Which seaweed suits you?

What is seaweed?

Seaweed is a collective name for various edible macroalgae — brown, green and red seaweeds. The main types in food and supplements are kelp/kombu (Laminaria/Saccharina), wakame (Undaria pinnatifida), nori (Pyropia/Porphyra), dulse (Palmaria palmata), arame (Eisenia bicyclis) and Irish moss (Chondrus crispus).

In Asian food traditions — especially in Japan, Korea and China — seaweed has been an everyday ingredient for thousands of years. Sushi rice rolled in nori, kombu in dashi soup, wakame in miso soup. On the Atlantic coasts of Ireland and Brittany, people know Irish moss ('sea moss'), a red seaweed traditionally used in puddings and drinks. For Western cuisine, seaweed is a relatively recent discovery, but growth is strong — both as a culinary ingredient and as a supplement category.

What sets seaweed apart nutritionally is its combination of iodine and minerals. It is by far the most concentrated natural source of iodine — kelp can contain up to 8,000 µg of iodine per gram of dry weight, while the RI is 150 µg/day. That makes seaweed intake a double-edged matter: a rich food source, but also an easy way to exceed the UL (upper limit) — especially with kelp and kombu.

For those considering seaweed, there are three points to watch. First: iodine content varies greatly by type. Second: avoid hijiki because of its arsenic content (advised against by the NVWA (Dutch food safety authority) and the UK Food Standards Agency). Third: B12 for vegans is complicated — many seaweeds contain pseudo-B12 that the human body does not recognise as active B12. Also take a look at our chlorella category and spirulina category, where the B12 aspect also plays a role.

Science, minerals and EFSA status

The minerals in seaweed each have their own authorised EFSA claims, applicable to seaweed supplements that meet the threshold for the mineral in question. For iodine: 'Iodine contributes to the normal production of thyroid hormones and normal thyroid function' and 'Iodine contributes to normal energy-yielding metabolism'.

Research into seaweed also focuses on antioxidant activity (due to fucoxanthin in brown seaweeds), gut health (alginates and carrageenan as prebiotic fibres) and cardiovascular research. Clinical evidence for specific health effects is predominantly small-scale.

We make no literal claims about effects specific to seaweed. With Hashimoto's, during pregnancy or when using thyroid medication: consult a doctor beforehand. Avoid hijiki because of the arsenic problem.

The main seaweeds — what are the differences?

Kelp / Kombu (Laminaria, Saccharina): Brown seaweed, highest iodine content. Classic in Japanese dashi soup. In supplements often standardised to iodine content.

Irish moss / Sea Moss (Chondrus crispus): Red seaweed from the Atlantic coast. A source of carrageenan (a gel-forming fibre), potassium, magnesium and calcium; lower iodine content than kelp. In supplements as capsules or gel.

Wakame (Undaria pinnatifida): Brown seaweed, mild in taste. Classic in miso soup. Lower in iodine than kelp, rich in minerals.

Nori (Pyropia/Porphyra): Red seaweed (dried, black-green). Used for sushi rolls. Some types contain genuine B12.

Dulse (Palmaria palmata): Red seaweed, soft taste. A good source of iron, moderate iodine.

Arame (Eisenia bicyclis): Brown seaweed, mild sweet taste. Rich in minerals.

Hijiki (Sargassum fusiforme): CONTAINS HIGH LEVELS OF ARSENIC. The UK Food Standards Agency and the NVWA advise against it. We do not stock hijiki.

The seaweed forms at Plenthera

Organic Sea Kelp capsules (KiKi Health)

Organic kelp in 500 mg vegetarian capsules. Kelp is the most concentrated natural source of iodine; the iodine content per capsule is stated on the label. For those who want iodine from a wholefood source instead of potassium iodide — dose consciously and stay below the UL of 600 µg/day.

Organic Irish Sea Moss capsules (KiKi Health)

Organic Irish moss (Chondrus crispus) in packs of 90 capsules, also as a value pack of 3×90. A red seaweed with a broad mineral profile and carrageenan fibres, with a lower iodine content than kelp. A practical alternative for those who do not want to prepare sea moss gel.

How do you use seaweed?

A simple basic approach to seaweed:

1. Decide on your goal: iodine supplementation (kelp), broad mineral supplementation (Irish moss), or culinary use (wakame, nori, arame from an Asian grocer or supermarket).
2. For kelp capsules: follow the label and check the iodine content per capsule — stay below the EFSA UL (600 µg/day), also if you eat seaweed as well.
3. For culinary use: wakame in miso soup (5-10 g fresh), nori for sushi (1-2 sheets per meal), arame in salads.
4. Avoid hijiki — advised against by government authorities (arsenic).
5. With Hashimoto's, thyroid medication or during pregnancy: consult a doctor before a high seaweed intake.
6. B12 for vegans: do NOT use seaweed as your only source of B12 — choose a methylcobalamin supplement.
7. Store capsules and dried seaweed in a dry place, away from light.

Myths and misconceptions about seaweed

Myth 1: 'All seaweeds are a good vegan source of B12.'

Incorrect. Many seaweeds contain pseudo-B12 that the human body does not recognise as active B12. Some types of nori do contain genuine B12, but to be sure, vegans use a methylcobalamin supplement.

Myth 2: 'The more seaweed, the better.'

Incorrect and risky. Kelp can quickly exceed the UL for iodine (600 µg/day). With Hashimoto's, a high iodine intake can make the condition worse. Dose consciously.

Myth 3: 'Hijiki is healthier than other seaweeds.'

Incorrect. Hijiki contains high levels of inorganic arsenic — advised against for regular consumption by the UK Food Standards Agency and the NVWA. Other seaweeds do not have this problem.

Myth 4: 'Seaweed for iodine is always more reliable than a supplement.'

Not automatically. The iodine content of seaweed varies greatly (a factor of 50-100 difference between types and batches). For exact dosing: a kelp supplement with a stated iodine content, or potassium iodide.

Combining seaweed with other supplements

Seaweed is often combined for the minerals theme. Popular combinations:

• Iodine: for exact dosing alongside wholefood seaweed.
• Multiminerals: for a broad mineral balance.
• Spirulina/chlorella: superfood stack.
• Vitamin D: vitamin D contributes to the normal function of the immune system.
• B12 methylcobalamin: for vegans (more reliable than seaweed B12).

Frequently asked questions

What is seaweed?

A collective name for edible macroalgae. Brown seaweed (kelp, wakame), green seaweed and red seaweed (nori, dulse, Irish moss).

Which seaweeds are there?

Main types: kelp/kombu, wakame, nori, dulse, arame, Irish moss. Avoid hijiki (arsenic).

How much seaweed per day?

Keep doses small: 1-3 nori sheets, 2-5 g of wakame, or kelp capsules according to the label (check the iodine content).

Seaweed and the thyroid

Seaweed is rich in iodine. Iodine contributes to the normal production of thyroid hormones and normal thyroid function. With Hashimoto's: be cautious.

What is the difference between kelp and Irish moss?

Kelp: brown seaweed, highest iodine. Irish moss: red seaweed, broader mineral profile with carrageenan, lower iodine.

Does seaweed contain vitamin B12?

Some types of nori contain genuine B12. Many seaweeds contain pseudo-B12. For vegans: a methylcobalamin supplement is more reliable.

Does seaweed have side effects?

With a high intake (especially kelp): the UL for iodine may be exceeded. With Hashimoto's/thyroid medication: be cautious.

Which seaweed for iodine?

Kelp is the most concentrated. For exact dosing: potassium iodide.

Is seaweed vegan?

Yes, it is plant-based. But as a source of B12 it is unreliable for vegans.

What is hijiki and can I eat it?

It contains high levels of arsenic. Advised against by government authorities. We do not stock it.

Sources

• EFSA — Scientific Opinion on the Tolerable Upper Intake Level of iodine.
• UK Food Standards Agency — advice on hijiki and inorganic arsenic.
• Watanabe F. et al., "Vitamin B12 sources for vegetarians", Nutrients (2014).
• Brown ES. et al., "Health benefits of seaweed", Nutr Bull (2014).