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Dietary Phytochemicals May Play Major Role in Cancer Prevention

Dietary Phytochemicals May Play Major Role in Cancer Prevention

WASHINGTON-Phytochemicals are naturally occurring substances found in plants. They differ from vitamins and minerals in that they have no known nutritive value, but many have been found to have a protective effect against cancer, according to reports from the American Institute for Cancer Research (AICR) annual research conference.

Although phytochemical research is in its infancy, scientists at the AICR conference expressed excitement about their potential ability to inhibit cell conversion from normal to malignant at a variety of stages. Many speakers expressed a belief that a lifelong diet rich in phyto-chemicals can have a significantly positive effect on cancer risk.

Tea as Chemopreventive

Hasan Mukhtar, PhD, said that the polyphenols found in green tea (and possibly those in black tea, although this has been less well studied) may have a chemo-protective effect against skin, lung, fore-stomach, esophagus, duodenum, pancreas, liver, and colon cancer.

"Polyphenols appear to effectively protect mice against all three stages of cancer: initiation, promotion, and progression," said Dr. Mukhtar, professor and research director, Departments of Dermatology and Environmental Health Sciences, Case Western Reserve University.

In one series of studies, green tea, administered to mice either topically or orally, inhibited skin tumor growth and
development. In other studies, administration of green tea reduced the tumor burden that occurs after exposure to ultraviolet-B radiation, which is the major risk factor for skin cancer in humans.

Polyphenols found in tea may inhibit binding of carcinogenic substances to genetic material produced in cells as a re-sult of carcinogenic exposure. They may also protect against cancer by inhibiting metabolism of the carcinogenic material.

Dr. Mukhtar said that, to date, experimental studies in humans have been inconclusive. However, in vitro studies on human prostate cancer cells indicate that green tea polyphenols may protect against cancer development in that organ. He said that further studies could define the population that might benefit from drinking green tea or by using tea in other edible substances.

Substances in Soybeans

Stephen Barnes, PhD, professor of pharmacology and toxicology, University of Alabama at Birmingham, said that the isoflavones genistein and daidzein have been linked to anticancer effects on mammary tumors in laboratory animals. He sees significant potential for these chemicals, which are found in soybeans.

Dr. Barnes is currently conducting pilot studies to gather data in advance of large clinical trials. In one study, healthy pre- and postmenopausal women are consuming diets with and without soy to look for properties of breast fluid that may indicate alterations in breast cancer risk. In another study, elderly men with elevated prostate-specific antigen (PSA) are drinking soy beverages to see if isofla-vones lower PSA levels.

Pamela Crowell, PhD, assistant professor of biology, Indiana University-Purdue University at Indianapolis, told the conference audience that perillyl alcohol and d-limonene (monoterpenes derived from cherries, lavender, and citrus peel) show chemopreventive activity against mammary, lung, liver, and skin cancers in mice.

Perillyl alcohol also induced contact inhibition in cultured human pancreatic carcinoma cells and inhibited their anchorage-independent growth. She said that these monoterpenes also appear to cause tumor cells to shift to a less malignant type.

Eat Your 'Cruciferous' Vegetables

In his presentation, Stephen Hecht, PhD, said that natural and synthetic isothiocyanates, occurring in conjugated forms called glucosinolates and found in cruciferous vegetables, are effective inhibitors of tumors of the lung, esophagus, mammary gland, liver, bladder, and forestomach in rats.

"When the vegetables are chewed, the glucosinolates are hydrolyzed to isothiocyanates by the action of the enzyme myrosinase, and chemopreventive agents are released," said Dr. Hecht, director of research, Naylor Dana Institute for Disease Prevention, American Health Foundation, Valhalla, NY.

Watercress and Smokers

Dr. Hecht is especially interested in the effects of watercress consumption on levels of carcinogens in smokers. "Forty-five million Americans are smokers and many are addicted to nicotine," he said. "We need to look for compounds that might prevent lung cancer in people who are not about to quit smoking."

Gary Stoner, PhD, said that tobacco smoking is a leading cause of esophageal cancer, which is rare but rapidly increasing in the United States and endemic in China, Russia, Iran, Afghanistan, and Rhodesia. According to Dr. Stoner, Lucius Wing Professor of Preventive Medicine and Pathology, Ohio State University, phenethyl isothiocyanate was found to be a potent inhibitor of preneo-plastic and neoplastic lesions in the rat esophagus.

Lung cancer and esophageal cancer in rats induced by certain compounds in cigarette smoke, such as the tobacco-
specific nitrosamine, 4-(methylnitros-amino)-1-(3-pyridyl)-1-butanone (NNK), can be inhibited by varying concentration of isothiocyanates, Dr. Stoner said. However, he added that these phyto-chemicals are effective only if administered before or concurrently with exposure of the rats to carcinogens.

Phytochemicals Being Studied as Possible Cancer Preventives

  • Isothiocyanates, found in cruciferous vegetables such as broccoli, cauliflower, cabbage, watercress, and Brussels sprouts, exert their protective effect via enzymes.
  • Saponins, found in beans and legumes, may prevent cancer cells from proliferating by influencing genetic material in cells.
  • Indoles, found in cruciferous vegetables, may have a dampening effect on estrogen, thus reducing breast cancer risk.
  • Allyl sulfides, found in plants of the garlic and onion family, increase levels of enzymes that rid cells of carcinogens.
  • Ellagic acid, found in strawberries and raspberries, may reduce the damage caused by tobacco smoke and other types of air pollution.
  • Isoflavones, found in soybeans, may inhibit cancer cell growth and division.

    Designer Foods as Chemopreventives?

    If, as many researchers believe, a diet rich in fruits and vegetables that contain phytochemicals can help prevent cancer, should society ensure their adequate intake by allowing companies to fortify foods with phytochemicals? Should the compounds be synthesized and sold in pill or powder form? Should we all have a phytochemical cocktail before sitting down to dinner?

    Probably not, at least in the foreseeable future, Mark Messina, PhD, said at the AICR conference (see story above).

    Dr. Messina, a consultant for the North Central Soybean Research Project, said that "although phyto-chemicals may be the 'vitamins and minerals' of the 21st century, we don't know enough about the risk of overexposure to these compounds." He also expressed concern about the safety of consuming phyto-chemicals that do not occur naturally in food.

    Agreeing with Dr. Messina was Jerianne Heimendinger, ScD, program director, NCI Prevention and Control Extramural Research Branch. "Until we have statistical proof of the chemoprotective effects of phytochemicals in humans, we need to focus on specific dietary recommendations rather than phytochemical pills and potions," she said.

    Johanna Dwyer, DSc, RD, director of the Frances Stern Nutrition Center, New England Medical Center, cautioned that before phyto-chemicals can be considered for use as cancer preventives, "we must study the cause and effect relationships of these chemicals to cancer cell development and metabolism, and develop surrogate markers for efficacy."

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