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SECOND INTERNATIONAL SYMPOSIUM
ON THE ROLE OF SOY
IN PREVENTING AND TREATING CHRONIC DISEASESeptember 15-18, 1996
Brussells, Belgium
SCIENTIFIC PROGRAM
(Oral Abstracts)
Cancer prevention: Food and Phytochemicals
John D. Potter, Fred Hutchinson Cancer Research Center and University of Washington, Seattle, Washington.
Consumption of particular vegetables and fruits has long been believed to be useful in the prevention and care of disease. Until recently, the practice of Western medicine largely involved the prescription of specific plants and foods, a practice that has its origins in ancient Egypt, Greece and Rome. The modern medical practices of China and India remain closer to their roots.
In 1981, Doll and Peto produced a wide estimate of 10 to 70 % of all cancer as being attributable to diet. Much of this interpretation was based on studies showing increased risk in association with particular foods- particularly foods of animal origin. Over the past decade, however, a substantial number of studies have examined the effect of plant food consumption on health and disease. That high consumption of vegetables and fruit is protective against cancers at many sites is better supported by the scientific literature than most of the other dietary hypotheses.
As early as 1933, a study by Stocks and Karn in Great Britain suggested an association between higher intake of certain vegetables and lower risk of cancer at all sites. Since then, over 200 epidemiologic studies have been conducted in many different parts of the world to investigate the role of vegetables and fruit in altering the risk of cancer in different organs of the body. Statistically significant inverse associations have been reported for one or more vegetable and/or fruit categories in a high proportion of these studies and in more than half of the studies of every cancer site except prostate. The evidence strongly suggests that it is not consumption of one or two varieties of vegetables and fruit that confer benefit but rather that intake of many different kinds of plant foods is higher in those at lower risk of cancer. Indeed, of all the vegetables studied, only legumes and potatoes appear to show no evidence of direct benefit. The fact that individuals who consume higher intakes of plant foods also have other healthy habits, such as a lower likelihood of smoking does not account for all of the differences seen.
There are many biologically plausible reasons why consumption of vegetables and fruit might deter the development of cancer. There are, in particular, a wide variety of chemical compounds present naturally in food that may lower risk of cancer. They include carotenoids such as -carotene, vitamin C, vitamin E, selenium, dietary fiber, as
well as substances such as dithiolthiones, isothiocyanates, indoles, phenols, and phytoestrogens. Many of these have been shown, in experimental studies, to be potentially anticarcinogenic. These are phytochemicals or bioactive compounds, chemicals of plant origin that can play a crucial role in our metabolism.
The cancer process from exposure to carcinogens or their precursors, through the changes that allow a cell with abnormal DNA to grow and multiply, to the appearance of a cancer involves many stages. At almost everyone of the stages, known phytochemicals can alter the likelihood of carcinogenesis, occasionally in a way that enhances risk, but usually in a favorable direction. For example, such substances as glucosinolates and indoles, isothiocyanates and thiocyanates (all present, particularly in crucerifous vegetables) can stimulate organs in the body to produce a multiplicity of enzymer that can inactivate carcinogens, vitamin C and phenols (present in wine and vegetables) block the formation of carcinogens such as nitrosamines, flavonoids and carotenoids (widespread in vegetables and fruit) can act as antioxidants, essentially disabling the carcinogenic potential of a number of cancer-causing compounds; some sulphur-containing compounds- such as those found in garlic and onions- and some carotenoids can suppress the machinery that allows the growth and division of cancer cells. Phytoestrogens (which are found in soybeans and also can be made from dietary fiber by bacteria in the colon) may be able to reduce the risk of hormone-related cancers but may alos have other benefits against chronic diseases. Colonic fermentation of dietary fiber (found in vegetables and grains) produces volatile fatty acids that may induce cancer cells to undergo programmed cell death.
In summary, there is substantial evidence that a diet high in vegetables and fruit can lower risk of a number of comon and important cancers. Experimental studies have provided evidence that vegetables and fruit contain a large number of substances that have the potential to slow down or reverse many of the steps that ultimately lead to cancer.
At present, it is not clear what quantity we should eat each day - it is clear that many people do not eat enough. Therefore, setting a population target of perhaps 400 gm may be a useful interim goal. Certainly, at this level, there are few dangers for any part of the population. Supplements and pills are likely not to provide the diversity of compounds available in food and certainly do not provide the taste and enjoyment. For any community or even nation, some specific incentives for the production of more vegetables and fruit would allow the changes at the individual level to made more easily and ultimately may prove to be a useful investment in lowering the burden of chronic disease.
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