Seaweed is a plant-based food enjoyed the world over. Many coastal civilizations have integrated seaweed into their local cuisines. Countries across East Asia have been enjoying green seaweed for millennia, while red and brown algae are well-loved in countries bordering the Nordic Sea. But, is seaweed good for you?

Seaweed provides an excellent source of many nutrients and antioxidants, many of which can also be processed into dietary supplements. Despite this rich nutrient profile and potential associated medicinal benefits, seaweeds do present certain health risks, as some types may contain toxic elements. In this article, we will first talk about what seaweed is and the compounds it contains. We’ll then talk about the health benefits of seaweed. Lastly, we’ll end by discussing things to consider when picking a seaweed product so you can enjoy this bountiful plant in your diet.

What is seaweed?

Seaweed comes from the algae that grow in aquatic environments around the world. However, only a subset of algal species is edible. Most marine species are edible, while most freshwater algae are toxic. Most edible algae are classified into three groups based on their color:

• Green algae are the second most diverse kind of algae; about 8,000 species of green algae exist worldwide. The bright green color that these algae give off comes from chlorophyll A and B, essential for the plant’s ability to perform photosynthesis. Green algae are found around the world, but most are found in freshwater environments such as rivers, lakes, and ponds.
  
• Red algae are members of the Rhodophyta group. In contrast with green algae, red algae are more commonly found in marine habitats. Red algae are the most diverse algae in the world, with an estimated 14,000 species. The red color that these algae give off comes from a special kind of light-harvesting pigment complex called phycoerythrin.
  
• Brown algae are almost universally found in marine environments, with less than 1% of brown algae species found in freshwater habitats. The greenish-brownish color that these algae give off comes from the production of the fucoxanthin pigment and pheophycean tannins.  

While the concentrations of macro- and micronutrients vary substantially by geographic location, season, and type of algae, all algae contain substantial concentrations of the following macromolecules:

• Proteins are abundant in all algae, with some seaweed species having protein levels similar to meat, eggs, soybean, and milk. The protein content in some algae is so high that species such as “spirulina” and Chlorella have become domesticated crops for adding protein to many foods. Glutamate, the most abundant amino acid in algae, is also responsible for the umami savory taste.
  
• Fatty acids are not a major component of algae; these plants typically contain no more than 2-4.5% of fatty acids by dry weight. Nevertheless, seaweed provides a healthy source of omega-3 fatty acids, which must be acquired from your diet.
  
• Polysaccharides are carbohydrates built from sugars such as glucose, and algae contain many diverse kinds of them, including many that are dietary fibers. Both soluble and insoluble fibers are present in algae — including fucoidans, alginate, and other polysaccharides.
  
• Vitamins are plentiful in algae, which obtain the vitamins when the algae interact with bacteria — similar to how bacteria produce essential vitamins in your gut. Multiple seaweed species are rich in vitamin C, with a single sheet of laver (a dried seaweed, sometimes called nori) containing as much as 10.1 mg of vitamin C. All edible algae are also good sources of other vitamins such as vitamin A, multiple B vitamins, vitamin E, and vitamin K.

Algae is also rich in mineral concentrations gathered from the seawater where marine seaweeds reside. Their plentiful supply of potassium, calcium, sodium, and phosphorus are harvested to make mineral supplements marketed as health products.  

Health benefits of eating seaweed

With the plentiful amounts of nutritious compounds in seaweed, it stands to reason that there may be certain seaweed benefits for your health:

• Improved gut function: A mixture of soluble and insoluble fibers can be found in all edible seaweed. Fibers are essential for nourishing the microbes that live in your gut and keep your bowel movements healthy. What makes seaweed special is that there are many dietary fibers unique to algae, such as alginate. In mouse models, sodium alginate can upregulate the expression of tight junction proteins, complexes that prevent harmful compounds from crossing from your intestines into your bloodstream.
  
• Type 2 Diabetes (T2D): Eating algae may also have protective effects against T2D. A 2022 study determined that rat models fed with algal extracts had reduced blood sugar levels at fasting state, while a meta-analysis of 27 randomized control trials (RCTs) determined that algal extracts may help reduce blood glycolipid levels.
  
• Anticancer benefits: Algae are also an excellent source of phytochemicals and antioxidant molecules. Antioxidants are compounds that protect against oxidative stress, a key driver of cancer. Antioxidants also include polyphenols, polysaccharides, and sterols. Brown algae contain high concentrations of phlorotannins, a class of polyphenol that protects cells from injury caused by UV radiation and oxidative stress. Future research should focus on examining the anticancer effects from other kinds of algae, such as green or red algae, since the research to date is insufficient to form reliable conclusions.
   
• Antibiotics: With the global threat of antibiotic resistance rapidly on the rise, a means to develop and identify new antibiotics is essential for treating bacterial infections. Algae extracts may provide one such promising source of antibiotics, as algaes appear to act against multiple pathogens.

In summary, while there is research suggesting that algal extracts can provide health benefits, more research is needed on multiple fronts. For instance, it remains to be seen how universal the benefits are for different kinds of algae. Variations in the composition of algal extract used provide a substantial confounding factor when teasing apart these benefits. Obtaining robust study populations across diverse ethnicities is also essential to determine seaweed benefits for your health.

Seaweed nutrition: Things to consider before buying

Before you jump into the aisles and grab the first seaweed product you see, be aware that some adverse effects are associated with excessive seaweed consumption. Certain types have high concentrations of the following potentially toxic compounds:

• Heavy metals such as lead, cadmium, chromium, and aluminum are well known for their toxic effects to the body. When ingested, heavy metals contribute to the dysfunction of different body organs such as the gut, kidneys, and the nervous system. Like all the other molecules mentioned, the heavy metal concentration in algae varies by type. For instance, brown algae tend to contain higher concentrations of heavy metals than red or green algae. The high concentrations of heavy metals stem from its ability to absorb them, so much so that they are used to remove toxic metals from other materials. Heavy metal concentrations in algae also change by season, making it difficult to know for sure whether a specific seaweed product contains excess heavy metals. 
  
• Arsenic is regularly taken up or consumed by aquatic species. Inorganic arsenic’s toxicity and health risks are well documented. Acute risks include vomiting, diarrhea, muscle cramping and death; chronic risks include increased risk of cancer and skin lesions. However, most marine and freshwater species convert inorganic arsenic into other compounds. Green and brown algae convert inorganic arsenic into arsenic-containing sugars, called arsenosugars, which can accumulate in tissues at similar levels as inorganic arsenic. While they may not pose an acute toxicity risk, arsenosugars may provide a chronic health risk. Nevertheless, more research is required to determine the risks of arsenic poisoning from consuming arsenosugars.
  
• Iodine: Elevated iodine levels are a significant problem for many algal-based products. Most of these foods contain iodine levels that exceed tolerable upper intake levels when taken as a serving or a portion. Having too much iodine in your system can lead to problems with your thyroid gland. Among middle-aged and elderly women in Japan, those who ate seaweed daily had an increased risk of thyroid cancer. Eating too much iodine can also make your thyroid overactive, contributing to unintentional weight loss and a rapid, irregular heartbeat. The concerns surrounding iodine consumption have led to recommendations that pregnant women refrain from eating seaweed.

In summary, the often uncertain concentrations of dangerous compounds in seaweeds makes it difficult to safely reap the health benefits of seaweed. However, if you follow these guidelines when selecting seaweed-based foods, you can enjoy many seaweed products while minimizing the risks.

• Do not eat seaweed if you have a condition, such as pregnancy, in which seafood consumption is contraindicated.  When in doubt, check with your medical provider before adding any seaweed or kelp food products to your diet.  
  
• Be vigilant about nutrient composition: Make sure you read the nutritional labels and check how much of each nutrient or molecule is present in the algal product you’re considering consuming. If you’re not sure what a compound is, be sure to check reliable, peer-reviewed online resources. 
  
• Advocate for accurate nutrient labeling and research: Beyond the individual level, speak to your representative and encourage legislative measures to ensure the monitoring and labeling of seaweed-based food products. Also take the time to support organizations that support research on heavy metal contamination in algae consumed by different demographic groups. In doing so, you will take the first steps to prevent the excess intake of salt, iodine, and heavy metals from eating algae-based foods. 

Key takeaways

So, is seaweed good for you? Enjoyed in many coastal societies around the world, seaweed is an excellent source of fiber, an important nutrient source for your gut microbes. Seaweed is also a good source of protein, omega-3 fatty acids and many healthy vitamins and minerals. However, the medical benefits of seaweeds remain somewhat unclear. More work needs to be done to study the extent of these benefits with different kinds of algae and for different demographic groups. Furthermore, seaweed consumption comes with potential health risks arising from taking in excess iodide or ingesting toxic compounds such as heavy metals and arsenic. Therefore, it’s prudent to err on the side of caution when considering adding a seaweed food product into your diet; while many are healthy for you, others may not be — so carefully read all nutritional labels and consume these foods in moderation.