The nutrition of seaweed

The weed of the sea

In the West, seaweed has long been seen as an annoyance washed up on the beach — the very word “weed” says it all. In Denmark we've started calling it “the sea's vegetable,” which sounds far more inviting. As Christian Bitz has written, seaweed is among the most nutritious things you can eat — and he is normally cautious about calling any food a superfood. Ole G. Mouritsen, author of Seaweed: Vegetables of the Sea, notes that seaweed offers far more than nutrition: it has been used for building materials, fuel and fertiliser.

Seaweed is far more established in the East, where it has been prized for centuries — it tops in Japan, which celebrates Seaweed Day every 6 February. In Danish, “tang” is an ancient Nordic word for the plants of the sea. Along our coasts you'll find toothed wrack, bladderwrack, sugar kelp, dulse and sea lettuce. Strictly speaking, seaweeds aren't plants at all, but multicellular macroalgae.

Global consumption

Most of the world's edible seaweed is grown on dedicated seaweed farms, with China, the Philippines, Korea, Indonesia and Japan as leading producers. This form of aquaculture is now worth tens of billions of Danish kroner each year. The most valuable single crop is Japanese nori — the red alga known from sushi. Seaweed in Denmark is still a small part of this, but the signs point to a market growing quickly.

The good kind of fat — brain food

Seaweed contains only about 2–5% fat by dry weight, but the balance is what matters: typically two to three times more unsaturated than saturated fat, including the essential omega-3 and omega-6 polyunsaturated fatty acids. The Western diet tends to have too much saturated fat and too little of these essentials.

In seaweed, the ratio of omega-3 to omega-6 sits in the range of 0.7–3.2 — close to the ideal of about 1. Our nervous system and brain are roughly 65% fat, much of it long-chain, highly unsaturated, with an omega-3 to omega-6 ratio near 1. That's why fish and seaweed are often called brain food. We typically eat at least ten times too much omega-6 relative to omega-3; omega-3 comes mainly from the sea — from algae, and therefore seaweed.

A diet food

Research suggests the vitamin and mineral content of seaweed can be many times higher than in some land plants. It is also packed with dietary fibre — more per 100 g than fibre-rich vegetables such as cabbage and broccoli — and is very low in calories.

Fish and seaweed

Fish cannot produce omega-3 fatty acids themselves; they get them from their diet, further down the food chain — from algae. Algae, and therefore seaweed, are the original source of the essential long-chain omega-3s DHA and EPA that we know best from fish oil.

All the good substances

The mix of substances varies greatly between species, and also with habitat and season. The iodine content can be very high in brown seaweeds, especially leaf kelps such as Saccharina latissima. Iodine is essential for thyroid function but can be harmful in excess — which is exactly why species, season and dosage matter, and why we work with researchers to measure them.

The salt in seaweed

Most seaweed species carry an excess of potassium salts relative to sodium. That makes seaweed and seaweed salt useful replacements for ordinary cooking salt when it comes to blood-pressure regulation — good for you and your everyday cooking.

Seaweed and umami

Seaweed delivers deep umami — the savoury fifth taste we usually associate with meat and aged products. A little seaweed makes a dish taste of more.

An untapped Nordic raw material

There's good reason to think seaweed was once part of the everyday diet all along the Nordic coasts — and in Iceland, the Faroes and Norway that memory survives. Demand is rising again, from wholefood shops and from restaurants up to Michelin level. Dulse, purple laver, winged kelp, toothed wrack, bladderwrack, sugar kelp, oarweed, cuvie and sea lettuce are the most obvious species for Nordic cooking — out of more than a hundred found in Denmark alone.

One caveat: we can't live on seaweed alone — it simply has too few calories. But as a “vegetable from the sea,” it's a welcome renewal of the Western diet, and there's hardly another ingredient with such breadth of use: raw, cooked, baked, toasted, dried, granulated or deep-fried, and in almost every case it keeps most of its good ingredients intact.

E-numbers — seaweed where you least expect it

Most people meet seaweed daily without realising it, in the form of additives used to thicken food. E-400 to E-407 cover the alginates, agar and carrageenan extracted from brown algae (alginate) and red algae (agar and carrageenan). These complex polysaccharides bind water remarkably well — a good agar can hold more than 99% water — and some carrageenans bind proteins, which is why they're used in dairy products.

Can you grow seaweed in Denmark?

Yes — and we do. We cultivate and harvest seaweed in the Isefjord and surrounding waters, and we're working towards more cultivation sites around the Isefjord, Kattegat and the Great Belt. Seaweed also suits combined cultivation alongside, for example, trout or mussels. The possibilities are many — and there's much more to come.

Sources: videnskab.dk, current science, Christian Bitz, Ole G. Mouritsen, and DTU.