Which Seaweed Should Be Avoided? Your Guide to Safe Consumption

According to a 2024 review, some seaweed species contain elevated levels of heavy metals like arsenic and cadmium, highlighting the importance of understanding which seaweed should be avoided. While often celebrated for its nutrient density, certain varieties can pose risks due to excessive iodine, harmful toxins, and environmental pollutants.

Quick Summary

This guide details specific types of seaweed to avoid due to high concentrations of iodine, arsenic, or heavy metals. It also covers the risks posed by toxic algae and advises on selecting safe, commercially sourced products, differentiating safe varieties like nori and wakame from more hazardous options.

Key Points

• Hijiki Seaweed: This brown seaweed contains dangerously high levels of inorganic arsenic and should be avoided.

• Kelp Varieties: Species like kombu and sugar kelp contain extremely high and variable amounts of iodine, posing a risk of thyroid issues with regular consumption.

• Wild-Harvested Seaweed: Harvesting from unverified or polluted waters increases the risk of heavy metal (lead, cadmium, mercury) and microplastic contamination.

• Freshwater Algae: Most freshwater algae are toxic and should never be consumed, unlike marine seaweed.

• Contaminated Seaweed: Always source commercially verified seaweed from reputable suppliers, as it is tested for contaminants that can be absorbed from the environment.

• Sensitive Individuals: Pregnant women, breastfeeding mothers, children, and those with thyroid disorders should be especially cautious about excessive iodine intake from seaweed.

• Harmful Algal Blooms (HABs): Seaweed can become contaminated with toxins from nearby HABs, affecting marine life and potentially humans.

Seaweed has been a dietary staple in many cultures for centuries, prized for its unique flavor and dense nutritional profile, which includes essential minerals, vitamins, and antioxidants. However, not all seaweed is created equal, and consuming the wrong kind or too much of a particular species can lead to health issues. It is crucial to be aware of which seaweed should be avoided, whether due to naturally high levels of certain compounds or contamination from polluted environments.

Seaweed Varieties High in Inorganic Arsenic

One of the most significant concerns with seaweed is the accumulation of inorganic arsenic, a known carcinogen. While most arsenic in seaweed is in less harmful organic forms (arsenosugars), certain species can accumulate dangerously high levels of the inorganic kind.

Hijiki (Hizikia fusiforme)

Hijiki is a type of brown seaweed that has been repeatedly identified as having hazardous levels of inorganic arsenic. For this reason, food safety agencies in countries like the UK and Australia have advised against its consumption. Studies have shown that hijiki can contain up to 67.5 mg/kg of inorganic arsenic, a level far exceeding safety thresholds in many regions. Despite this, it is still sold in some markets and used in Japanese cuisine, so careful identification is essential.

Laminaria digitata

Some inedible or less commonly consumed brown seaweeds, such as certain types of kelp, have also been shown to accumulate high levels of inorganic arsenic. This highlights the risk of wild harvesting or consuming seaweed from unverified sources, as you may not be able to accurately identify the species or its contamination level.

Seaweeds with Excessive Iodine Content

While iodine is a vital nutrient for thyroid function, excessive intake can lead to thyroid disorders, including hyperthyroidism or hypothyroidism. Brown seaweeds, particularly kelp species, are notorious for their extremely high and variable iodine content.

Kombu (Laminaria japonica)

Kombu, a type of kelp used to make dashi broth, has one of the highest iodine concentrations of all seaweeds. Regular or high intake can easily lead to iodine overdose, which is particularly risky for individuals with pre-existing thyroid conditions, as well as pregnant and breastfeeding women. The iodine levels can vary so widely that it is not considered a reliable or safe source for consistent intake.

Other Kelp Varieties

Many other species of kelp and brown seaweed also carry a high risk of excessive iodine intake. For instance, sugar kelp (Saccharina latissima) contains very high levels and should be consumed in extreme moderation. For most healthy adults, occasional, small servings are not an issue, but consistent use, especially in supplements, poses a significant risk.

Risks from Algal Blooms and Other Toxins

Not all dangers come from a seaweed’s natural composition; environmental factors also play a critical role. Harmful algal blooms (HABs) can contaminate marine ecosystems with potent toxins, posing a severe risk to humans and animals.

Freshwater Algae

Most edible seaweed is marine algae, while most freshwater algae are toxic. Harmful freshwater cyanobacteria (blue-green algae) produce dangerous toxins like microcystins, which can cause liver damage. Never consume algae from freshwater sources unless it is a commercially verified food product, like spirulina or chlorella, grown under controlled conditions.

Marine Biotoxins

Certain marine seaweeds, or the fish and shellfish that feed on them, can be contaminated with biotoxins from HABs.

• Ciguatera Poisoning: Caused by toxins from dinoflagellates consumed by reef fish, which can then be ingested by humans. While the seaweed itself is not the direct cause, it's part of the trophic transfer. Some species, like Lyngbya, can be toxic.
• Amnesic Shellfish Poisoning: Caused by domoic acid from diatoms like Pseudo-nitzschia, which accumulates in shellfish. This can cause neurological effects in marine mammals and humans.
• Neurotoxic Shellfish Poisoning: Caused by brevetoxins from dinoflagellates like Karenia brevis, which is responsible for Florida's red tides.

Environmental Contamination and Wild Harvesting

Beyond natural risks, seaweed can absorb pollutants like heavy metals (mercury, lead, cadmium) and microplastics from its environment. Wild-harvested seaweed from polluted coastal areas, industrial zones, or near sewage outlets is at particularly high risk. Unless you are certain of the water quality, it is safest to avoid wild-harvested seaweed entirely. Brown algae are especially prone to heavy metal accumulation.

Comparison of Seaweed Risks

Feature	Low-Risk Varieties (e.g., Nori, Wakame)	High-Risk Varieties (e.g., Hijiki, Kombu)	Wild-Harvested Seaweed	Harmful Algal Blooms	Freshwater Algae
Iodine Content	Generally lower, more stable.	Very high and variable, poses significant thyroid risk.	Variable depending on species and location.	Not a primary concern for toxins, but can be contaminated.	Not typically consumed, but can be toxic.
Heavy Metals (Arsenic, Cadmium)	Low to moderate, depending on source.	Hijiki is particularly high in inorganic arsenic.	High risk if harvested from polluted waters.	Accumulation in shellfish and fish is the primary risk vector.	Very high risk; can produce hepatotoxins.
Biotoxins	Very low risk from commercially sourced products.	Generally low risk, unless contaminated by HABs.	Potential risk from surrounding environment.	Primary source of toxins like brevetoxins and domoic acid.	High risk, produce cyanotoxins.
Recommended Consumption	Safe for regular consumption in moderation.	Should be limited or avoided, especially kelp and hijiki.	Avoid unless source is confirmed pristine and species is verified.	Do not consume fish or shellfish from bloom-affected areas.	Never consume.

Conclusion

Understanding which seaweed should be avoided is essential for safe consumption. The key takeaway is to choose commercially sourced seaweed from reputable suppliers who test for contaminants, especially avoiding species known for high arsenic or iodine accumulation like hijiki and kombu. Be extra cautious if you have a pre-existing thyroid condition, are pregnant, breastfeeding, or preparing food for children, and always refrain from harvesting wild seaweed from unverified or potentially polluted areas. For those seeking the nutritional benefits of seaweed, options like nori and wakame offer safer choices with lower risk profiles. By staying informed and making deliberate choices, you can safely enjoy the health benefits that seaweed has to offer.

Here is a list of other potentially harmful seaweeds and algae to be cautious about.

Frequently Asked Questions

Is all seaweed safe to eat?

No, not all seaweed is safe to eat. Some varieties contain high levels of toxins, inorganic arsenic, or excessive iodine, while wild-harvested seaweed can be contaminated with heavy metals and other pollutants.

Which seaweed has the highest iodine content?

Kelp varieties, such as kombu and sugar kelp, are known for having extremely high levels of iodine, which can be harmful if consumed in large or frequent amounts.

Why is hijiki seaweed considered dangerous?

Hijiki seaweed is considered dangerous because it contains hazardous levels of inorganic arsenic, a known carcinogen. Food safety agencies in several countries have advised against its consumption.

Can I eat seaweed that I collect from the beach?

You should not eat seaweed collected directly from the beach or shoreline. It can be contaminated by pollutants, microplastics, or sewage, and may also be rotting.

Are there any specific seaweeds to avoid during pregnancy?

Pregnant and breastfeeding women should limit or avoid brown seaweeds like kombu and other kelps due to their very high and variable iodine content, which can negatively affect thyroid function.

How can I tell if an algal bloom is harmful?

You cannot tell if an algal bloom is harmful just by looking at it. Many blooms appear harmless but produce dangerous cyanotoxins. Avoid contact with water where a bloom is suspected and do not consume seafood from these areas.

Which seaweeds are generally considered safe for consumption?

Commonly consumed and generally safe varieties include nori (used for sushi) and wakame, as they typically have lower levels of problematic contaminants like arsenic and more moderate iodine content compared to kelp.