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.
NEW YORK--Researchers are accumulating more evidence that diets richin tomato products may prevent several types of cancer. Although the chemicalcomponents of tomatoes that have anticancer properties remain to be identified,investigators are hopeful that lycopene, a little-known carotenoid responsiblefor the tomato's red color, will prove an effective tool in cancer prevention.
Presentations at the International Symposium on the Role of Lycopeneand Tomato Products in Disease Prevention, cosponsored by the AmericanHealth Foundation, Valhalla, NY, and the Tomato Research Council, suggestthat lycopene has several chemical and biological effects that may contributeto cancer prevention.
Helmut Sies, MD, professor and chairman, Department of PhysiologicalChemistry, University of Dusseldorf, Germany, said that "among naturalcarotenoids, lycopene is the most efficient singlet oxygen quencher andhas been shown to scavenge peroxyl radicals," meaning that it is highlyeffective in deactivating reactive oxygen, thought to play a role in initiatingmalignant transformation of cells.
In addition to its antioxidant properties, Dr. Sies said, in vitro studiessuggest that lycopene may play a role in the control of proliferation oftumor cells. "It appears that lycopene in some way induces channelsof 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 bloodand tissues. It is the predominant carotenoid in the liver, lung, kidney,and testis, and is also found in high concentrations in the adrenal andprostate glands.
Uptake of lycopene is enhanced by the processing and heating of tomatoes,which releases it from its biologic matrix. Furthermore, consuming tomatoproducts with some fat or oil enhances absorption. For these reasons, hesaid, tomato paste, ketchup, tomato sauce, and other cooked tomato productsare better vehicles for lycopene than fresh, raw tomatoes.
Although the preventive value of lycopene remains to be definitivelydemonstrated through epidemiologic, in vitro, and in vivo studies, LeonardA. Cohen, PhD, said that researchers are trying to avoid the errors madewhen beta-carotene was trumpeted as a potent cancer preventive.
For example, no animal studies of beta-carotene were done prior to humanstudies, said Dr. Cohen, head, Section of Nutritional Endocrinology, AmericanHealth Foundation.
Dr. Cohen and his colleagues have studied the uptake and transport oflycopene to organ sites in male and female rats. It was found to be absorbedand 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 tumormodels, including breast, colon, prostate, and lung.
Elizabeth Johnson, PhD, of the Jean Mayer USDA Human Nutrition ResearchCenter on Aging, Tufts University, said that serum levels of lycopene donot differ between the sexes, but there is an age-associated decrease inserum levels.
Interestingly, smoking does not appear to lower lycopene levels, thoughit 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, lycopeneis found in significant levels only in tomatoes. Markedly lower amountsof lycopene are available from watermelon, pink grapefruit, and guava,Dr. Johnson said, and most other red fruits and vegetables derive theircolor from other sources. Whereas lycopene accounts for 40% of the totalserum carotenoids in Americans, it represents only 10% in Asians, reflectingthe comparative role of tomatoes in the Western and Asian diets.
The so-called Mediterranean diet, rich in tomatoes and olive oil, canbe seen as a nutritional model for studying lycopene. Carlo La Vecchia,MD, MSc, head, Laboratory of Epidemiology, Mario Negri Institute for PharmacologicalResearch, Milan, presented a case-control study of 2,709 gastrointestinalcancers (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 colorectalcancer, 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. LaVecchiasaid that "tomatoes constitute one of the most specific features ofthe Mediterranean diet, a fact that has both scientific and public healthrelevance. Tomatoes should serve as a simple focal point and tool for GIcancer prevention campaigns."
Studies of American men, summarized by Dr. Edward Giovannucci, of theHarvard School of Public Health, show that diets rich in tomatoes, tomatopaste, tomato sauce, and pizza are associated with a lower risk of prostatecancer.
High concentrations of lycopene in the prostate suggest a preventiverole in prostate malignancy, which Steven K. Clinton, MD, PhD, termed epidemicin the United States. "It is believed that environment and life-stylehave 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 Mediterraneandiet, are associated with reduced risk of prostate cancer," whereasdiets high in fat, meat, and dairy products are associated with higherrisks.
Dr. Clinton has studied the cellular and molecular aspects of lycopeneand prostate cancer. "The presence of lycopene in the human prostateat biologically active concentrations supports the hypothesis that lycopenemay have direct effects within the prostate," he reported.
However, he added that his team has observed from 15 to 20 differentchemical forms of lycopene in the human prostate, making it very difficultto suggest which isomer of lycopene may actually exhibit cancer-fightingproperties.
Dr. Clinton is currently evaluating tomato products, lycopene, and otherchemicals derived from tomatoes in prostate carcinogenesis using cell androdent models. Although his research is in its early stages, he believesit is appropriate to recommend to the public "a diverse diet richin fruits and vegetables, including tomatoes."
The general dietary recommendations that emerged from the symposiumare five to seven servings of tomato products each week within a diversearray of foods according to the USDA's food pyramid.