Lycopene rich tomatoes or tomato-based foods may not help prevent prostate cancer, according to a study published in the May 2007 issue of cancer Epidemiology, Biomarkers & Prevention. The finding would further compound the confusion over the role of tomatoes and tomato products in preventing one of the most common cancers in the United States.The study, conducted by researchers at the National Cancer Institute and Fred Hutchinson Cancer Research Center, actually found a correlation between high serum levels of beta-carotene, an antioxidant related to lycopene, and an increased risk of aggressive prostate cancer.
“It is disappointing, since lycopene might have offered a simple and inexpensive way to lower prostate cancer risk for men concerned about this common disease,” said Ulrike Peters, Ph.D., M.P.H., of the Fred Hutchinson Cancer Research Center. “Unfortunately, this easy answer just does not work.”
Previous epidemiologic studies proved inconsistent with regard to the role lycopene may play in prostate cancer prevention. Ulrike Peters, Ph.D., lead author of the study, and colleagues carried out the study in the hope that their study may result in more evidence to prove or disprove any effect of lycopene of the prostate cancer.
For the study, Peters and team investigated the association between pre-diagnostic serum carotenoids including lycopene, alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein and zeaxanthin and risk of prostate cancer among some men who took part in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial, which included a total of 38,000 men age 55 to 74 who had no diagnosed prostate cancer when entering the study.
The eight-year study between 1993 and 2001 involved 692 incident prostate cancer cases, diagnosed 1 to 8 years after subjects, and 844 randomly selected matched controls. Prostate cancer cases included 270 aggressive cases, with regional or distant state in 90 people and or Gleason score more than seven in 235 people.
At the start of the study, the men gave a blood sample and completed a questionnaire on their health, diet and lifestyle. Blood samples were analyzed for the serum levels of various carotenoids. Then the researchers tried to establish any possible association between individual carotenoid such as lycopene and risk of prostate cancer.
The researchers focused on non-Hispanic Caucasian men as the numbers of cases for other ethnic groups were not large enough for meaningful statistic analysis. They found an association which indicated that highest levels of lycopene may raise the risk of prostate cancer by 14 percent compared to the lowest levels, but the researchers did not seem to consider that statistically significant. “Our results do not offer support for the benefits of lycopene against prostate cancer,” Peters said.
The researchers also found subjects with beta-carotene in the highest quintile were 67 percent more likely to develop aggressive prostate cancer than those who had the compound in the lowest quintile. In particular, those who had serum beta-carotene in the highest quintile were 216 percent more likely to develop regional or distant stage disease than those who had the antioxidant in the lowest quintile.
Peters and team concluded that "these results suggest that lycopene or tomato-based regimens will not be effective for prostate cancer prevention." They have early investigated the role of lycopene based on dietary intake of this antioxidant and found that high intake of lycopene was not associated with lower risk of prostate cancer.
Still, the study may not have the final say about the role of lycopene in the prevention of prostate cancer, commented a scientist affiliated with foodconsumer.org who was not involved in the study. Peters acknowledged in a statement that this unexpected observation “may be due to chance.”
The foodconsumer.org scientist said two things need to be considered when readers look at the results of the current study. For one thing, the subjects enrolled in the study were initially screened through a prostate-specific antigen or PSA test and digital rectal exam, and were then followed through routine exams and screenings until first occurrence of prostate cancer, death or the end of the trial in 2001. He asked, “is it possible that those who were at high risk for prostate cancer indicated by a high PSA count might take beta-carotene and or lycopene supplements in an effort to reduce their risk of the disease?” If that is true, a higher risk might lead to consumption of high beta-carotene in the subjects, not vice versus.
Another thing that needs to be considered is that the blood test for carotenoids was conducted only once at the beginning of the study. The data derived from the single test could not show the subjects’ dietary habits before nor after they enrolled in the study, the foodconsumer.org scientist commented. This means that the results of the current study may not be applicable to people who use lycopene rich vegetables such as tomatoes or tomato based foods for a long period.
The current study was consistent with some previous studies including one study conducted by Peters's group, which found no overall association between lycopene intake through more than 25 tomato-based foods and prostate cancer.
But many studies examining the effect of dietary intake of lycopene and serum lycopene levels have suggested that high intake of lycopene-rich tomatoes and or tomato-based foods may reduce the risk of prostate cancer. In addition to the possible preventive effect, some laboratory studies have even suggested that tomatoes along with other vegetables such as broccoli may effectively inhibit progression of prostate cancer by more than 70 percent.
Among others, two major studies established a positive association between increased intake of tomatoes and reduced risk of prostate cancer. In one six-year study involving 14,000 men, Mills P.K. and colleagues in 1989 found tomato, bean, lentil and pea consumptions were related to lowered prostate cancer risk, but no association was found for intake of beta-carotene rich foods such as carrots. The study was published in Cancer 1989; 64:598-604.
Another study conducted at the Harvard School of Medicine involving 47,894 prostate cancer free male health professionals showed that high lycopene consumption had a statistically significant 21 percent risk reduction. Among 46 food items that contain carotenoids, tomato sauce and tomatoes containing high levels of lycopene were associated with a lower risk of prostate cancer. The researchers found that 10 servings per week of tomatoes and tomato-based products were linked with a 35 percent reduced risk of prostate cancer compared to 1.5 servings per week. The risk reduction was even more significant in the cases of aggressive prostate cancer, a 47 percent reduction. The study was conducted by Giovannucci E and colleagues and published in American Journal of Epidemiology. 151(2):119-123, January 15, 2000.
Evidence has merged earlier indicating that lycopene may prevent or even help treat prostate cancer. Early studies conducted by Rao A. V. and colleagues at the University of Toronto, which were based on the blood and tissue measurements of lycopene, found that prostate cancer patients had lower levels of lycopene both in the blood and prostate tissue compared to control subjects. The results were published in Nutrition and Cancer 1999; 32:159-164.
In a cell culture study, Pastori M and colleagues found lycopene together with vitamin E inhibited growth of prostate cancer. Their study was reported in Biochemical and Biophysical Research Communications 1998; 250:582-585.
Earlier, American Longevity and ketchup maker H. J. Heinz Company petitioned with two scores of studies to the Food and Drug Administration for a dozen of health claims over the potential benefits of lycopene and tomato products in the prevention of prostate cancer. According to American Longevity, the FDA had delayed its response to the petition six times. But the agency said it needed more time to review the petition. In November 2005, the FDA rejected all claims. Instead, the agency suggested an extremely weak claim as follows cited from the FDA letter to the organization.
"Very limited and preliminary scientific research suggests that eating one-half to one cup of tomatoes and/or tomato sauce a week may reduce the risk of prostate cancer. FDA concludes that there is little scientific evidence supporting this claim."
The controversy over the role of lycopene in the prevention of prostate cancer will continue, the foodconsumer.org scientist said. Regardless, he suggested that consumers should eat a nutrition-balanced diet including carotenoid-rich vegetables such as lycopene rich tomatoes or tomato products or beta-carotene rich carrots since these foods are important part of a health diet even if they may not have any protective effect against prostate cancer.
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