The benefits of eating seaweed (in moderation)

Consumption of seaweed is on the rise in the United States. Perhaps you have enjoyed seaweed wrapped around sushi rolls, as furikake sprinkled over poke bowls (or as popcorn seasoning!), or in seaweed snack packs. 

International seaweed production has increased a thousand-fold since the 1950s, according to the U.S. Department of Agriculture—much of it gathered from the wild—and although Europe and Asia are larger producers, seaweed farming is the fastest-growing sector of aquaculture in the U.S. As seaweed becomes more common in diets here, scientific research is revealing the unique health benefits of its consumption—and exposing potential drawbacks. 

Marine veggies

The foods mentioned above all feature a species of Pacific seaweed called laver, often called by its Japanese name, nori, or gim in Korean. This is just one of thousands of edible types of seaweed harvested and consumed for millennia by coastal communities around the world, from East Asia to the United Kingdom. 

Seaweed is the common term referring to the broad categories of red, brown, and green macroalgae. These large, plant-like structures grow predominantly in saltwater environments and are as beautifully varied in their appearance and nutrient composition as our land plants. 

Though we often think about the human health benefits of consuming vegetables, it’s helpful to recall that the nutrients they contain are there to support plant growth and function. Fibers provide structure. Starch stores energy. Vitamin C provides antioxidant protection against UV light exposure. Green leafy plants are rich in magnesium—the central atom of the chlorophyll molecule, which is responsible for the process of photosynthesis. Each species of plant has unique physiological needs, which helps to explain the range of nutrient profiles among the plant kingdom. In addition, the axiom “you are what you eat” applies just as much to plants as it does for us. Plants’ ability to incorporate minerals into their structure is constrained by which minerals are available in their environment. Nutrient profiles of seaweeds differ from that of land plants because they are surrounded by and built to thrive in very different environments. 

Iodine accumulation

More than any other nutrient, iodine exemplifies this pattern. Iodine is easily washed out of agricultural soils, which is why most of the earth’s iodine is found in the ocean and why land plants rarely contain it, unless grown in coastal areas where soil iodine content is replenished by ocean mists. The concentration of iodine in the ocean is dilute: a liter of seawater contains roughly the iodine mass of a single grain of Morton’s salt. However, seaweeds accumulate iodine from ocean water, making them a potent source of this micronutrient. 

The amount of iodine accumulation varies widely depending on the species of seaweed, location, and even the point in the season in which it is harvested. Brown seaweeds such as kombu are the highest accumulators, with one species (horsetail kelp) concentrating iodine at levels between 30,000-50,000 times greater than the surrounding ocean water. Red seaweeds such as nori and dulse are mid-range accumulators, while green seaweeds such as wakame and sea lettuce have the lowest levels of iodine. Researchers postulate that iodine might provide antimicrobial functions in seaweed, just as it does in our doctors’ offices (that orange-y swab of betadine disinfectant on your skin before an injection: iodine-based) and in dairy farms (sterilization of milking equipment and cow teats inadvertently makes dairy products a source of iodine).  

In humans, iodine is an essential mineral required in trace amounts to produce thyroid hormones, our metabolic regulators. It’s been added to table salt in the U.S. since the 1920s. Not enough iodine and your metabolic processes slow, leading to symptoms such as sluggishness, weight gain, thinning hair, difficulties learning, and painful swelling of the thyroid gland (called goiter). Deficiencies while pregnant or breastfeeding can have devastating impacts on the growth and development of the baby. For individuals who prefer sea salt rather than iodized for home cooking, particularly vegans, seaweed may be a viable whole-food source of iodine to ensure adequate amounts. But a “more is more” philosophy doesn’t hold water when it comes to iodine. Too-high levels of this micronutrient can result in the same symptoms as iodine deficiency.  

Seaweeds’ ability to accumulate minerals has a downside, too. Heavy metals such as lead, cadmium, silver, arsenic, and aluminum are found in trace amounts in the ocean and can concentrate in seaweeds. Just how much is hard to assess, and there are no federal regulations governing how much seaweed is recommended in a daily diet. As with iodine, accumulation of heavy metals depends on the species of seaweed, time of harvest and the surrounding marine environment. There is a positive scenario for this ability, too: ecological researchers suggest certain seaweeds have tremendous potential to sustainably clean up marine regions polluted by heavy traffic or industry. (Seaweeds cultivated for this purpose, needless to say, would not be meant for human consumption.)

Ecologists aren’t the only researchers with eyes fixed on the unlocked algal potential waving in the ocean expanses. Compounds unique to seaweeds already have medical, pharmaceutical, industrial, agricultural, food science, and cosmetic applications, and research into seaweeds continues to grow at a rate that rivals the most prolific species of kelp (up to 18 inches a day). Iodine may receive most of the attention, but the unique array of phytochemicals present in seaweeds functions together like instruments in a symphony, their collective impact far greater than the sum of the solitary components.

Erin Cazel is pursuing a master’s degree in nutrition at Bastyr University.