Seaweed is an increasingly popular food choice, recognized for its density of nutrients. This rise in consumption has been met with concerns about arsenic, an element seaweed can accumulate from its surroundings. This raises valid questions for consumers about the safety of including seaweed in their diet.
The Source of Arsenic in the Marine Environment
Seaweeds absorb the minerals they need to grow directly from seawater, a process that also causes them to take up other substances like arsenic. The arsenic found in the ocean comes from a combination of natural and human-related activities. The primary natural sources are geological, including the erosion of arsenic-containing rocks and sediments on the ocean floor and emissions from underwater volcanic activity.
These natural processes establish a baseline level of arsenic in the marine environment. Human activities, however, can increase these concentrations in certain areas. Industrial effluent, atmospheric deposition from the burning of fossil fuels, and agricultural runoff can introduce additional arsenic into coastal waters where many edible seaweeds are harvested.
Because of their biology, seaweeds can accumulate arsenic to levels thousands of times higher than the surrounding water. This ability to hyperaccumulate the element is influenced by factors like the specific seaweed species, water temperature, and the time of year it is harvested. This explains why arsenic content is not uniform across all types of seaweed.
Distinguishing Between Arsenic Forms
Not all arsenic is the same; the element exists in different chemical forms with different implications for human health. The two main categories are inorganic arsenic and organic arsenic. This distinction is a primary factor when evaluating the potential health risks of eating seaweed.
Inorganic arsenic is classified as a carcinogen and is the form associated with adverse health effects from chronic exposure. In contrast, the majority of arsenic found in most seaweeds is in organic forms. These organic compounds are considered to be significantly less toxic.
The body processes these forms differently. Organic arsenic compounds are water-soluble and are excreted from the body relatively quickly, posing little risk. The transformation of inorganic arsenic into organic forms within the seaweed is thought to be a natural detoxification process for the plant itself. Therefore, knowing the total arsenic content of a seaweed product is not enough; the important information is the concentration of the more harmful inorganic arsenic.
Arsenic Content Across Seaweed Varieties
The amount and form of arsenic present can vary dramatically among different types of seaweed. Brown seaweeds tend to accumulate higher levels of total arsenic compared to red or green varieties. Within the brown algae category, there are significant differences, making some species a greater concern than others for consumers.
Hijiki is a type of brown seaweed that has been found to contain exceptionally high levels of inorganic arsenic. In some studies, the inorganic arsenic in hijiki was more than 20 times higher than in other brown seaweeds. For this reason, food safety agencies in several countries, including the United States, Canada, and the United Kingdom, have advised consumers to avoid eating hijiki seaweed.
Kombu, another popular brown seaweed often used to make broths, has high total arsenic levels, a significant portion of which can be inorganic. Wakame, commonly used in miso soup and salads, contains moderate levels of arsenic. Of the most commonly consumed seaweeds, nori—the dried sheets used for sushi—has the lowest concentrations of total and inorganic arsenic, making it a safer option for more frequent consumption.
Safe Consumption and Preparation Practices
For those who enjoy seaweed, there are practical steps to minimize potential exposure to inorganic arsenic. The most straightforward approach is moderation. Occasional consumption of seaweed as part of a balanced diet is unlikely to pose a significant health risk.
Consumers can also take other steps to minimize exposure:
- Choose seaweed varieties with naturally lower levels of inorganic arsenic, such as nori.
- Look for products from reputable brands that may conduct testing for heavy metal contaminants.
- Source seaweed from regions known to have lower levels of marine pollution, although this information is not always readily available to the consumer.
- Use preparation methods that can help reduce inorganic arsenic content. Research has shown that rinsing dried seaweed and then cooking it in a large volume of fresh water can significantly lower the concentration of water-soluble inorganic arsenic. This technique is particularly effective for tougher seaweeds like kombu, and the cooking water should be discarded.