Seaweed is a common ingredient in Asian cuisine that's rapidly gaining popularity among health-conscious Westerners.
And for good reason — eating seaweed is a super healthy and nutritious way to add extra vitamins and minerals to your diet.
Eating it regularly may even boost your health and protect you from certain diseases.
This article takes a close look at seaweed and its many benefits.
Seaweed is a general term used to describe many different species of algae and marine plants.
It can grow in a variety of waters, including the sea, lakes and rivers. Algae from the sea is generally edible, whereas freshwater varieties tend to be toxic.
Edible seaweed is classified by color. The most commonly eaten types are red, green, blue-green and brown ().
It can also range in size dramatically. Phytoplankton can be microscopic, whereas kelp can grow up to 213 feet (65 meters) in length, rooted in the ocean floor.
Seaweed plays a vital role in marine life and is the primary source of food for a variety of creatures in the ocean.
It has also been an integral part of human diets for thousands of years and is especially popular in Chinese and Japanese cuisines.
Bottom Line: Seaweed refers to many species of algae and other marine plants. Edible seaweed can range in color and size and is a popular ingredient in Asian cuisine.
There are many varieties of edible seaweed in the world. Here are a few of the most common:
- Nori: A red algae commonly sold in dried sheets and used to roll sushi.
- Sea lettuce: A type of green nori that looks like lettuce leaves. Commonly eaten raw in salads or cooked in soups.
- Kelp: A brown algae usually dried into sheets and added to dishes during cooking. Can also be used as a gluten-free alternative to noodles.
- Kombu: A type of kelp with a strong flavor. It's often pickled or used to make soup stock.
- Arame: A different type of kelp with a mild, sweet flavor and firm texture. It can be incorporated into a variety of dishes, including baked goods.
- Wakame: A brown algae commonly used to make fresh seaweed salad. It can also be cooked in stews and soups.
- Dulse: A red algae with a softer, chewier texture. It is used to add flavor to a variety of dishes and may also be eaten as a dried snack.
- Chlorella: A green, edible freshwater algae often sold as a supplement in powdered form.
- Agar and carrageenan: These jelly-like substances obtained from algae are used as plant-based binding and thickening agents in a variety of commercially sold food products.
Spirulina is often referred to as an edible, blue-green freshwater algae and is sold in tablet, flake or powdered form.
However, spirulina has a different structure than other algae and is therefore technically considered a type of cyanobacteria.
That said, since spirulina is often categorized with other types of algae in scientific research, it will be discussed alongside the other varieties in this article.
Bottom Line: There are various types of edible seaweed available. These can be consumed fresh, dried, cooked or as a powdered supplement.
Seaweed is rich in various minerals and trace elements. In fact, it often contains higher levels of these nutrients than most other foods.
For this reason, many consider seaweed to be vegetables of the sea.
Seaweed's nutrient content can vary based on where it was grown. Therefore, different types will contain different amounts of nutrients.
Generally, 3.5 ounces (100 grams) of seaweed provides you with (, , ):
- Calories: 45
- Carbs: 10 grams
- Protein: 2 grams
- Fat: 1 gram
- Fiber: 14–35% of the RDI
- Magnesium: 27–180% of the RDI
- Vitamin K: 7–80% of the RDI
- Manganese: 10–70% of the RDI
- Iodine: 1–65% of the RDI
- Sodium: 10–70% of the RDI
- Calcium: 15–60% of the RDI
- Folate: 45–50% of the RDI
- Potassium: 1–45% of the RDI
- Iron: 3–20% of the RDI
- Copper: 6–15% of the RDI
- Smaller amounts of other nutrients: Omega-3 and omega-6 fatty acids, vitamins A, C, E, phosphorus, B vitamins and choline
Dried algae is more concentrated in nutrients. One tablespoon (8 grams) is sufficient to provide most of the nutrient amounts listed above (, , ).
Spirulina and chlorella contain twice as much protein per portion. Unlike other types of algae, they also contain all of the essential amino acids required by the human body. This makes them complete sources of protein (, ).
Some claim that seaweed is a great plant source of vitamin B12, a vitamin naturally found in meat, poultry, eggs and dairy.
However, there's still debate on whether the form of vitamin B12 found in algae is active in humans (, , , , ).
Finally, seaweed is a rich source of antioxidants. It also contains a good amount of sulfated polysaccharides (sPS), which are beneficial plant compounds thought to contribute to seaweed's health benefits (, , , ).
Bottom Line: Edible seaweed contains a wide spectrum of vitamins and minerals. Dried seaweed varieties such as spirulina and chlorella are especially rich sources of complete protein.
The thyroid plays several important roles in the body, including in the regulation of your metabolism (, ).
Your thyroid requires a good intake of iodine to function properly. Luckily, iodine is readily available in most varieties of seaweed.
Other sources of iodine include seafood, dairy products and iodized salt.
Failure to get enough iodine from the diet can lead to hypothyroidism.
This can create symptoms such as low energy, dry skin, tingling in the hands and feet, forgetfulness, depression and even weight gain (). Adding seaweed to your diet can help you consume sufficient iodine for your thyroid to function optimally ().
The RDI of iodine for adults is 150 micrograms per day. Most people can meet this requirement by eating several servings of seaweed per week.
That said, certain varieties such as kelp, kombu and dulse tend to contain very high amounts of iodine and should not be eaten frequently, or in high amounts.
Others, such as spirulina, contain very little, so don't rely on them as your only source of iodine.
Bottom Line: Seaweed is a great source of iodine, which can help promote proper thyroid function.
Seaweed contains certain beneficial nutrients that may help keep your heart healthy.
In addition, several animal studies report that the sulfated polysaccharides (sPS) found in seaweed may have the ability to reduce blood pressure and prevent blood clotting (, , , ).
They may also help reduce LDL (the "bad") cholesterol and total cholesterol levels (, , , , ).
A few studies have also been performed on humans.
For instance, several studies report that high seaweed intakes may reduce blood pressure levels in preschoolers, adults and the elderly (, , , ).
A two-month study gave type 2 diabetics either a spirulina supplement or a placebo every day. The supplement group's triglyceride levels dropped by 24% ().
Participants in the spirulina group also improved their LDL-to-HDL cholesterol ratio, whereas the ratio in the placebo group worsened ().
In another study, a daily spirulina supplement reduced participants' total cholesterol levels by 166% more than the placebo group over the two-month study period ().
Participants in the seaweed group also reduced their LDL cholesterol levels by 154% more than the placebo group ().
Although these results seem promising, not all studies found similar results and more human studies are needed before strong conclusions can be made ().
Bottom Line: Seaweed is a good source of heart-healthy nutrients and may help reduce risk factors for heart disease.
Adding seaweed to your diet may reduce your risk of developing diabetes.
Researchers believe that certain compounds found in seaweed may play a beneficial role in stabilizing blood sugar levels and preventing type 2 diabetes (, , ).
One of these is fucoxanthin, an antioxidant that gives brown algae its characteristic color. This compound is thought to help reduce insulin resistance and stabilize blood sugar levels ().
In addition, the type of fiber found in seaweed may slow down the speed at which carbs are absorbed from a meal. This can make it easier for your body to stabilize your blood sugar levels (, ).
In one study, type 2 diabetics who took a large amount of powdered seaweed every day had 15–20% lower blood sugar levels at the end of the four-week study than those given a placebo ().
In another study, healthy participants who were given seaweed extract 30 minutes before a carb-rich meal benefited from an 8% higher insulin sensitivity than those given a placebo ().
Higher insulin sensitivity is beneficial because it helps your body respond better to insulin and regulate your blood sugar levels more effectively.
Another group of type 2 diabetics who were given a daily powdered seaweed supplement for two months experienced a 12% decrease in blood sugar levels. No changes were observed in the control group ().
The treatment group also reduced their hemoglobin A1C levels by 1% ().
Hemoglobin A1C is used as a measure of your average blood sugar levels over the last 2–3 months. A 1% decrease in A1C represents an average blood sugar decrease of 130 mg/dl (1.5 mmol/l).
Overall, seaweed may be beneficial for blood sugar control, but optimal dosage levels remain unclear. More research is also needed to study the effects of raw versus powdered varieties.
Bottom Line: The antioxidants and soluble fiber found in seaweed may help increase insulin sensitivity and stabilize blood sugar levels. More studies are needed to determine optimal intake levels.
Eating seaweed regularly may help you get rid of unwanted weight.
Researchers believe this may be due, in part, to seaweed's ability to affect your levels of the weight regulating hormone leptin. Combined with seaweed's high fiber content, this may help decrease hunger and enhance feelings of fullness ().
In addition, fucoidan, a type of sPS found in seaweed, may enhance fat breakdown and prevent its formation (, , ).
Studies in obese participants report that those given a seaweed supplement for 12–16 weeks lost around 3.5 pounds (1.6 kg) more than those given a placebo (, ).
What's more, seaweed is low in calories, but rich in glutamate, an amino acid thought to give it a savory, umami taste ().
Therefore, seaweed snacks may help boost weight loss by providing a satisfying alternative to more calorie-rich snack options.
Bottom Line: Seaweed may boost fat loss by reducing hunger, increasing feelings of fullness and preventing the accumulation of fat. Its savory taste makes it a great low-calorie snack option.
Seaweed may also help protect you from certain types of infections.
That's because it contains marine plant compounds believed to have antioxidant, anti-allergenic and disease-protecting properties (, , ).
Research shows that these compounds may have the ability to fight viruses such as herpes and HIV by blocking their entry into cells ().
Unfortunately, not many high-quality studies have been done in humans to support these effects.
Two often-cited studies report that taking seaweed supplements may have the ability to reduce symptoms of the herpes virus and increase levels of immune cells in HIV patients (, ).
However, neither of these studies had a placebo group, which makes it difficult to interpret their results.
A more recent study looked at the effects of taking seaweed supplements in HIV-positive women. Those given 5 grams of spirulina per day developed 27% fewer disease-related symptoms, compared to the placebo group ().
However, no differences in immune cell levels were observed over the 12-week study period ().
Additional studies are needed before strong conclusions can be made.
Bottom Line: Seaweed may have some beneficial effects on your immune system. However, more research is needed.
Seaweed may help improve the health of your gut in various ways. For one, it is rich in fiber, which can help prevent constipation and ensure smooth digestion.
It also contains agars, carrageenans and fucoidans, which are thought to act as prebiotics (, ).
Prebiotics are a type of non-digestible fiber that feed the beneficial bacteria in your gut. The more good bacteria you have in your gut, the less space there is for harmful bacteria to thrive.
Accordingly, animal studies show that taking seaweed supplements may improve the amount of healthy bacteria and reduce the amount of harmful bacteria in the gut more effectively than other types of prebiotics (, ).
Researchers also believe that the prebiotics found in seaweed may have certain anti-inflammatory and antibacterial effects.
This may be partly because, when feeding on prebiotics, the bacteria in your gut produce butyrate. This short-chain fatty acid is believed to have anti-inflammatory effects inside the colon ().
In addition, certain prebiotics may have the ability to block harmful bacteria such as H. pylori from sticking to the gut wall. In turn, this may prevent the formation of stomach ulcers (, ).
Bottom Line: Seaweed contains certain compounds that may help smooth digestion, improve the health of your gut and decrease your risk of infection with certain harmful bacteria.
The presence of seaweed in your diet may help reduce your risk of developing certain types of cancer.
For instance, researchers believe that seaweed may help decrease estrogen levels, potentially reducing women's risk of developing breast cancer (, ).
The soluble fiber found in seaweed may also help protect against the development of colon cancer ().
What's more, some studies suggest that a class of compounds found in brown varieties, such as kelp, wakame and kombu, may help prevent the spread of cancerous cells (, , ).
That said, very few human studies have investigated the direct effects of seaweed in cancer patients. Very high intakes may also increase the risk of certain cancers, particularly thyroid cancer ().
Therefore, more studies are needed before strong conclusions can be drawn.
Bottom Line: Seaweed may offer protection against certain types of cancer. However, more research in humans is needed.
Seaweed may also offer some protection against:
- Metabolic syndrome: Seaweed's potential ability to lower weight and reduce blood pressure, blood sugar and cholesterol may lower the risk of developing metabolic syndrome ().
- Skin damage: Compounds in seaweed may help protect the skin from damage caused by UVB rays from the sun. They may also help prevent wrinkles, sun spots and premature skin aging (, , ).
- Bone and inflammatory diseases: Seaweed's antioxidant and anti-inflammatory effects may help reduce the risk of developing rheumatoid arthritis and osteoporosis (, ).
Bottom Line: Seaweed may offer some additional protection against metabolic syndrome, skin damage, bone disease and rheumatoid arthritis.
Eating fresh seaweed is considered to be safe for most people.
That said, consuming it regularly or in high amounts may cause some side effects.
It May Contain High Levels of Heavy Metals
Depending on where they're grown, some varieties of seaweed can contain high levels of mercury, cadmium, lead and arsenic.
The US Food and Drug Administration (FDA) regulates the levels of these chemicals and heavy metals in fresh seaweed. However, supplements are not regulated and may contain levels that are detrimental to health ().
A High Intake May Interfere With Kidney Function and Blood Thinners
Certain varieties of seaweed may contain high levels of sodium and potassium, which can be harmful to individuals suffering from kidney disease ().
Seaweed also contains vitamin K, which may interfere with blood-thinning medications. Those taking blood thinners should make sure to check with a doctor before making it a regular part of their diet.
Some Are Very High in Iodine and May Interfere With Thyroid Function
While iodine is necessary for proper thyroid function, getting too much iodine can be harmful (, , ).
Kelp, dulse and kombu are types of seaweed with the tendency to contain very high levels of iodine. For instance, 25 grams of fresh kombu can contain close to 22 times more iodine than the safe daily limit (, ).
Therefore, these varieties should not be consumed too often, nor in large quantities.
Bottom Line: Seaweed is considered safe for most people. Limit your intake if you tend to prefer high-iodine varieties, or if you take blood thinners or have kidney issues.
Seaweed can be purchased fresh or dried from most Asian supermarkets. Nori, the type commonly used to roll sushi, may also be available at regular grocery stores.
In addition to their use for sushi, nori sheets can also easily be used to replace tortilla bread when making wraps.
Fresh wakame and sea lettuce can be easily tossed with a little rice vinegar, sesame oil and sesame seeds to make a delicious salad.
Dried nori or dulse make for nice savory snacks. Or, try crumbling them over salads to add a dash of umami flavor.
Spirulina and chlorella can be incorporated into smoothies, while kelp can be used instead of salt to add flavor to just about anything.
Many types of seaweed can also be incorporated into warm dishes, including soups, stews and baked goods. There's no right or wrong way to go about it.
Bottom Line: Seaweed can be purchased in most Asian supermarkets. It can be incorporated into a wide variety of dishes including soups, salads, smoothies, stews and even baked goods.