Lycopene and prostate cancer risk. Methodological considerations in the epidemiologic literature

Abstract

Prostate cancer is the most common cancer in U.S. males. Among potentially beneficial natural compounds is lycopene, which is derived largely from tomato-based products. Recent epidemiologic studies have suggested a potential benefit of this carotenoid against risk of prostate cancer, but not all of the studies have been supportive. The largest prospective dietary study, the Health Professionals Follow-Up Study (HPFS), had found that 2-4 servings of tomato sauce per week were associated with about a 35 % risk reduction of total prostate cancer and a 50 % reduction of advanced prostate cancer in follow-up from 1986 to 1992. Tomato sauce was by far the strongest predictor of plasma lycopene levels in this study. In the largest plasma-based study, high lycopene levels were associated with similar risk reductions for total and advanced prostate cancer. Results from other studies, mostly dietary case-control studies, have been mixed. The reasons for these inconsistencies are unclear. Because lycopene may come from a number of sources, and the bioavailability of lycopene may vary profoundly across these sources, dietary questionnaires are likely to vary markedly in their utility to estimate true variation in body lycopene stores across individuals. With further follow-up in the HPFS, we addressed some possibilities for apparently conflicting results. We confirmed our initial findings with the independent 1992­1998 follow-up period. Our results also indicated various factors may contribute to some of the inconsistencies, including insufficient sample size, low intake of lycopene, failure to account for bioavailability, reliance on a single dietary assessment, and heterogeneity of prostate cancer.