Chemopreventive herbs

Chemoprevention can be defined as the use of chemical agents to prevent the occurrence of cancer. Animal studies have demonstrated that several hundred different compounds have shown chemopreventive properties, and the consequence of this large number of compounds is that they prob- ably act through a variety of different mechanisms. Epidemiological data on cancer indicate that chemopreventive agents in the diet may play an important role in preventing cancer. It is known that vegetarian popu- lations have significantly lower incidences of breast, colon, and prostate cancer. Whereas the cause of this lower incidence may be related to the decreased amount of fat in the vegetarian diet, another factor is the abun- dance of chemopreventive agents in vegetarian diets, including terpenes, organosulfides, isothiocyanates, indoles, dithiolethiones, polyphenols, fla- vones, tannins, carotenoids, and vitamins. 55,56

Specific mechanisms for chemoprevention are varied. Some dietary agents may block carcinogen activation by inducing phase II enzymes involved in precarcinogen xenobiotic detoxification, 55 leading to a more rapid excretion from the body. Other dietary compounds—for example, carotenes, vitamins C and E, and some flavonoids—have antioxidant activity that prevents the formation of free radicals (see Figure 6.3 , Chapter 6). Free radicals can induce mitochondrial and nuclear DNA damage leading to carcinogenesis. Several herbal products appear to have potential as effective chemopreventive agents. 57

Green tea The leaves and leaf buds of Camellia sinensis (L.) Kuntze (family Theaceae), an evergreen shrub indigenous to mountainous regions of Southeast Asia, are carefully processed in China and Japan to produce green tea. It is pre- pared by rapidly drying the freshly harvested leaves in copper pans over

a mild artificial heat or by first subjecting the fresh leaves to steam for at least thirty seconds and then drying. Both processes inactivate oxidative enzymes (primarily polyphenol oxidase), which prevent the destruction of polyphenols (catechins), the antioxidant constituents of tea. Fermentation of the withered, rolled, and crushed fresh leaves at high humidity pro- duces black tea, which contains the less active oxidation and condensation products of catechins. 58

The chemopreventive constituents of green tea are the polyphenols, which are complex flavonoid structures also called catechins. The main polyphenols are epicatechin, epicatechin gallate, epigallocatechin gallate

210 Tyler's herbs of choice: The therapeutic use of phytomedicinals (EGCG), and proanthocyanidins. Because they are water soluble, a cup of

green tea contains 80–140 mg of polyphenols as well as 50 mg of caffeine. Commercially available green tea extracts are standardized to 60–80 per- cent polyphenols.

Green tea and green tea polyphenols have been shown to demonstrate chemopreventive and anticancer effects in a number of animal tumor mod- els. The polyphenols were found to scavenge hydrogen peroxide and super- oxide anions ( Figure 6.3 , Chapter 6) and reduce oxygen free-radical damage.

In addition, they increase antioxidant activity and phase II enzymes in mouse skin, liver, lungs, and small bowel. Green tea polyphenols—particularly EGCG—were more potent scavengers of polycyclic aromatic hydrocarbon free radicals (carcinogens) than the widely used food antioxidant BHT. 59

Although epidemiological studies and clinical trials indicate a potential benefit of green tea consumption in the prevention of cancer, a direct cause-and-effect relationship has not been proved through clinical trials in humans. Because of the lack of side effects and even though the chemopre-

ventive action of green tea still must be determined, the use of green tea or green tea extract at dosages normally consumed by humans may have ben-

eficial effects. The chemopreventive daily dose of green tea has not been established. Overconsumption may produce effects associated with exces- sive caffeine intake, such as insomnia, nervousness, and tachycardia.

Chemopreventive herbs