the hippocampus.17
It has been noted that seaweed extracts exert a stimulatory effect on B lymphocytes and macrophages, which may be used clinically for the modulation of immune responses.18, 19 In a randomized, placebo-controlled, double-blind study (n = 70) in elderly Japanese people living in a nursing home, 300 mg/day of mekabu fucoidan (sulfated polysaccharide seaweed extract) given 4 weeks before the 2009-2010 seasonal influenza vaccine demonstrated an immunostimulatory effect. The seaweed extract group had significantly higher B strain antibody titers at 5 weeks that were maintained over 20 weeks. Additionally, natural killer cell activity in the seaweed group was not significantly different at week 9 or 24 compared with baseline; however, it was lower at week 24 versus week 9 in the placebo group.30
As a source of calcium, magnesium, selenium, and other minerals, seaweed extracts have been evaluated in osteoarthritis. Methodological problems have limited the validity of the studies, but there may be potential value in the supplementation.20, 21
Clinical trials have evaluated the following dosages.
Reduction of cholesterol
5 g/day seaweed powder.12
12 g seaweed per day.13
4 to 6 g dried seaweed daily for 2 months.5
Effects on estrogen
5 g/day seaweed for 7 weeks.10
Pregnancy / Lactation
Information regarding safety and efficacy in pregnancy and lactation is lacking.
Effects on serum and urinary estrogen levels have been demonstrated experimentally in small clinical trials, with a dose-dependent relationship demonstrated with seaweed supplementation.10, 11 An increase in menstrual cycle length has also been demonstrated with the brown kelp, Fucus vesiculosus.11
A single case report describes a change in INR in a patient taking warfarin who consumed a large quantity of sushi that contained seaweed. This was thought to be caused by the high vitamin K content of the seaweed.22, 23
Minor gastric adverse events have been reported in some clinical studies, including soft feces and feelings of fullness after 4 to 6 g doses of dried seaweed.10
Seaweed is a rich source of iodine. Increased serum TSH levels have been demonstrated experimentally, and goiters have been reported in Japan where seaweed is widely consumed. The goiters appear to be only cosmetics and elimination of seaweed from the diet generally leads to shrinkage or disappearance of the goiter. Iodine concentrations in seaweed, as well as in dietary supplements, vary widely. In some cases, the United States maximum safe iodine intake of 1,000 mcg/day could be exceeded easily. Whether this would prove harmful is difficult to ascertain because susceptibility to the effects of a high intake of iodine appears to vary among individuals.24, 25
Excessive intake of dried seaweed has been reported to cause carotenodermia (yellowing of the skin). Hypercarotenemia is caused by excessive intake of carotene-rich vegetables or drinks.26 Arsenic is considered a human carcinogen and certain forms have been found in large amounts in seafood and seaweed; however, increases in urinary arsenic excretion have not been demonstrated in human studies.27
Certain blue-green algae found in the Pacific, Indian, and Caribbean oceans have been associated with contact dermatitis. An isolate from the algae, debromoaplysiatoxin, is a very potent inflammatory agent, producing a follicular, papular, and pustular reaction in minute concentrations.28 Two other seaweeds, Gracilaria coronopifolia and Gracilaria tsudai, usually are considered nontoxic, but occasionally may produce poisons associated with GI symptoms when ingested. Extreme cases have resulted in death.29
- Brown algae
- Green algae
- Red algae
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