The health benefits of... seaweed

The health benefits of seaweed

Slimy, slippery seaweed has been heralded as the new, must-have superfood but is it truly healthy or just media hype? Registered Nutritionist Jo Lewin explains all.

What is seaweed?

Seaweed is the term given to a variety of sea vegetables ranging from algae to marine plants.  Despite its trendy, superfood status, seaweed has been used for thousands of years, most notably in Asian cuisine, particularly in Japan, Korea and China.

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There are thought to be over 10,000 species of seaweed, reflecting its immense diversity, both in flavour and nutritional value. The most popular seaweed species are nori, which is dried in sheets and widely used to make sushi. Other common varieties include dulse, arame, wakame, kelp and spirulina.

Seaweed can be enjoyed fresh, dried, cooked or in powder form.

Discover our full range of health benefit guides and check out some of our favourite seaweed recipes, from our rice and quinoa prawn sushi bowl to our miso soup.

Square sheets of dried seaweed

Nutritional benefits

A 5g portion of nori (dried) contains approximately:

7 Kcal / 29 KJ

1.5g Protein

0.1g Fat

3g Fibre

22mg Calcium

0.98mg Iron

0.32mg Zinc

73.5mcg Iodine

1.4mcg Vitamin B12

It’s worth noting that nutritional values vary depending on the species, geographical location, the season of harvest and how the seaweed is stored and processed. For example, red and green varieties have a higher protein content than brown, but the latter is richer in the mineral iodine.  All types contain a supply of minerals, most notably calcium, iron, zinc and iodine.

Top 5 health benefits

1. May support thyroid health

Our thyroid gland is responsible for releasing hormones to help govern our growth, energy, reproduction and repair. In order to do this the thyroid has a need for a number of nutrients, one of which being iodine and another the amino acid tyrosine. Both of these nutrients are found in seaweed, although in variable levels depending on the variety and how it’s stored and prepared.

2. May support heart health

A 2011 review of 100 studies on the benefits of seaweeds, published in the American Journal of Agricultural and Food Chemistry, reported seaweeds may be used to help lower blood pressure and promote heart health. These benefits are in part thanks to high levels of soluble fibre as well as heart-healthy nutrients like omega-3 fatty acids.

3. May support gut health

Seaweed is rich in dietary fibre, making up anywhere from 25-75% of its dry weight. Much of this fibre is in the form of polysaccharides, a type we can’t digest but the bacteria in our gut can, in this way the fibre acts as a prebiotic, a food source for our beneficial gut microbes. By so doing it helps create a favourable environment in the gut promoting the microbes which offer us greater benefits. It’s these gut microbes which, among other things, help shape our immune response through their production of beneficial compounds called short chain fatty acids.

Interestingly, how effective our gut microbes are at breaking down these polysaccharides may depend, in part, on our genes, which is why some cultures are better able to optimise their effects.

4. May support immune function

Some studies suggest compounds in seaweed may reduce viral load and potentially shorten the duration of a cold and minimise the risk of a secondary infection. More recent studies have looked at the potential of seaweed as an aid in our fight against Covid-19 (SARS-COV- 2).

In this regard, several components in seaweed are thought to offer benefits including ACE inhibitory peptides, antioxidants such as fucoxanthin (found in varieties like wakame), the prebiotic fibres as well as contributions of vitamins D and B12.  These components offer protective properties including being anti-inflammatory and anti-viral.

5. May balance blood sugar levels

Studies suggest that including seaweed as part of a healthy, balanced diet may support blood sugar management and potentially reduce the risk of type 2 diabetes. This is thanks, in part, to compounds such as the carotenoid, fucoxanthin, which helps to reduce insulin resistance and supports better blood sugar control. The high fibre content in seaweed also plays its part in helping to slow the speed of digestion.

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Is seaweed safe for everyone?

Being rich in the mineral iodine, makes seaweed especially useful for those following a vegan or largely plant-based diet, however, while the iodine content makes seaweed beneficial for thyroid health, consuming too much may have a detrimental effect. Seaweeds vary in their iodine content with some varieties, like kelp, supplying higher amounts. For this reason, the British Dietetic Association recommends seaweed is not eaten more than once a week, especially during pregnancy.

If you are on blood thinning medication such as warfarin, your GP or dietician may suggest you monitor the vitamin K-rich foods, like seaweed, in your diet to ensure you eat similar amounts consistently. Another nutrient which seaweed is high in is potassium, certain medication works by raising the potassium levels in the blood, if you are prescribed medication of this nature you should take care when eating foods rich in potassium, such as seaweed.

Seaweed can soak up and store minerals in high amounts and as such may accumulate toxic heavy metals like cadmium, mercury, aluminium and lead. If possible, check the source of the seaweed you buy to ensure it is organic and sourced from clean waters.

If you’re on prescription medication, have a relevant medical condition or have concerns speak to your GP for guidance.

Recipe suggestions

Fancy rolling your own sushi? Our step-by-step video guide will show you how, then put your new skills to use in some of our recipes:

Simple sushi
Salmon & cucumber sushi rolls
Smoked salmon & avocado sushi
Japanese-style bento box
Baked new potato pebbles & seaweed mayonnaise
Miso soup

Now, discover the health benefits of more of your favourite (and soon-to-be favourite) foods.


This article was last reviewed on 7th July 2021 by Kerry Torrens.

A registered Nutritional Therapist, Kerry Torrens is a contributing author to a number of nutritional and cookery publications including BBC Good Food. Kerry is a member of the The Royal Society of Medicine, Complementary and Natural Healthcare Council (CNHC), British Association for Applied Nutrition and Nutritional Therapy (BANT).

Jo Lewin is a registered nutritionist (RNutr) with the Association for Nutrition with a specialism in public health. Follow her on Twitter @nutri_jo.

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