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
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
"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
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.
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,"
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."