Seaweed Benefits and Nutrition: What a Portion Actually Gives You

By Jayson Byles, forager & chef · September 2026

Seaweed is a low-calorie food that gives you fibre, minerals, some protein and, above all, iodine: more of it, weight for weight, than anything else you can eat. What a portion actually gives you depends on the species and on how much you use, and that is the part most articles skip. A teaspoon of dried dulse and a teaspoon of dried kelp are not the same food.

We harvest and cook nine species on the Fife shore, so we are asked this every week: is seaweed good for you, can you eat it every day, which one should I use. This page answers with numbers from the NHS, the British Dietetic Association and the peer-reviewed literature, and with how we use each plant in the kitchen. It is the nutrition spoke of the other uses of seaweed; the sister pages cover kelp and bladderwrack supplements and how Scotland came to burn the stuff. We sell seaweed as food and as raw material, not as medicine or as a supplement, and nothing here is medical advice.

What is in seaweed

Start with the unit. Nutrition tables quote seaweed per 100 g of dry weight, and nobody eats 100 g of dried seaweed. A realistic portion is about 5 g dried, a heaped tablespoon of flakes. Divide the table by twenty and the picture changes.

Protein. Brown seaweeds run at 5 to 20 per cent protein by dry weight, greens 3 to 30 per cent and reds anywhere up to 45 per cent, with dulse usually quoted at 12 to 14 per cent (Cherry and colleagues, Nutrition Reviews, 2019). A 2024 analysis of Irish dulse found 25.8 per cent, knotted wrack 5 per cent and carrageen 9.2 per cent (Life, 2024). So a 5 g portion of dulse gives you around a gram of protein.

Fibre. This is where a small portion counts. The 2019 review calculates that 5 g of dried brown, red or green seaweed can supply up to 14, 11 or 12 per cent of a day's fibre. Much of it is the soluble kind, alginate in the browns and carrageenan in carrageen, which is why seaweed thickens a stew.

Minerals. The 2024 Life paper measured dulse at 1,080 mg of calcium, 18 mg of iron and 2,170 mg of potassium per 100 g dry weight, and carrageen at 5,190 mg of calcium and 61 mg of iron. Per 5 g portion that is roughly 54 mg of calcium from dulse, 260 mg from carrageen, and about a milligram of iron from dulse. Seaweed is salty, but the same arithmetic applies: knotted wrack at 3,730 mg of sodium per 100 g works out at under 200 mg in a portion, less than half a gram of salt; dulse gives about 50 mg. Fat is minimal, though what there is includes the omega-3 EPA, 11 per cent of dulse's fatty acids in the same study.

Vitamin B12. Handle with care. Laver (nori) really does carry active B12, at 32 to 134 µg per 100 g dry weight (Cherry, 2019), and in 2024 a randomised trial in 30 vegetarians found that 5 g of roasted nori a day for four weeks improved four blood markers of B12 status (Huang and colleagues, European Journal of Nutrition, 2024). That is one small trial, and it concerns laver, which we do not supply. For dulse, kelp and the wracks the B12 evidence is not there.

Iodine: the number that matters

Everything else is a footnote to iodine. Adults need 140 µg a day, according to the NHS, which adds that taking high doses for long periods "could change the way your thyroid gland works" and that 0.5 mg a day or less from supplements is unlikely to cause harm. The European Scientific Committee on Food set a tolerable upper level of 600 µg a day for adults, and European guidance flags dried seaweed products above 20 mg of iodine per kilogram as capable of causing "dangerously excessive" intakes (Commission Recommendation 2018/464). Most seaweed is above 20 mg per kilogram. The kelps are above it by a hundredfold and more.

The table shows what that means for a 5 g dried portion of the species we cut, using published measurements rather than our own. Content varies with season, shore and the part of the frond, which is why the ranges are wide; testing and a certificate of analysis are available on request for anything we supply.

Three things follow. First, the kelps are a condiment. Sugar kelp and oarweed go into stock, a strip in the pot and out again, or as a pinch of flakes in seaweed salt; a gram is plenty. Second, water takes iodine out. Nitschke and Stengel soaked and boiled Irish seaweeds and found dabberlocks lost 62 per cent of its iodine to soaking alone, dulse 15 per cent and gutweed 10 per cent; after rehydration and twenty minutes' boiling all three still held 66 to 165 µg per gram (Journal of Applied Phycology, 2016). Discard kelp's soaking water and most of the iodine goes with it, which for kelp is the point. Third, the reds and greens, dulse, sea lettuce and gutweed, pepper dulse and carrageen, are the ones to eat by the handful, and even they reach the upper level at a full dried portion on a rich day.

Is it OK to eat seaweed every day?

Red and green seaweeds in normal culinary amounts, most days, are fine for most people. Brown seaweed is different. The British Dietetic Association's iodine fact sheet says plainly: "It is not recommended to eat brown seaweed or kelp more than once a week, especially during pregnancy," adds that seaweed "is not a reliable source of iodine" because the content varies so much, and says kelp supplements should not be used as an iodine source (BDA, November 2022).

The UK Committee on Toxicity looked at the same question for women before and during pregnancy and while breastfeeding. Its 2022 statement estimated mean iodine exposure from a serving of seaweed at anywhere from 95 µg for sea lettuce to 11,512 µg for sugar kelp or kombu, and concluded that "high consumers of seaweed may be exposed to levels of iodine that could pose a toxicological risk to maternal health" (COT statement 02/22, August 2022). If you are pregnant, planning to be, or feeding a baby, the kelps are the ones to leave out, and infants should not be given them at all.

Anyone with a thyroid condition, or taking medicine for one, should talk to their GP before making seaweed a habit. The iodine numbers above are why.

What we are allowed to say, and what we are not

In Great Britain the only health wording that may be attached to a food is on the GB Nutrition and Health Claims Register. For iodine the authorised claims are that it contributes to normal thyroid function, the normal production of thyroid hormones, normal cognitive function, normal energy-yielding metabolism, normal skin and the normal functioning of the nervous system, and they may be used only where the food is a source of iodine, at least 15 per cent of the reference intake per 100 g. Every species in the table qualifies many times over; the kelps qualify so hard that the sensible message is restraint. There are no authorised claims for detoxing, immunity, weight loss or treating any condition, and a food sold with those words is being sold unlawfully. Extracts are a separate matter: fucoidan from Undaria, for example, is an authorised novel food only up to 250 mg a day (FSA register), and the supplements page deals with that world.

What the research says now

• The UK market has outrun its labelling (2023). Martha Redway and Emilie Combet at the University of Glasgow surveyed 37 to 40 UK retailers in three waves between 2018 and 2021 and found 2.3 times as many seaweed foods as in 2015, a median iodine dose per serving above 400 µg in every year, iodine labelling still uncommon, and some products, supplements especially, above the tolerable upper level in a single dose (British Food Journal, 2023). Evidence level: a market survey with modelled intakes, not a trial.
• Europe's monitoring data (2023). EFSA pooled 1,002 iodine analyses and several thousand metal analyses from 2011 to 2021. Dried kombu averaged 3,529 mg of iodine per kilogram, dulse 249, sea lettuce 348 and laver 85; consumers of kombu and laver were estimated above 20 µg of iodine per kilogram of body weight a day; and seaweed supplied 10 to 30 per cent of dietary inorganic arsenic and lead among people who eat it, with hijiki singled out (EFSA Journal, 2023). Evidence level: exposure assessment from official monitoring. Hijiki is not a Scottish species and we do not supply it.
• Stopping weekly seaweed lowered TSH (2026). A Norwegian team followed 49 healthy adults who ate seaweed every week. Their estimated iodine intake fell from a median of 658 µg a day to 189 µg when they stopped, and median TSH fell from 1.4 to 1.1 mIU/L, with the biggest drop in the heaviest eaters and no change in free T3 or T4 (European Journal of Nutrition, 2026). Evidence level: a small non-randomised before-and-after study; the authors call for a proper randomised trial.
• Nori as a B12 source (2024). The 30-person randomised trial above: 5 g of roasted nori a day for four weeks improved four B12 markers; 8 g a day did no better (European Journal of Nutrition, 2024). Evidence level: one small human trial, one species.

How we eat it on the Fife shore

People here have eaten seaweed for a very long time: dulse since the monks of Columba's era, according to Slow Food's Ark of Taste, and the kelp we are so careful with today was once burned for glass and iodine on these shores, a story told on our history page. What the kelpers never did was eat the tangle by the bowlful, and neither do we.

In practice: dulse and pepper dulse are eaten fresh or dried as they come, in a salad, worked into seaweed butter or milled into seaweed salt with sea lettuce; sea spaghetti is blanched and dressed like a vegetable; carrageen sets a pudding; dabberlocks goes into a pickle; a whole frond wraps a potato in the coals; and the kelps season stock and a cup of tea, a pinch at a time, dried and stored for the year. To see the plants in the water and taste them cooked the same afternoon, that is what a workshop is for; the complete guide to seaweed foraging in Scotland is the reading to do first.

Sources

• Iodine: vitamins and minerals (NHS)
• Iodine food fact sheet (British Dietetic Association, November 2022)
• Statement on the potential effects that excess iodine intake may have during preconception, pregnancy and lactation, COT 02/22 (Committee on Toxicity, 2022)
• Commission Recommendation (EU) 2018/464 on the monitoring of metals and iodine in seaweed (Official Journal, 2018)
• Great Britain nutrition and health claims register (Department of Health and Social Care)
• Fucoidan extract from Undaria pinnatifida, novel food authorisation NOVEL-64 (Food Standards Agency)
• Risks and benefits of consuming edible seaweeds (Cherry et al., Nutrition Reviews, 2019)
• Seaweed nutritional value and bioactive properties: insights from Ascophyllum nodosum, Palmaria palmata and Chondrus crispus (Life, 2024)
• Quantification of iodine loss in edible Irish seaweeds during processing (Nitschke and Stengel, Journal of Applied Phycology, 2016)
• Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European population (EFSA Journal, 2023)
• Seaweed as food: survey of the UK market and appraisal of opportunities and risks in the context of iodine nutrition (Redway and Combet, British Food Journal, 2023)
• Impact of habitual seaweed consumption on iodine nutrition and thyroid function: a non-randomized pre-post clinical study (Aakre et al., European Journal of Nutrition, 2026)
• Effect of roasted purple laver (nori) on vitamin B12 nutritional status of vegetarians: a dose-response trial (Huang et al., European Journal of Nutrition, 2024)
• Dulse, Ark of Taste (Slow Food UK)

Questions we get asked

Is seaweed good for you?

In small portions, yes. A 5 g dried portion gives useful fibre, minerals such as calcium, iron and potassium, a little protein and a large amount of iodine, which contributes to normal thyroid function. The kelps carry so much iodine that they belong as a seasoning; dulse, sea lettuce and carrageen can be eaten more freely.

Is it OK to eat seaweed every day?

Red and green seaweeds in normal culinary amounts are fine most days for most people. The British Dietetic Association advises against eating brown seaweed or kelp more than once a week, especially in pregnancy, because a single portion can carry many times the 600 µg adult upper level for iodine. Anyone with a thyroid condition should ask their GP first.

How much iodine is in a portion of seaweed?

It depends entirely on species. Published measurements put a 5 g dried portion of oarweed at around 31,000 µg, dabberlocks at about 3,350 µg, dulse and sea lettuce at roughly 500 to 1,700 µg and laver at about 425 µg, against an adult need of 140 µg a day. Soaking and boiling remove a large share, most of all from the kelps.

Which seaweed has the most protein?

Red seaweeds generally. Dulse is usually quoted at 12 to 14 per cent protein by dry weight and one 2024 analysis found 25.8 per cent, while knotted wrack came in at 5 per cent and carrageen at 9 per cent. Because a portion is only about 5 g dried, that is roughly a gram of protein from dulse, so treat it as a bonus rather than a source.

Can I eat seaweed when pregnant?

Small amounts of red and green seaweed are generally considered fine, but the UK Committee on Toxicity concluded in 2022 that high seaweed consumers could take in enough iodine to pose a risk to maternal health, and the BDA advises against brown seaweed or kelp more than once a week, especially in pregnancy. Leave the kelps out, and never give them to infants.

This page is about seaweed as food. It is not medical advice, and we do not sell supplements or make medicinal claims. If you have a thyroid condition, are pregnant or breastfeeding, or take iodine in any form, talk to your GP or a registered dietitian before making seaweed a regular habit.