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THE NUTRITIONAL AND MEDICINAL VALUE OF SEAWEEDS
USED IN CHINESE MEDICINE
There are four seaweeds commonly used in Chinese medicine:
These seaweeds will be discussed briefly in this article.
Seaweed draws an extraordinary wealth of mineral elements from the sea that can account for up to 36% of its dry mass. The mineral macronutrients include sodium, calcium, magnesium, potassium, chlorine, sulfur and phosphorus; the micronutrients include iodine, iron, zinc, copper, selenium, molybdenum, fluoride, manganese, boron, nickel and cobalt.
Seaweed has such a large proportion of iodine compared to dietary minimum requirements, that it is primarily known as a source of this nutrient. The highest iodine content is found in brown algae, with dry kelp ranging from 1500-8000 ppm (parts per million) and dry rockweed (Fucus) from 500-1000 ppm. In most instances, red and green algae have lower contents, about 100-300 ppm in dried seaweeds, but remain high in comparison to any land plants. Daily adult requirements, currently recommended at 150 µg/day, could be covered by very small quantities of seaweed. Just one gram of dried brown algae provides from 500-8,000 µg of iodine and even the green and red algae (such as the purple nori that is used in Japanese cuisine) provides 100-300 µg in a single gram.
The amounts of seaweed ingested as food in Japan, or in supplements, is often considerably more than 1 gram a day. Studies show that the human body adapts readily to higher iodine intake, where the thyroid gland is the main tissue involved in use of iodine (it is a component of thyroid hormones). Huge portions of the world population get insufficient iodine because the land, plants, and animals that serve as common dietary sources are very low in iodine. In many countries, iodine is added to table salt to assure adequate levels are attained. However, some developing countries are still catching up and suffering from the effects of low iodine intake. China is has the largest population with a history of low iodine intake, followed by India.
Aside from iodine, seaweed is one of the richest plant sources of calcium, but its calcium content relative to dietary requirements pales in comparison to the iodine. The calcium content of seaweeds is typically about 4-7% of dry matter. At 7% calcium, one gram of dried seaweed provides 70 mg of calcium, compared to a daily dietary requirement of about 1,000 mg. Still, this is higher than a serving of most non-milk based foods.
Protein content in seaweed varies somewhat. It is low in brown algae at 5-11% of dry matter, but comparable in quantitative terms to legumes at 30-40% of dry matter in some species of red algae. Green algae, which are still not harvested much, also have a significant protein content, i.e., up to 20% of dry matter. Spirulina, a micro-alga, is well known for its very high content, i.e., 70% of dry matter.
Seaweed contains several vitamins. Red and brown algae are rich in carotenes (provitamin A) and are used, in fact, as a source of natural mixed carotenes for dietary supplements. The content ranges from 20-170 ppm. The vitamin C in red and brown algae is also notable, with contents ranging from 500-3000 ppm. Other vitamins are also present, including B12, which is not found in most land plants.
Seaweed has very little fat, ranging from 1-5% of dry matter, although seaweed lipids have a higher proportion of essential fatty acids than land plants. Green algae, whose fatty acid make-up is the closest to higher plants, have a much higher oleic and alpha-linoleic acid content. Red algae have a high EPA content, a substance mostly found in animals, especially fish. Seaweed has a high fiber content, making up 32% to 50% of dry matter. The soluble fiber fraction accounts for 51-56% of total fibers in green (ulvans) and red algae (agars, carrageenans and xylans) and for 67-87% in brown algae (laminaria, fucus, and others). Soluble fibers are generally associated with having cholesterol-lowering and hypoglycemic effects.
|Seaweeds: Ecklonia (upper left); Porphyra (middle left);|
Sargassum (lower left); Laminaria (right)
Probably the most widely known seaweed used for food is Porphyra, which literally means purple (see sample leaf below), reflecting its color in nature. The Chinese name is zicai, which means purple vegetable. It is classified among the red algae, which have red to purple pigments. Upon processing to yield the food, which is known in Japan as nori, the red pigments are lost and the final product has a dark greenish color. Nori is used to wrap sushi and for making numerous snacks. The other common food item is the low cost but highly nutritious kelp (kunbu in Chinese; kombu in Japanese). Kombu is usually sold in 5-6-inch dried pieces and can be found in health food stores and Japanese groceries (see sample package below). It is also sold as kombu that cooks quickly, vinegared, shaved kombu that needs little or no cooking, boiled, soy sauce flavored kombu, lightly pickled kombu, and powdered kombu that can be sprinkled on food or used in drinks. Dried kombu needs to be simmered for at least 20 minutes to soften it and flavor the liquid. If used only for flavoring stock, the kombu itself is removed from the liquid at the end of cooking and discarded. A third seaweed widely used in Japan is known as wakame (from Undaria pinnatifida). See the Appendix for information about seaweed utilization in Japan.
Seaweeds have a salty taste that is an indication that the material can disperse phlegm accumulation, particularly as it forms soft masses, include goiter, the thyroid swelling that indicates severe iodine deficiency. Following are the descriptions of the seaweeds from Oriental Materia Medica (2):
Kunbu (Laminaria and Ecklonia)
The descriptions for kunbu and haizao are quite similar. Yang Yifan (3) wrote about the differences between these commonly used seaweeds:
Haizao and Kunbu are salty and cold, and enter the liver, lung, and kidney meridians. Both can clear heat, transform phlegm, soften hardness, and dissipate nodules. They can also promote urination and reduce edema. In clinical practice, they are often used together to treat nodules such as goiter and scrofula.
There are some differences between the two herbs. Haizao is stronger in transforming phlegm and dissipating nodules, and it is more suitable for treating goiter and scrofula. Kunbu is stronger in softening hardness and reducing congealed blood; it is more suitable for treating liver-spleen enlargement, liver cirrhosis, and tumors.
One of the best known formulas with the seaweeds is Haizao Yuhu Tang, or the Sargassum Decoction for the Jade Flask (4). This formula of 12 ingredients includes Sargassum, Ecklonia, and Laminaria. It was used to treat a condition of goiter which was so severe it made the throat look like a large flask. However, these seaweeds have been adopted into formulas for treating other soft swellings, including ovarian cysts, breast lumps, lymph node swellings, lipomas, and fat accumulation from simple obesity.
Following is an outline report on Japanese utilization of seaweed and seaweed products, with data from 1998, except as noted, thanks to the Fisheries Information Newsletter (1).
Seaweed farming is highly developed in Japanese coastal areas. The main species grown there are Porphyra (nori), Laminaria (kombu) and Undaria (wakame). These alone have accounted for 98% of overall Japanese seaweed production since 1984. The balance is made up of minor traditional (Monostroma, Enteromorpha and Cladosiphon) or experimental (Meristotheca and Grateloupia) crops or wild stock harvests.
Nori is a traditional food used, for example, to make sushi, which has been a highly profitable crop over the past century. Since the reproduction cycle's summer phase was discovered by Dr Kathleen Drew-Baker, farming has become easier and more lucrative - so much so that no imports have occurred since 1976. Porphyra cultivation is the largest sub-industry in Japanese aquafarming, employing 16,800 workers. In 1998, output stood at 10,326 million nori sheets, i.e. equivalent to 396,615 tons of fresh produce.
Kombu is the most widely sold seaweed in Japan. It is an "all-purpose" product although it is most commonly used for bean-curd soup with kombu.
Wild stock still accounts for a major share of output. Several species of the genus Laminaria are used in the Japanese food industry. Laminaria japonica takes the lion's share of kombu production with a raw-material tonnage of 141,875.
Undaria pinnatifida cultivation is a relatively recent development in Japan and wakame is served as a luxury food on Japanese and Korean tables. It is highly sought after for bean-curd soup or salads. Raw-material production, standing at 73,508 tons, is unable to meet Japanese demand for wakame.