Lycopene-Rich Tomatoes May Play Role in Cancer Prevention
Lycopene-Rich Tomatoes May Play Role in Cancer Prevention
NEW YORK--Researchers are accumulating more evidence that diets rich in tomato products may prevent several types of cancer. Although the chemical components of tomatoes that have anticancer properties remain to be identified, investigators are hopeful that lycopene, a little-known carotenoid responsible for the tomato's red color, will prove an effective tool in cancer prevention.
Presentations at the International Symposium on the Role of Lycopene and Tomato Products in Disease Prevention, cosponsored by the American Health Foundation, Valhalla, NY, and the Tomato Research Council, suggest that lycopene has several chemical and biological effects that may contribute to cancer prevention.
Helmut Sies, MD, professor and chairman, Department of Physiological Chemistry, University of Dusseldorf, Germany, said that "among natural carotenoids, lycopene is the most efficient singlet oxygen quencher and has been shown to scavenge peroxyl radicals," meaning that it is highly effective in deactivating reactive oxygen, thought to play a role in initiating malignant transformation of cells.
In addition to its antioxidant properties, Dr. Sies said, in vitro studies suggest that lycopene may play a role in the control of proliferation of tumor cells. "It appears that lycopene in some way induces channels of cell-to-cell communication that attenuate cell proliferation," he said, adding that further study is needed to elucidate this mechanism.
Dr. Sies explained that lycopene is a major carotenoid in human blood and tissues. It is the predominant carotenoid in the liver, lung, kidney, and testis, and is also found in high concentrations in the adrenal and prostate glands.
Uptake of lycopene is enhanced by the processing and heating of tomatoes, which releases it from its biologic matrix. Furthermore, consuming tomato products with some fat or oil enhances absorption. For these reasons, he said, tomato paste, ketchup, tomato sauce, and other cooked tomato products are better vehicles for lycopene than fresh, raw tomatoes.
Although the preventive value of lycopene remains to be definitively demonstrated through epidemiologic, in vitro, and in vivo studies, Leonard A. Cohen, PhD, said that researchers are trying to avoid the errors made when beta-carotene was trumpeted as a potent cancer preventive.
For example, no animal studies of beta-carotene were done prior to human studies, said Dr. Cohen, head, Section of Nutritional Endocrinology, American Health Foundation.
Dr. Cohen and his colleagues have studied the uptake and transport of lycopene to organ sites in male and female rats. It was found to be absorbed and to be deposited in all tissues, with the majority stored in the liver. Future studies will look at its chemopreventive role in a variety of tumor models, including breast, colon, prostate, and lung.
Elizabeth Johnson, PhD, of the Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University, said that serum levels of lycopene do not differ between the sexes, but there is an age-associated decrease in serum levels.
Interestingly, smoking does not appear to lower lycopene levels, though it does have that effect with other carotenoids. Alcohol consumption, however, does adversely affect lycopene levels.
Unlike other carotenoids, which are found in a variety of foods, lycopene is found in significant levels only in tomatoes. Markedly lower amounts of lycopene are available from watermelon, pink grapefruit, and guava, Dr. Johnson said, and most other red fruits and vegetables derive their color from other sources. Whereas lycopene accounts for 40% of the total serum carotenoids in Americans, it represents only 10% in Asians, reflecting the comparative role of tomatoes in the Western and Asian diets.
The so-called Mediterranean diet, rich in tomatoes and olive oil, can be seen as a nutritional model for studying lycopene. Carlo La Vecchia, MD, MSc, head, Laboratory of Epidemiology, Mario Negri Institute for Pharmacological Research, Milan, presented a case-control study of 2,709 gastrointestinal cancers (oral cavity and pharynx, esophagus, stomach, colon, and rectum) and 2,879 controls.
The study found "a consistent pattern of protection for all sites" among those whose consumption of tomatoes was high. A second study of colorectal cancer, based on 1,953 cases and 4,154 controls, found a similar consistency.
Citing epidemiologic evidence of low rates of cancers of the intestines, pancreas, breast, and prostate in Mediterranean countries, Dr. LaVecchia said that "tomatoes constitute one of the most specific features of the Mediterranean diet, a fact that has both scientific and public health relevance. Tomatoes should serve as a simple focal point and tool for GI cancer prevention campaigns."
Studies of American men, summarized by Dr. Edward Giovannucci, of the Harvard School of Public Health, show that diets rich in tomatoes, tomato paste, tomato sauce, and pizza are associated with a lower risk of prostate cancer.
High concentrations of lycopene in the prostate suggest a preventive role in prostate malignancy, which Steven K. Clinton, MD, PhD, termed epidemic in the United States. "It is believed that environment and life-style have a larger role than inheritance in the development of prostate cancer," said Dr. Clinton, of Dana-Farber Cancer Institute and Harvard Medical School.
He added that "diets rich in tomato products, such as the Mediterranean diet, are associated with reduced risk of prostate cancer," whereas diets high in fat, meat, and dairy products are associated with higher risks.
Dr. Clinton has studied the cellular and molecular aspects of lycopene and prostate cancer. "The presence of lycopene in the human prostate at biologically active concentrations supports the hypothesis that lycopene may have direct effects within the prostate," he reported.
However, he added that his team has observed from 15 to 20 different chemical forms of lycopene in the human prostate, making it very difficult to suggest which isomer of lycopene may actually exhibit cancer-fighting properties.
Dr. Clinton is currently evaluating tomato products, lycopene, and other chemicals derived from tomatoes in prostate carcinogenesis using cell and rodent models. Although his research is in its early stages, he believes it is appropriate to recommend to the public "a diverse diet rich in fruits and vegetables, including tomatoes."
The general dietary recommendations that emerged from the symposium are five to seven servings of tomato products each week within a diverse array of foods according to the USDA's food pyramid.