Seaweeds are a rich nutritional food that have a wide array of health benefits.

Despite being considered the plant foods of the future, seaweeds are not true plants — they are actually primitive multicellular marine macroalgae. Seaweeds live in seawater attached to rocks or hard ocean floors in coastal areas. Macroalgae, or seaweeds, belong to three different major groups, with unknown thousands of species: brown algae (Phaeophyta), red algae (Rhodophyta) and green algae (Chlorophyta). The red and brown seaweeds live exclusively in seawater, while the green algae are also common in freshwater rivers and lakes and can even live out of water attached to damp rocks, houses, walls and tree bark.

The term seaweed lacks a formal definition and the different types also lack a common ancestor. Some of the blue-green algae, or Cyanobacteria, are also considered seaweeds.

Seaweeds are very ancient organisms, and being very rich in vitamins and other nutrients they have been used by humans for millennia — as food, medicines, fertiliser and today for many industrial products such as carrageenan, agar and other gelatinous substances.

Seaweeds absorb water and nutrients, and unlike terrestrial plants can photosynthesise in all their tissues, whereas land plants do so only in their leaves. They have simple structures without the complex organs such as leaves or roots of land plants. There are three environmental requirements for seaweed ecology: they need seawater or brackish water, enough light to support photosynthesis and a solid surface for attachment. They therefore most commonly inhabit waters near the shoreline, known as littoral areas, and are more likely to be found on rocks than sand or shingle. Some live very close to the surface, only being wet by sea spray, and others can live several metres deep, the deepest living ones being the red algae.

Active ingredients

Seaweeds are a rich source of novel complex polysaccharides, polyphenols, fatty acids (while being low in fats) and carotenoids, along with a high mineral and vitamin content. The novel complex polysaccharides have also been shown to be prebiotic, fermentable in the gut to improve gut microbial populations and therefore overall health.

Studies in Japan have shown that eating 5.3 grams of seaweed per day confers a decreased incidence of chronic disease.

The vitamin and mineral content includes vitamins A, C, E and the B group in higher concentrations than land plants. With their ability to bioaccumulate minerals such as sodium, potassium, calcium, magnesium and the trace elements zinc, manganese and copper, seaweeds can have a mineral content 10 to 100 times higher than that of land vegetables. Interestingly, even with their high sodium content, the ratio of sodium and potassium is significantly skewed to higher potassium, thus supporting cardiovascular health and lowering high blood pressure. The mineral content is higher in the red and brown algae, which are stronger tasting, than in the green.

Seaweeds as food

Seaweed is consumed as food across the world, particularly in Asian countries. Studies in Japan have shown that eating 5.3 grams of seaweed per day confers a decreased incidence of chronic disease, protecting against cardiometabolic and inflammatory risk factors. Seaweeds have been a staple food item in Japan, Korea and China since prehistoric times. Up to 21 species are used in everyday cookery in Japan, and account for more than 10 per cent of the Japanese diet.

Medicinal uses

Immune system

Sulphated polysaccharides from red and green algae inhibit DNA- and RNA-enveloped viruses, including HIV, HSV and influenza-A, working through various mechanisms. They also show antibacterial activity against H. pylori, E. coli and staphylococcus.

Digestion and weight control

Seaweed extracts are used in some diet pills, expanding in the stomach to make the person feel less hungry. Improving thyroid function can also assist in weight management.

Anti-inflammatory

The polysaccharides in seaweeds interfere with the migration of leukocytes to the sites of inflammation and have anti-inflammatory and immunomodulatory properties. They also inhibit the COX-2 pathways of inflammation.

Blood sugar regulation

Research has shown that seaweeds improve lipid status, reduce inflammatory effects, attenuate weight gain and regulate blood glucose, thereby reducing the risk of metabolic syndrome and diabetes.

Anticancer

Cancer incidence is much lower in populations who consume a seaweed-rich diet. Research is showing that seaweeds may confer protection against breast cancer (possibly due to the iodine content), lung cancer and pancreatic cancer.

Cardiovascular health

Seaweeds have antioxidant, anti-angiogenic, anticoagulant and anti-adhesive properties, and are good sources of the anti-inflammatory long-chain omega-3 fatty acids, thus playing a significant role in the prevention of cardiovascular disease and strokes.

Thyroid support

The high iodine and mineral content of seaweeds supports thyroid function, particularly when a person is iodine deficient — hypothyroidism and goitres are a common problem worldwide. So consuming seaweeds as food can be important for thyroid health. However, there are some precautions with this approach as the daily intake of more than 600µg of iodine can cause symptoms of an overdose. Excess iodine can adversely affect thyroid function, causing a suppression of thyroid function, or conversely triggering hyperthyroidism in susceptible people.

Cauions

The great ability of seaweeds to bioaccumulate minerals from seawater can be beneficial or risky for human health, particularly when they also bioaccumulate toxic metals. To use seaweeds as either a food or a medicine it is therefore essential to obtain them from an unpolluted source.