and the Uses of Tannins in Chinese Medicine
Chinese gall, also referred to as Chinese gallnuts (or nutgalls), is a plant excretion produced when irritants are released by the larvae of gall insects, such as those of the Cynipidae family, the gall wasps. A major commercial source of medicinal gallnuts is oak trees, which yield the Chinese herb moshizi, but a similar material is found on Chinese sumac (Rhus species), produced by the activities of a type of aphid; these galls are called wubeizi by Chinese herbalists. The plant secretes the liquid gall, comprised mainly of tannins, that hardens to become the "nut." Gallnuts are a native product of China and Southeast Asia, but are also produced in small amounts in Turkey, India, Japan, and Korea. The annual yield of gallnuts in China is about 95% of the total world yield. Guizhou and Yunnan are the biggest gallnut-producing areas in China
Oak galls are shown in the upper right photo; the other pictures are sumac and its galls.
Gallnuts from oak and sumac contain the highest naturally occurring level of tannin (gallotannin): 50-75%. They also contain 2-4% each of the smaller molecules gallic acid and ellagic acid (see structures next page) that are polymerized to make tannins. The tannin of gallnuts has been used for centuries for tanning of leather (a process involving coagulating proteins). Now, gallnut extracts are widely used in pharmaceuticals, food and feed additives, dyes, inks, and metallurgy. Gallotannin (comprised of molecules of gallic acid attached to a central glucose) is obtained from natural gallnuts by extraction with hot water; gallic acid is obtained by the hydrolysis of tannic acid with sulfuric acid. When heated above 220°C, gallic acid loses carbon dioxide to form pyrogallol; other chemicals in demand, such as propyl gallate, octyl gallate, dodecyl gallate, syringic acid, and trimethoxybenzoic acid, can all be made from gallnuts.
There are additional sources of these desired components. Since 1989, Chinese manufacturers of gallnut products began using tara (spiny sappan; Caesalpinia spinosa) imported from Peru for production of gallic acid. While sappan heartwood (from Caesalpinia sappan) is used in Chinese medicine as a blood vitalizer, tara is an extract from the fruits pods of the related C. spinosa. The tannin concentration is high in the pods; a typical analysis is: gallotannin 53.1%, gallic acid 9.5%, ellagitannin 6.9%. The shift to use of tara resulted from over-collection of gallnuts in China leading to a high price for them. But, this shift to alternative sources of tannins led to a recovery of gallnut supplies and their reduction in price and restoration as major market items; in addition, Caealpinia spinosa was also cultivated in Southwest China (e.g., Yunnan Province), reducing the need for imports from Peru. Other major sources of tannins are chestnut, mimosa, quebracho, valonea, and myrobalans (fruits and twigs).
Tannins comprise a large group of natural products widely distributed in the plant kingdom. They have a great structural diversity, but are usually divided into two basic groups: the hydrolyzable type and the condensed type. Hydrolyzable tannins include the commonly-occurring gallic acid and the ellagic acid.
As their name infers, hydrolyzable tannins are readily degraded into smaller molecules. Hydrolyzable tannins are present in many different plant species but are found in particularly high concentrations in nutgalls growing on Rhus semialata (Chinese and Korean gallotannins) and Quercus infectoria (Turkish and Chinese gallotannins), the seedpods of Caesalpinia spinosa (Tara tannins), and the fruits of Terminalia chebula. The hydrolyzable tannins react with proteins to produce the typical tanning effect; medicinally, this is important for treatment of inflamed or ulcerated tissues. They also contribute most of the astringent quality that is noted when drinking tannin-containing beverages.
The condensed tannins, also known as proanthocyanidins, are much more resistant to decomposition and merely yield polymers or precipitates when acidified. The basic monomer of condensed tannins is epicatechin and catechin (identical except for orientation of the molecules, see below); these are then extended by the successive addition of similar phenol units to produce polymers (polyphenols). Traditionally, important commercial sources of condensed tannins are the heartwood of Schinopsis spp. (quebracho tannins), the bark and/or heartwood of Acacia catechu (catechu tannins) and Acacia mollisima (mimosa tannins), and the bark of Rhizophora (mangrove) and Eucalyptus species. The application of proanthocyanidins as health protective antioxidants was popularized through extracts of pine bark (particularly the branded product Pycnogenol) and grape seeds, which can be used interchangeably. In addition, proanthocyanidins were recognized as beneficial for vision, and are active components of bilberry extract sold for this purpose.
Although both types of tannin have been used to treat diseases in traditional medicine, the hydrolyzable tannins have long been considered official medicinal agents in Europe and North America. They have been included in many pharmacopoeias, in the older editions in particular, and are specifically referred to as tannic acid. These were recommended for treatment of inflammation and ulceration, including topical application for skin diseases and internal use for intestinal ulceration and diarrhea. Now, the condensed tannins also have important medicinal roles, such as stable and potent antioxidants. In China, tannin-containing substances, such as galls, pomegranate rinds, and terminalia fruits, are used in several medicinal preparations.
Herbs that have tannins as their main component are astringent in nature and most such medicinals are listed in the Materia Medica category of astringents. The most common applications are treating:
The following table illustrates some of the herbs of Chinese medicine with tannins as a major component and their uses; these are listed in the Materia Medica category of astringents (except catechu is listed with topical therapies):
|Herb (Botanical Name/Pinyin)||Taste, Nature||Applications|
|Acacia catechu gall|
|bitter, astringent, neutral||cough, red and white dysentery; topically for skin ulceration|
|Cedrela sinensis root bark |
|bitter, astringent, cool||red-white dysentery, hematochezia, morbid leucorrhea, functional bleeding, involuntary emission|
|Punica granatum rind|
|astringent, warm||chronic diarrhea and dysentery, hematochezia, rectal prolapse, involuntary emission, functional bleeding, morbid leucorrhea, intestinal parasites|
|Quercus acutissima fruit|
|astringent, mildly warm||diarrhea, rectal prolapse, hemorrhoidal bleeding|
|Quercus infectoria gall|
|bitter, warm||red-white dysentery, hyperhidrosis, oral ulceration, leucorrhea, hemorrhoids, rectal prolapse; topically for skin lesions|
|Rhus semialata gall|
|sour, salty, cold||cough, rectal prolapse, spontaneous sweating, night sweating, epistasis, functional bleeding; topically for wound bleeding, ulcerous dermatitis, toxic skin swelling|
|Rosa laevigata fruit|
|sour, astringent, neutral||enuresis, frequent urination, morbid leucorrhea, persistent diarrhea, involuntary emission|
|Terminalia chebula fruit|
|bitter, sour, neutral||chronic diarrhea and dysentery, rectal prolapse, aphonia due to longstanding cough, hematochezia, leucorrhea, night sweating, involuntary emission|
There are tannins in other herbs that should be mentioned; for example: cornus fruit, also categorized as an astringent, has tannins but they are probably not the main active component. Others include tea (listed with diuretics); rhubarb root (listed with purgatives); geranium (listed with antirheumatics); sanguisorba (listed with hemostatics), and both Polygonum bistorta (heat clearing) and Polygonum multiflorum (blood nourishing). Because tannins are widely distributed in nature, many herbs have them to some extent, but they may have only a small or no significant influence on their therapeutic activity. A Chinese-Tibetan herb recently popularized as an "adaptogen," rhodiola, is high in tannins (about 16-18% of the root, but as much as 40% of the hot water extract). In Ayurvedic medicine, the tannin-rich myrobalans fruits, such as amla (Emblica officinalis), are used like the Chinese Polygonum multiflorum (heshouwu) as anti-aging remedies.