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Constituents of Herbal Medicines

Through photosynthesis, plants manufacture carbohydrates and give off oxygen, and through this process they create metabolic pathways that provide building blocks for the production of a vast array of compounds.

In medicinal plants these include minerals, vitamins and trace elements, and a vast assortment of substances known to have specific therapeutic actions in the body. The more widely known of these are as follows:

Phenols

Phenols, sometimes called phenolic compounds, are a large class of secondary plant compounds. They are aromatic alcohols and the building blocks of many plant components, and generally have antiseptic, antibacterial and anthelmintic actions. The simplest of the class is the antimicrobial phenol (C₆H₅OH). Another simple phenolic compound is salicylic acid, which forms glycosides found in willow, cramp bark, and meadowsweet, and has antiseptic, painkiller and has anti-inflammatory properties. It forms the basis of aspirin.

Other compounds include hydroxycinnamic acids, including caffeic, ferulic and sinapic acids, which form the basis of phenolic esthers, coumarins, glycosides and lignans; also cynarin, the main constituent of globe artichoke, with its liver-protective and cholesterol-lowering actions, and curcumin, the main component from turmeric, which is famous as an anti-inflammatory agent and also for its ability to lower blood pressure and protect the liver (Pengelly, 2004).

Other phenolic compounds include stilbenes, which occur in grape skins and red wine, with antioxidant, anti-inflammatory, anti-clotting and anti-allergy actions, and quinones, including anthraquinones (see overleaf) and naphthaquinones. The latter have antimicrobial and anti-tumour properties, for example juglone in walnut bark, and lapachol in pau d'arco.

Coumarins

Coumarins occur widely in plants, including black cohosh, wild oats, angelica, and horse chestnut, and they are generally antimicrobial and antifungal. The evocative smell of hay is due to the presence of coumarins that are lactones of hydroxycinnamic acids. They generally occur as glycosides, for example aesculin from horse chestnut. Dicoumarol that is originally derived from sweet clover (Melilotus officinalis) is used a strong anti-clotting agent, in the form of Warfarin, in allopathic medicine.

Furanocoumarins include angelican and archangelican from angelica roots, which are antispasmodic. These need to be used cautiously as they can cause photosensitivity, increasing the effect of sunlight on the skin, but they can also be used therapeutically for vitiligo and psoriasis. (Pengelly, 2004).

Anthraquinones

Anthraquinones occur as glycosides, and have a yellow-brown colour that has often been used for producing commercial dyes. They are found in senna, aloes, yellow dock, and cascara, and they pass unaltered through the stomach and small intestine, to be converted to their active form by micro-organisms. 8–12 hours after ingestion, they stimulate peristalsis and inhibit water reabsorption in the large intestine, producing a laxative effect. Their peristaltic action can sometimes cause griping in the bowel, so they are best combined with herbs such as peppermint, ginger or fennel. They are contraindicated in spastic bowel problems and pregnancy, and should not be used over a long period of time as they can reduce the normal bowel reflexes and cause habituation.

Tannins

Tannins occur widely in nature, often as glycosides, and they represent the largest group of polyphenols. Tannins are the main therapeutic constituents in witch hazel, agrimony, raspberry leaf, and meadowsweet. Their main therapeutic action is astringent, brought about by their ability to bind albumin, a protein in the skin and mucous membranes, to form a tight, insoluble protective layer that is resistant to infection. On the skin and in, the delicate linings of the mouth, the respirary, digestive, urinary and reproductive systems, tannins can separate bacteria that threaten to invade from their source of nutrition.

Tannins occur either as hydrolysable or condensed tannins. Sxq hydrolysable tannins have the ability to protect the skin and mucosa from irritation and to reduce swelling and inflammation. They have a drying effect that is useful for curbing over-secretion of mucous, bleeding and diarrhoea. Herbs rich in tannins make useful mouthwashes for infected and bleeding gums, gargles for sore throats, eyewashes for inflamed or infected eyes, remedies for catarrh, inflammation of the GI tract, diarrhoea, and heavy menstrual bleeding, compresses to heal burns, abrasions and cuts, and lotions to bathe haemorrhoids and inflammatory skin conditions.

Condensed tannins include oligomeric procyanidins, which have become widely known for their antioxidant and cardiovascular properties. They are found in green and black tea, red wine and grape seeds. Grape seed extract has been demonstrated to have strong antioxidant activity, protecting against free radical damage and cardiovascular disease, and preventing degeneration of connective tissue.

Filipendula, or meadowsweet, is a source of tannin and can be used as an astringent in fighting infection.

Flavonoids

Flavonoids and flavonoid glycosides occur widely in nature and impart a yellow, orange and red colour to fruits, vegetables and flowers. With their antioxidant actions, they are an important part of our daily diet, having a beneficial effect on the heart and circulation, strengthening and healing blood vessel walls and enhancing our resilience to stress. They act synergistically with ascorbic acid to enhance the body's ability to metabolise it. Flavonoids are also anti-inflammatory (e.g. quercitrin), hepato-protective (e.g. silymarin and quercitrin), anti-tumour, anti-viral and hypotensive. Herbs rich in flavonoids (e.g. kaempherol, myricitin, quercitrin) protect against cardiovascular disease and are used to treat vascular problems, including for example venous insufficiency, bruising, piles and nose bleeds.

Isoflavones, such as genistein from soya, have a similar structure to oestrogen. They bind to oestrogen receptors in the body and are known as phytoestrogens. They occur in herbs includingare shatavari, wild indigo, liquorice, red clover and black cohosh. They have been found to help prevent tumours and breast cancer, and to relieve menopausal symptoms.

Anthocyanins and anthocyanidins are found in red, blue and black fruits, and particularly found in high amounts in grape skins, elderberries and bilberries. They also occur in herbs, including ginkgo, cat's claw and corn silk. They have antioxidant actions that protect against oxidative damage, particularly in the eyes and connective tissue.

Terpenes

Terpenes, or terpenoids, occur widely in a variety of forms, including monoterpenes, sesquiterpenes and triterpenes.

Monoterpenes are the main components of volatile oils (see next page), and include bitter iridoids, as in the sedative valepotriates in valerian, hypotensive asperulsides in cleavers, and paeoniflorin in peony, which has anti-inflammatory, febrifuge and sedative actions.

Cimicifuga, or black cohosh, contains isoflavones, which have been found to prevent tumours.

Sesquiterpenes are also found in volatile oils or as lactones, and have a bitter taste, anti-inflammatory and antimicrobial actions. Sesquiterpenes are found in myrrh, hops, German chamomile and vitex, and sesquiterpene lactones occur in boneset, feverfew, yarrow, wormwood, globe artichoke and elecampane.

Triterpenes have a similar structure to steroids (see below).

Bitters

Bitter principles is a term for a group of chemicals that have a very bitter taste and a cooling effect. They are diverse in structure but have certain therapeutic actions in common, and include mostly terpenes, flavonoids and some alkaloids. Through their effect on the bitter receptors on the tongue, they promote secretion of digestive enzymes from the stomach and intestines, flow of bile from the liver and release of hormones.

Bitters are prescribed for poor appetite and digestion, gastritis, heartburn, to regulate blood sugar, to relieve allergies and inflammation and to aid convalescence. Many bitter herbs have other actions; some are relaxant or sedative like hops and valerian, others are anti-inflammatory like devil's claw, and some, like marigold, exert a beneficial action on the immune system, acting as natural antibiotics and anti-neoplastics. Well known “bitter tonics” include dandelion, cleavers, blessed thistle, wormwood, dock root and gentian.

Triterpenoids and Saponins

Triterpenoids represent a large and diverse group that includes phytosterols, triterpenoid saponins, steroidal saponins and cardiac glycodises.

The leaves of Cynara scolymus, or globe artichoke, contain sequiterpene actones, which can reduce inflamation and protect against infection.

Phytosterols

Phytosterols such as sitosterol and stigmasterol are vital to the formation of cell membranes and help regulate cholesterol, for example guggulsterones in guggulu which lower cholesterol and triglycerides by their regulatory effect on the thyroid are a good example. Phytosterols have been used as building blocks for making steroid drugs and may have the ability to inhibit tumour formation; withanolides in ashwagandha, for example, have antitumour and hepatoprotective properties (Pengelly, 2004).

Saponins

Saponins are glycosides that form a soap-like lather when they are mixed with water and precipitate cholesterol. Herbs containing saponins have a bitter taste and haemolytic activity. They can dissolve red blood cells walls so should never be injected into the blood stream. Taken orally, however, they are hardly absorbed through an intact intestine and help to promote digestion and absorption, for example of nutrients including calcium and silicon. Some, like horse chestnut, have a beneficial action on blood vessel walls, some decrease blood coagulation, blood sugar and cholesterol levels (Tillotson, 2001). Some, like goldenrod and horsetail, are duiretic. Others like mullein are expectorant, and several have hepato-protective and immuno-modulating effects, for example Korean ginseng and liquorice.

Triterpenoid saponins

These help regulate steroidal hormonal activity and counter the effects of stress, and often have antifungal properties. Herbs containing these hormone-regulating properties are known as adaptogens, the most famous of which is panax ginseng. Others include liquorice, wild yam and fenugreek. Some, like wild yam and liquorice, act as anti-inflammatories.

Solidago virgaurea, or goldenrod, contains saponins, which promote digestion and absorption.

Steroidal saponins

These, such as diosgenin from wild yam, are used in the body as building blocks for the production hormones secreted by the testes, ovaries and adrenal glands, and for vitamin D.

Cardiac glycosides

Discovered in 1785 in foxglove, these have been widely researched for their ability to increase cardiac output by effecting the force and speed of heart contractions, which is beneficial in dealing with heart failure. Herbs containing these are generally for use by practitioners only.

Volatile Oils

The exotic perfumes and delicious tastes of aromatic herbs are derived from volatile oils, which are complex combinations compounds, and the different combinations of these produce the wide variation in scent and therapeutic effects in the herbs containing them. Up to 60 different chemical constituents have been identified in some oils. Categories of volatile oils include terpenoids, including monoterpenes and sesquiterpenes and phenylpropanoids. Herbs rich in volatile oils have to be stored carefully in sealed bottles or containers as volatile oils can evaporate easily.

All volatile oils are antiseptic, stimulating the production of white blood cells and enhancing immunity. Many oils have antibacterial, antifungal and antiviral actions, as well as anti-inflammatory and antispasmodic properties, especially those containing sesquiterpenes, such as for example azulene in German chamomile which is particularly applicable for the relief of inflamed and irritated conditions of the digestive tract, while those in dill relax spasm and colicky pain in the gut. Some oils have an expectorant action such as in thyme and hyssop, clearing mucous from the chest, while others are diuretic, useful for fluid retention and urinary infections. While they exert their beneficial effects on the physical body, they also reach the brain and nervous system and have a wide range of mento-emotional applications.

Fixed Oils

These are the lipids found in all plants, especially in the seeds, and they contain fatty acids which are either saturated, monounsaturated or polyunsaturated. They are vital for growth and health, the formation of cell membranes andthe healthy functioning of the immune and cardiovascular systems. Two fixed oils that exist in every cell, particularly in the nervous system, known as essential fatty acids, are linoleic (found in evening primrose oil, and saw palmetto berries), and linolenic acid (found in flax seed). These essential fatty acids are not able to be synthesised in the body and so need to be taken in through our diet. In the body linoleic acid is converted into gamma linolenic acid (GLA); atopic allergies such as eczema and asthma and other immune problems have been related to the lack of the enzyme responsible for this conversion in some individuals. Evening primrose oil contains GLA and for this reason is very useful for treating such problems.

Polysaccharides

These large sugar molecules are found widely in the plant world, for example in fructose, glucose, and cellulose, and consist of chains of sugars linked to other molecules. They include mucilage, gums, and fructans. Some polysaccharides, particularly beta-glucans found in, for example, reishi and shiitaki mushrooms, have immune-stimulating properties. They achieve this through throughactivating cytokines which enhance the production of white blood cells and antibodies, and with their anti-inflammatory and anti-tumour properties. Liquorice, codonopsis, aloe vera, rehmania and cinnamon also contain immune-stimulating polysaccharides.

Mucilage

This is a sugary, gel-like substance with the ability to draw water to it to form as viscous fluid. When taken orally, mucilage coats the mucous membranes of the digestive, respiratory and genito-urinary tracts and protects them from irritation and inflammation. Herbs rich in mucilage including slippery elm, marshmallow and plantain are prescribed for their cooling and soothing properties. They can relieve diarrhoea by reducing peristalsis caused by irritation of the gut lining, but can also be used as laxatives as they absorb water into the bowel and bulk out the stool, e.g. psyllium seeds.

Aloe Barbadensis, or aloe vera, contains polysaccharides, which stimulate the immune system.

Gums

These are protective and healing exudates composed of monosaccharides which are released when a plant is damaged. Those in guggulu enhance the liver's metabolism of cholesterol by promoting the uptake of LDL cholesterol from the blood (Tillotson, 2001). Marigold is high in gums with antimicrobial, antifungal and anti-inflammatory effects.

Fructans

These are composed of fructose and occur especially in the Asteraceae family as inulin. Herbs rich in inulin include elecampane, globe artichoke, goldenrod, gentian, codonopsis and burdock. Inulin helps regulate blood sugar and enhances the immune system by acting as a prebiotic to nourish the beneficial flora in the gut.

Calendula officinalis, or marigold, is high in gums.

Alkaloids

A diverse group of chemicals that contain a nitrogen-bearing molecule and are pharmacologically very potent. Many of the more toxic plants contain alkaloids, such as atropine in belladonna and morphine from the opium poppy, the first alkaloid to be isolated in 1806 (Tillotson, 2001). Caffeine, ephedrine, quinine, strychnine, piperine, nicotine and codeine are all alkaloids with diverse actions, ranging from stimulants, bronchodilators, antimicrobials and anti-inflammatories, to narcotics and painkillers.

Sources

Pengelly, A. (2004), The Constituents of Medicinal Plants. Oxford: CABI Publishing.

Huang, K. (1993), The Pharmacology of Chinese Herbs. Boca Raton: CRC Press.

Tillotson, A. K., et al (2001), The One Earth Herbal Sourcebook. New York: Kensington Publishing Corps.