Classified as marine algae, seaweeds are chlorophyll- containing plants without true stems, roots, or leaves that live in the sea or brackish water, often attached to rocks or other surfaces. Successful and resilient species, they’ve been around for more than two billion years. Most seaweeds are photosynthetic, relying on sunlight as an energy source from which to produce food.
Although all have chlorophyll, many also contain other pigments in order to better absorb various wavelengths of light and capture more of the sun’s energy. As a result, they occur in three main color groups—red, green, and brown—which is a handy way of classifying them.
- Green seaweeds, such as sea lettuce, mainly contain chlorophyll, similar to their land-based brethren.
- Red seaweeds, which include dulse, laver, nori, agar, and Irish moss, primarily have red pigments, although they can look purple as well as a whole range of other related colors depending on the specific kinds of carotene pigments present. Because this class of pigments can be water soluble and heat sensitive, seaweed that looks red when uncooked may change to a dark green after cooking.
- Brown seaweeds, such as kelp, kombu, alaria, arame, wakame, sea palm, and hijiki, depend on brown pigments from other carotenoid pigments, fucoxanthin in particular. Although chlorophyll is also a component of brown seaweeds, its green color is masked by the brown.
Seaweeds have long been known for their nutritional attributes.
Traditional Chinese medicinal texts as far back as 2700 BCE mention seaweed’s medicinal qualities, including reference to its ability to reduce goiter. The Ebers Papyrus, the ancient Egyptian dissertation on medical care thought to have been written in 1550 BCE, also specifically includes the therapeutic use of seaweeds, as do Ayurvedic medicinal texts from the fourth century CE.
Current scientific research has keyed in on the phytonutrients in seaweeds, including lignans that may help prevent certain forms of cancer, including breast cancer. Tumor reduction, inhibition of cancer cell proliferation, free radical scavenging, and significant antioxidant activity have also been exhibited by red and brown seaweeds.
In addition, sulfated polysaccharides, a type of carbohydrate found in some of the brown seaweeds, are being explored as antiviral agents and as aids in preventing blood clots. Studies exploring polysaccharides in bladderwrack and kelp have also shown them to be a highly effective agent for reducing the effects of radiation toxicity.
With their ocean origins, sea vegetables are a valuable source of a wide array of trace minerals and, depending on the variety, small amounts of calcium, magnesium, and potassium.
Sodium and iodine are the most predominant, and it is iodine that’s responsible for the long history of use of seaweeds to treat goiter, even though this element was not identified and isolated until fairly recently.
Iodine was discovered by accident in 1811 and isolated as a specific component of seaweed when Bernard Courtois, a manufacturer of saltpeter (potassium nitrate), was in the process of making gunpowder for France. Rather than using wood ashes, as he usually did, to extract sodium carbonate, a necessary component when making saltpeter, he decided to use burned kelp instead. When the characteristic violet-colored vapors we now know to be iodine rose from the mixture, he quit the saltpeter industry and spent several months investigating the chemical reactions and properties of the newly discovered element.
Most of the iodine used commercially is obtained as a by-product of various mining operations because this is an inexpensive source, but seaweeds are still noted as the most concentrated food source of the mineral, and they have the benefit of providing an array of nutrients beyond iodine itself. The amount of iodine in different types of sea vegetables varies, depending on the age and condition of the plant, season and geographic location of harvest, the part of the plant consumed, how it is prepared, and how the seaweed is stored.
While iodine is an extremely important nutrient, it is possible to get too much if excessive amounts of seaweed are consumed.
This can have disruptive effects on the thyroid and cause a host of other problems. Recommended daily iodine intake for most healthy adults with normal thyroid function aged nineteen years and older is 150 micrograms, and the upper limit for this group of people is 1,100 micrograms. Recommended daily amounts and tolerable upper intake levels for children and adolescents are lower and vary depending on age group. Within these ranges, the amount that’s safe to ingest depends on the individual.
Some people are very sensitive to iodine and need to keep their daily intake on the low side. On the other hand, in some cultures where sea vegetables are a consistent part of the everyday diet, people may have adapted, making them able to tolerate higher levels.
As a class, red varieties of sea vegetables are consistently much lower in iodine than are brown ones, with nori containing the least (around 16 micro- grams per gram), in contrast with brown varieties, of which kelp and kombu contain the most iodine (averaging 1300 to 1500 micrograms per gram, although sometimes as much as 2500 micrograms per gram). Even within the brown varieties, there is a wide range of iodine content, with alaria as low as 110 micrograms per gram, a marked contrast to the much higher levels found in kelp.
All in all, a little seaweed goes a long way, especially the brown varieties because of their higher iodine levels. Use common sense and moderation in deciding how much to eat and how frequently, and also be aware that how they are cooked and what is also included in the meal can affect the amount of iodine ingested. The iodine in sea vegetables is mostly water soluble, which means that boiling them in water will result in much of the iodine being extracted into the cooking water. The roasting method appears to release the least amount of iodine, while steaming and frying may release moderate amounts.
Eating sea vegetables in a meal that also includes goitrogenic foods may also help to lessen the effects of their high iodine content.
Goitrogenic foods are those that contain substances that interfere with the thyroid gland’s uptake and utilization of iodine, which is necessary for thyroid hormone production.
They include soybeans (and products made from them), millet, peanuts, and cruciferous vegetables, such as broccoli, cauliflower, bok choy, brussels sprouts, rutabaga, turnips, and cabbage. For most people with healthy thyroid function, the potential negative effect of these foods is of no concern unless excessive amounts are consumed.
Although some of the goitrogenic compounds in these foods may also be deactivated when they’re cooked, eating goitrogenic foods with seaweeds not only provides a tasty combination but may help provide a healthy balance in terms of iodine as well.
Sea vegetables can be quite high in sodium, not surprising given that they grow in seawater. Excess salt can be removed by rinsing them with water. When you use them in a dish, you can reduce the amount of other salty seasonings or forgo them altogether.
Seaweed can also contain heavy metals, such as lead, arsenic, mercury, and cadmium, which are unhealthful to the human body at certain levels.
Given that these heavy metals occur naturally worldwide because they may be leached from bedrock, sea vegetables often contain at least trace amounts. But to minimize your exposure, it’s important to buy sea vegetables only from areas unpolluted by industrial waste.
Good sea vegetable companies are very particular about where and how their seaweed is harvested, dried, and stored and test it regularly for heavy metals, as well as herbicides, pesticides, and microbial contaminants. Always purchase sea vegetables from companies that have a commitment to providing a high-quality product.
Reprinted with permission from The Essential Good Food Guide by Margaret Wittenberg, copyright (c) 2013. Published by Ten Speed Press, a division of Random House, Inc.
Photo Credit: (c) 2013 Jennifer Martine