Seaweed, or macroalgae, encompasses a broad range of edible marine plants that predominantly grow in oceans but are also found in rivers, lakes, and other bodies of water. Think of seaweed as aquatic leafy greens; instead of arugula, kale, and spinach, you have things like wakame, kombu, and nori. Seaweed has even been called the “kale of the sea.”
Seaweed is popping up on menus and grocery store shelves more frequently, thanks to its wide availability and umami flavor. However, many varieties have been culinary staples in coastal cultures (particularly in Japan, China, and Korea) for thousands of years.
Some restaurants serve fresh seaweed, but in stores, you’ll typically come across dried, shelf-stable varieties that have been washed and then sun-dried, oven-dried, or freeze-dried. All seaweed falls into three main categories: green, red, and brown algae. These are further broken down into specific seaweed types. Acquainting yourself with the wide varieties of seaweed can feel overwhelming, but these five popular picks are a great place to start:
- Nori: Pleasantly salty, crispy-but-pliable nori sheets are sushi rolls wrappers. They’re made by shredding red seaweed and pressing it into sheets, then drying or toasting.
- Wakame: This brown seaweed has a subtly sweet taste and silky texture. It’s often sliced thin and used in seaweed salads and miso soup.
- Kombu: This brown seaweed comes in wide, thick, rigid strips with loads of umami flavor. It’s often simmered with other ingredients to make broths and stocks.
- Dulse: Ultra savory, salty, and almost meaty, this red seaweed is often sold in ground or flaked form and used as a seasoning.
- Arame: This brown seaweed resembles twiggy strands of loose black tea and is relatively mild and subtly sweet in flavor, making it a versatile addition to many dishes.
Because seaweed absorbs nutrients from the surrounding water, its nutrient profile varies depending on the type and where it was harvested. However, it’s one of the best sources of iodine, which plays a crucial role in thyroid health and overall metabolism. Seaweed also contains fiber, protein, omega-3 fatty acids, and antioxidants. It is rich in vitamins and minerals, including magnesium, calcium, potassium, iron, zinc, B vitamins, and vitamins A, E, D, and C.
Seaweed can also absorb pollutants from water—but research findings are mixed, and some studies, primarily in Asia, suggest eating seaweed likely poses a low risk. However, hijiki seaweed is known for high levels of inorganic arsenic, a carcinogen, so skip it or consume it cautiously.
Seaweed’s Metabolic Punch
Seaweed is low in calories and carbohydrates, making it a great way to add flavor and texture to meals without spiking your blood sugar. For example, a two-tablespoon serving of dried wakame has just 10 calories and two grams of carbs, while ⅓ cup of dried arame has 15 calories and three grams of carbs—and all of those carbs are in the form of fiber. (If these servings seem small, keep in mind that dried seaweed doubles in size when rehydrated.)
Seaweed is perhaps best known nutritionally for its iodine content. Iodine is an essential trace mineral crucial for producing thyroid hormones, which, in turn, are vital for metabolic health. When you eat iodine-containing foods, cells in the thyroid absorb iodine and combine it with the amino acid tyrosine to create two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). From there, T3 and T4 are released into the bloodstream and travel to cells throughout the body to regulate metabolism, growth, and other physiologic processes. Iodine is also an antioxidant that helps counteract oxidative stress in the body.
A lack of dietary iodine can lead to insufficient thyroid hormone production and sluggish metabolism, which can result in widespread symptoms such as fatigue, weight gain, dry skin, cold intolerance, and constipation. Thyroid hormones also play a role in glucose metabolism by acting as both insulin agonists and antagonists—that is, they can increase and decrease their action as needed to maintain glucose homeostasis. Research has established that hypothyroidism can disrupt this balance and alter glucose metabolism, which can lead to insulin resistance.
It’s also possible to consume too much iodine. While excess iodine is generally well tolerated in healthy individuals, it can result in autoimmune thyroid disease in some. Out of all seaweed varieties, kombu contains the most iodine, delivering several thousand times more iodine per serving than the recommended daily amount (RDA) of 0.15 mg/day. And while nori contains one of the lowest amounts of iodine, a single sheet can still provide 62 percent or more of the RDA. Aim to stay below the tolerable upper limit of 1,100 micrograms of iodine per day. Depending on where the seaweed was harvested, this might equate to 12 sheets of nori, 3 tablespoons of dried wakame, and less than a serving of kombu. But if you’re not eating seaweed or other iodine-rich foods daily, you may be able to get away with a larger serving now and then.
Seaweed can also be a good source of metabolism-friendly nutrients like magnesium and vitamin B12. Magnesium helps your cells create energy from glucose, and research shows that greater magnesium intake is associated with lower fasting glucose levels and reduced insulin resistance. Vitamin B12 supports cardiovascular health by assisting in the breakdown of the amino acid homocysteine, which can damage blood vessels and increase the risk for blood clots, heart attack, and stroke if levels get too high.
Additionally, the polyphenol and carotenoid compounds in seaweed (such as phlorotannins, fucoxanthin, and others) function as antioxidants, helping protect cells from oxidative damage that might otherwise contribute to insulin resistance and overall metabolic dysfunction.
Seaweed Buying Advice
Different varieties of seaweed are available in the international aisle of supermarkets and online. Asian markets generally carry the widest selection of seaweed products.
- Typically seaweed is sold in dried form in plastic packaging along with a silica gel packet to keep the product moisture-free and fresh. This seaweed should always appear dry and feel somewhat crisp (never spongy) with no signs of moisture within the packaging. Brands like Eden Foods offer several varieties of whole-leaf or pre-cut seaweed that have been sun-dried or toasted. You can also buy dried seaweed salad mixes that you rehydrate with water.
- Dried seaweed flakes and granules are often available in shaker canisters. Thanks to seaweed’s umami flavor, these make a great addition to seasoning blends and can serve as a nutrient-dense, lower-sodium salt substitute.
- Drying methods influence nutritional profiles. Freeze-dried seaweed contains the highest amount of certain nutrients, such as polyunsaturated fatty acids and vitamin C. In contrast, oven-dried seaweed loses the most nutrients but retains the most minerals.
- Read the labels of seaweed snacks made from roasted, seasoned sheets of nori. Many brands are made with refined seed oils, which can contribute to inflammation when consumed in excess. Others favor avocado and olive oil, which are rich in heart-healthy monounsaturated fats.
- Some markets offer prepared seaweed salads and other products made with fresh seaweed. These run the gamut in terms of quality and may include added sugars, refined seed oils, and artificial colors, so always read the label. One brand we love is Maine-based Atlantic Sea Farms, which offers a variety of fermented kelp seaweed products (including seaweed salad, kimchi, and sauerkraut) with naturally occurring probiotics, as well as frozen pureed kelp cubes that you can add to smoothies, soups, and sauces.
Tips for Storing & Using Seaweed
Here are some essential storage and prep strategies for dried seaweed:
- Keep packages of dried seaweed in a cool, dark, dry place like a kitchen pantry.
- Unopened seaweed is typically safe to eat well beyond the “best by” date, as long as there are no signs of discoloration or mold.
- Once you open seaweed, transfer it to an air-tight container or zip-top bag along with the silica gel packet from its original packaging to keep things dry. At room temperature, it can last anywhere from several weeks to several years. Thin varieties such as nori have the shortest shelf-life (about two to three weeks), as they quickly absorb moisture when exposed to air, while sturdy varieties like kombu can last years.
- To further extend shelf life, store leftover seaweed in the freezer. Thin varieties like nori should last about 10 months. When you’re ready to use your seaweed, allow it to come to room temperature before opening to prevent moisture accumulation.
- Proper prep depends on the seaweed type and your recipe. Certain varieties (such as nori) can be used immediately in their dried form; others, like wakame, may require a five-minute or more soak in cold water before incorporating into salads, soups, and other dishes. Kombu is typically simmered to make broths like dashi.
Ideas for Eating Seaweed
Seaweed is easy to work into meals. Here are some accessible ways to start experimenting:
- Use nori for DIY sushi or a low-carb sandwich alternative. You can pack it with nutrient-rich ingredients such as salmon, avocado, and thinly sliced veggies for a satisfying crunch.
- Assemble seaweed salad by rehydrating dried wakame strips and toss with rice vinegar, soy sauce, mirin, yuzu juice, and unrefined oil. While wakame is typically the go-to for seaweed salad, you can experiment with other varieties such as arame or thinly sliced kombu. For a dose of protein, add some silken tofu.
- Simmer kombu, fish flakes, and shiitake mushrooms in water to make umami-packed traditional Japanese dashi—the backbone of miso soup, noodle bowls, and more.
- Create a traditional miso soup by combining kombu-based dashi with rehydrated wakame, miso paste, tofu, and scallions.
- When making beans, add a half strip of dried kombu to the cooking water for a smoother mouthfeel and extra flavor.
- Rehydrate twiggy strands of mild-flavored arame to use in stir-fries, salads, and grain dishes.
- Crumble any type of dried seaweed (or buy seaweed flakes or granules) and sprinkle it onto soups, salads, bowls, broths, and popcorn for a salty, umami punch. Savory and smoky dulse makes a great addition to seasoning blends.
- Experiment with low-carb, grain-free kelp noodles in stir-fries, ramen, pho, and noodle salads. These low-calorie, low-carb, grain-free noodles are made from the clear inner flesh of kelp, sodium alginate (sodium derived from seaweed), and water. You can enjoy them hot or cold.