Is Seaweed Healthy? Here’s What Experts Say

Seaweed is a staple in many Asian cuisines, but many Americans still only encounter it when they go out for sushi. As a result, many diners don’t know what to make of it: Is seaweed healthy? Does it count as a vegetable? Are all kinds nutritionally the same?

Miho Hatanaka, a Japanese-born registered dietitian who is based in Oregon, says the sea vegetable is worth getting to know. In the Japanese tradition, “people really attribute their health and longevity to seaweed, and even healthier skin and hair,” she says.

Here’s what you need to know about eating seaweed.

There are many types of edible seaweed

There are multiple varieties of seaweed, all with different nutrient profiles. The three main categories are brown algae, such as kombu, which is used to make dashi; green algae, such as sea lettuce; and red algae, such as nori, which is often used to wrap sushi rolls and garnish soups. There are also plenty of edible seaweeds you may not have tried, including dulse, a red algae that researchers from Oregon State University bred to taste like bacon when cooked.

As seaweed becomes more mainstream in the U.S., it’s also showing up in new forms, like crunchy seaweed snacks and algae oil. Seaweed snacks, like any processed food, can be high in sodium and additives, but Hatanaka says they can be a healthier replacement for chips and crackers. Algae oil can be a good alternative to fish oil for those who don’t eat animal products (or simply don’t like the taste), says Nancy Oliveira, a senior nutritionist at Brigham and Women’s Faulkner Hospital in Massachusetts. Like fish oil, algae oil contains heart-healthy fatty acids.

Seaweed is a low-calorie way to get nutrients

Since there are so many kinds of seaweed, it’s difficult to issue a blanket statement about their nutrition, Oliveira says. But “overall, [it’s] fairly low in calories,” she says, and many varieties are lower in sodium than their salty tastes would suggest. “People will use dried kelp instead of the salt shaker. That can be useful if people are trying to cut down on actual table salt.”

Many types of seaweed have as much protein and as many amino acids per gram as beef, according to a research review recently published in Nutrition Reviews — but since seaweed servings are typically quite small, it may not be realistic to eat equivalent amounts. The digestibility of seaweed proteins also varies by type, according to the paper.

Many types of seaweed are also rich in fiber. A five-gram serving of brown algae, for example, has about 14% of a person’s recommended daily fiber, according to the Nutrition Reviews paper. Fiber aids healthy digestion and keeps you feeling full longer, and research also suggests that fiber-rich foods can help prevent chronic conditions including heart disease and some types of cancer. In addition to fiber, some research has shown that a compound in seaweed, called alginate, may help control appetite and help people eat fewer calories. Many varieties also contain polysaccharides, which can improve gut health and “help to give you a feeling of satiety,” Oliveira says.

Seaweed also comes with many nutrients, Hatanaka says. “Even with a smaller amount of seaweed, you can get a lot more nutrients out of it than ground vegetables,” Hatanaka says, such as magnesium and iron. Many seaweeds also contain vitamin A, vitamin K, folate and some vitamin B12, though not all types contain B12 that can be digested by humans.

But there are some health drawbacks

Research has found that heavy metals lurking in contaminated water, including arsenic, aluminum, cadmium, lead, rubidium, silicon, strontium and tin, can taint some types of seaweed, though the type and amount of contamination can vary drastically depending on the plant’s natural habitat. Hijiki, a thin seaweed that looks black when cooked and is often used in Japanese and Korean appetizers, is the most commonly affected by arsenic exposure. Health officials in the U.S. and Australia, as well as some countries in Europe and Asia, have issued public-health warnings about this type of seaweed, but it’s often still possible to buy and order it.

Some nutrients in seaweed, while healthy for many diners, can also pose health risks for some people. Seaweed can also absorb iodine from sea water, which may be problematic for people who have thyroid disorders, since it can throw off the thyroid’s ability to make hormones, Oliveira says. Seaweed tends to be high in vitamin K, which can interact poorly with blood thinners, and potassium, which can be dangerous for people with heart and kidney conditions that prevent them from filtering excess potassium out of the body, she says.

For those reasons, Oliveira says people should eat seaweed in moderation. While periodic seaweed salads or sushi rolls are likely nothing to worry about, Oliveira recommends thinking of seaweed more as a condiment than a main dish. “My fear is that people are going to take it as a superfood…and eat a lot of it,” Oliveira says. “Eating bowls of seaweed every single day could be too much of these trace minerals.”

Hatanaka agrees, noting that seaweed isn’t a daily staple of a typical Japanese diet either. “I would definitely limit consumption,” she says. In Japanese cuisine “it’s more of a side dish, where you would eat it once or twice a week — or we would sprinkle some on miso soup, but those are very minimal amounts.”