Lycopene, tomato products, and prostate cancer prevention. Have we established causality?

The relationship between tomato products and prostate cancer prevention has been the subject of increasingly intense research activity over the past decade. The hypothesis that tomato products contain phytochemicals, perhaps lycopene and others, that modify prostate carcinogenesis warrants investigation. However, scientists, regulatory agencies, marketers of products, and those defining public health policy have differing opinions regarding the strength of the data when applied to criteria for inference and causality. It is useful to briefly review these criteria and discuss the relative strengths and weaknesses of the published data in order to better define future directions for research. Accepted categories of criteria include: consistency, strength of association, biological gradient, temporality, specificity, biological mechanisms and coherence, and experimental evidence. We believe that continued support for research regarding tomato products, lycopene and other phytochemicals, and prostate cancer risk is warranted and may ultimately serve as the basis for more consistent public health recommendations for the consumer. But, what do we say to the public when the data is still inconclusive? The authors support the recommendation that a minimum of five servings of fruit and vegetables should be consumed daily from a variety of sources. We believe that the hypothesized benefits of tomato products for prostate cancer prevention may be achieved with approximately five servings of tomato products per week. The consumption of lycopene supplements is not currently recommended for prostate cancer prevention or therapy. Future research will allow us to provide more definitive guidelines.