Phyllis Bowen, Ph.D., of the University of Illinois at Chicago refers to the complexities of food studies as the "haystack of diet." Protein, fat, vitamins, flax, selenium, and flavonoids are only a few of the innumerable components of food. When researchers talk about the interaction between various diet components, and then add genetic and environmental factors to the equation, the enormity of pinpointing the anti-cancer effects of specific substances within specific foods becomes clear.
Yet out of the haystack has come evidence that phytochemicals, the natural substances found in grains, fruits, vegetables, soy foods, herbs and spices, may not only protect against chronic disease, but also may help to fight cancer. In recent years phytochemicals have been shown to have anti-oxidant, anti-inflammatory, and anti-carcinogenic effects.
Exploration of these substances, which number in the thousands to date, brought researchers, dietitians, physicians, and nurses together recently in Washington, D.C., for the American Institute for Cancer Research's annual conference. Speakers focused specifically on the effects of soy foods, tea, garlic, selenium, and resveratrol.
"We are here to exchange ideas, report results, and share our convictions,"said Ritva Butram, Ph.D., vice president for research at AICR, at the outset of the 2-day meeting.
Each phytochemical showcased at the conference boasted some promising results whether from animal or human studies with evidence of cellular mechanisms supporting epidemiologic findings. Still, study conclusions were sometimes inconsistent and investigators cited several challenges for the future.
Not a Has-Bean
Take soy, for example. Asian countries, where soy and soy products such as miso and tempeh
are diet staples, report relatively low incidences of breast, prostate, colon, and endometrial
cancers. The low rates led researchers to examine the relationship between soy intake and the risk
of those cancers.
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But Birt cited challenges ahead, including uncertainty in categorizing soybean properties; the unknown interaction between soy and genetic components; and the differences in gender response to soy products because isoflavones, prominent in soybeans and soy products, are weak estrogens.
Researchers are awaiting the results of a study led by Anna Wu, Ph.D., of the University of Southern California. The project will not be completed for another 2 years, but preliminary results of this case-control study involving over 2,400 people of Chinese, Filipino, and Japanese descent living in Los Angeles county, show a decrease in breast cancer risk associated with the highest isoflavone intake level.
Study results of soy's anti-carcinogenic effects range from generally unsupportive to highly encouraging, depending on the type of cancer, according to Wu. Studies generally support the conclusion that soy reduces the risk of breast, prostate, and endometrial cancers.
Studies of colorectal cancers, however, have not been as encouraging, although animal and epidemiologic data suggest that eating soy might delay the onset of colon cancer, according to Maurice Bennink, Ph.D., of Michigan State University, Lansing. Bennink said that if tumors do develop in the colon, a soy diet might lead to fewer tumors and slower growth of those tumors.
Research on tea, garlic, and resveratrol, which is found in wine and peanuts, bolstered support for anti-cancer effects, but human studies revealed inconsistencies.
Green and black teas' potential anti-carcinogenic effects have shown promising results in the laboratory, for example (see News, Oct. 21, 1998). But the sparse epidemiologic data does not support anti-cancer claims, and there is a need for well-designed, large-scale human studies.
Research on garlic has also not led to straightforward answers. Garlic studies in the laboratory, such as those of John Milner, Ph.D., of Pennsylvania State University, University Park, have been encouraging. Milner, currently on sabbatical at the National Cancer Institute, has identified possible mechanisms by which garlic acts as an anti-carcinogen.
Population studies cited by Lenore Arab, Ph.D., of the University of North Carolina, Chapel Hill, also link garlic consumption to a statistically significant reduction in cancer risk. But studies investigating the effects of garlic supplements in clinical trials were not promising. Most studies showed no effect, with the exception of one cohort study in the Netherlands in which the group that consumed garlic supplements actually had a higher risk of developing gastric cancer.
Clearly, much work remains to uncover the therapeutic effects of what so many refer to as "that stinking rose."
Garlic researchers at the conference called for future work to address biases that could arise from memory/recording practices and from the possibility that those who take garlic supplements are a self-selected group not representative of the average population.
Selenium, found predominantly in meats, fish, and grains, is, strictly speaking, not a phytochemical but a nutrient that is essential in trace amounts for survival. In contrast to the phytochemicals discussed at the conference, the main question regarding selenium is: How much?
Go With the Grain
According to Gerald Combs, Jr., Ph.D., of Cornell University, Ithaca, N.Y., a large body of epidemiological studies have shown that for various cancers, selenium intake is inversely related to the risk of cancer. In both animal and epidemiologic studies, increased selenium intake was associated with statistically significant reductions in the risk of lung, colorectal, liver, esophageal, and prostate cancers. But the United States recommended daily allowance of selenium is 55 to 70 micrograms and the typical U.S. daily intake is 75 to 150 micrograms.
It may be, researchers agreed, that the amount needed to prevent nutritional deficiency and the amount needed to reduce cancer risk are two different figures. While Combs reported that he has found a dose response reduction in cancer risk corresponding to selenium intake up to 200 micrograms a day, and Howard Ganther, Ph.D., of the University of Wisconsin, Madison, stated that it is possible to achieve selenium efficacy without selenium toxicity, researchers remain cautious in recommending selenium supplementation until further studies confirm its safety and efficacy.
Resveratrol, a compound abundant in grape skin and peanuts, among other sources, has long been linked to a reduced risk of heart disease. Only recently has resveratrol been examined for its anti-cancer effects.
David Goldberg, M.D., Ph.D., of the University of Toronto, spoke of various experimental approaches that have given evidence of resveratrol's antioxidant activity in animal models. Highlighting resveratrol's relative infancy in phytochemical cancer research, Dr. Goldberg said: "We do not know for certain the anti-cancer effects [of resveratrol] in vitro . . . but the alcohol in red wine will ensure risk reduction in coronary heart disease, and on the question of alcohol absorption and bioactivity, there is absolutely no doubt whatsoever."
Just as the combination of animal and human studies has helped researchers to negotiate their way through the haystack of diet, Bowen called for coordination as a top priority for future phytochemical research.
Richard S. Rivlin, M.D., of Memorial Sloan-Kettering Cancer Center and Weill Medical College of Cornell University, both in New York, said that researchers still need to better understand the mechanisms involved in phytochemical activity, learn dosage and organ specificity, find or develop biomarkers, carve out a role for alternative medicines, and continue to study genetics so that healthcare professionals can recommend a diet to patients based on their estimated risks.
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