Herbal Medicine The Scientific Evidence S. Sotheeswaran University of the South Pacific, Suva, Fiji Herbal medicine uses herbal extracts in the treatment of human illnesses. This system of medicine is popular, even today, among the indigenous peoples of Africa, America, Asia, and the South Pacific. Even Westerners who usually have recourse to modern Western medicine are not strangers to herbal medicine. How effective are these herbs and herbal extracts in treating human illnesses? It is well known that an antimicrobial drug, quinine (11,was shown to be the active constituent of the bark of the Cinchona tree used by the people of Peru to treat malaria (1).The leaves of the foxglove plant, Digitalispurpurea, figure in an African herbal remedy to treat heart failure and dropsy. Two cardiac glycosides, digitoxin (2) and digoxin (31, have been found to be the active substances in foxglove and both are now official drugs in the British pharmacopoeia (2). The two examples of the efficacy of herbal medicines cited above are known to many chemistry educators. This paper discusses less well-known information on the efficacyof some herbs.
Garlic Garlic is the bulb of Allium satiuum and is used as a spice and seasoning in foods. Garlic has been recognized as an antibiotic medicine both against bacteria and fungi for centuries. More recently it was used to treat gangrene and dysentery in the first and second world wars, and modem research has provided new evidence that taking garlic regularly can help maintain a healthy heart and circulation (3). Laboratory investigations (4) have shown that garlic juice diluted to one part in 125,000 inhibits the growth of bacteria of the genera Staphylococcus, Streptococcus, Vibria, andBacillus. Garlicjuice also exhibits (4) a broad spectrum of activity against zoopathogenic fungi and many strains of yeast, including some that cause vaginitis. The antibacterial activity of garlic has been traced by Carvallito and Bailey (5)to the ethanol solubles of garlic. The main component of the ethanol. extracts has been named allicin and was characterized as 2-propenethiosuliinate (4). The antifungal activity of allicin was subsequently demonstrated by Yamada and Azuma in 1977 (6).Allicin
CH,O digitom
digitoxom
I I
I digitoxom I
digitoxose digitoxose
digitoxose
Quinine 1
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Journal of Chemical Education
Digitoxin 2
Digoxin 3
accounts for the odor of garlic. Antithrombotic activity in garlic extracts was detected by Apitz-Castro (7)and the antithrombotic substance was shown to be ajoene (5). Ajoene (5) and allicin (4) are both unstable substances and are best obtained from fresh garlic (4).
Allicin 4
Ajoene 5 Onion Onion, Allium cepa, is botanically related to garlic and is used world wide in salads, curries, and other preparations. Onion bulbs are reported (8)to have diuretic, aphrodisiac, and carminative properties. It is also claimed (8) that onion bulbs, when eaten raw with salt, relieve stomachache. Though these claims have not yet been verified it has been established that freshly cut onion contains the antifungal and the antibiotic substance allicin (4) (8).It is also reported that onions contain a substance which increases the coronary flow. It is likely that this substance is ajoene (5),which is biosynthesized from allicin (4) (4). Ephedra siniw Chinese herbal medicine uses Ephedra sinica i n the treatment of asthma (9).This plant was subsequently shown, by scientific investigation (101, to contain ephedrine. Ephedrine (6)is known to dilate the air passages of the lungs.
Ephedrin 6
Phmbago zeylanica The leaves and roots of Plumbago zeylanica are a popular remedy (2) for parasitic skin diseases in Africa and Asia. Strong antimicrobial and antifungal activities have been detected (2) in the leaf and root extracts of Plumbago zeylanica. Anaphthaquinone, plumbagin (71, was isolated from this plant and was shown to be responsible for the antimicrobial and antifungal activities. A very dilute solution (1:50,000) of plumbagin was shown to be lethal to a wide spectrum of bacteria and to pathogenic fungi (21, and hence the use of the mot and leaf extracts to treat skin diseases is considered justified (2) and explicable on a scientific basis. Ageratum conyzoides and Ocimum gratissimum The leaves of some plants such as Ageratum conyzoides and Ocimum gratissimum are used to treat diarrhea and dysentery (2, 11). The wound healing property of A. conyzoides also is well documented (2). The curative powers of
Thymol
Plumbagin 7
8
these two herbs are traced (12,131 to the volatile oils present in these plants. The antimicrobial activity in the leaves of 0. gratissimum has been shown to be due to the major component of the volatile oil, thymol(8). Montanoa tomentosa In many parts of South America the extracts of the leaves and twigs ofMontanoa tomentosa are used as a contraceptive, to stimulate menstruation, facilitate labor, or to terminate an early pregnancy (2). Phytochemical investigation of this plant has resulted (14) in the isolation of a very potent fertility-regulating agent called zoapatanol(9).
Zoapatanol 9 These are only a few of the cases where evidence is available for the scientific basis for herbal remedies. Out of the 6,000 or so medicinal herbs used by traditional healers all over the world. onlv about 5% have been scientificallv examined. Out of these only a few have been shown to-contain active natural products responsible for the medicinal properties claimed for the extracts. In some cases, the natural products isolated had medicinal properties which were totally different to those of which the herb was used. For example, the rosyperiwidde plant, which is botanically described as Cathamnthus roseus has been used (15) as an antidiabetic, emetic, diaphoretic, purgative, hemostatic. emmenaeoeue. - - . antimalarial drne and for the treatment of sore throat, chest complaints, and laryngitis. Phytochemical investigation of rosyperiwinkle showed it to be a treasure house of alkaloids which have antileukemic, antiviral, diuretic, and CNS active properties (16).lbenty years ago, four children out of every five suffering from leukemia died. Now it is the reverse and four out of every five survive by using the antileukemic d r u ~ leurocristine (or vincristini) (10;isolated from rosyperiwinkle. Leurocristine (10, is a vcw minor alkaloid (-0.03 dkgofdried leaves, of rosyperiwinfie. It would never be po&gle to cure leukemia in children bv " eivine - them extracts of rosweriwinkle because the active principle is present in only very small amounts. This is one of the major problems of using herbal medicine. The active principle will be present in varying amounts in the herbs depending on the place where the herbs are mown and also on the age of the herbs. Certain natural pmducts are not found inplants at all times but are formed or lust duringmaturitv The identitv ofthe herb re collected withthe help is also important. ~ e r b ~ ausuaily
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Volume 69 Number 6 June 1992
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