Shedding new light on the etiology of prostate cancer?

Prostate cancer has the highest incidence rate of any cancer among men in the United States (1 ). It is estimated for 1993 that 1 65,000 new prostate cancer cases will be diagnosed in the country, representing 28% of all new cancer cases among men (2). Although its incidence is rising, little is known about the causes of this disease. Researchers suspect that genetic, hormonal, and dietary factors, among others, have a role in the development of prostate cancer (3). Dietary studies of prostate cancer have focused aliention mainly on fat (both saturated and unsaturated) and vitamin A. Most case-control studies of dietary fat have reported a positive association, whereas recent cohort studies have been unable to confirm these results (3). However, if dietary fat exerts its effect at a late stage in the development of prostate cancer, then case-control studies, in which diet histories are based on food consumption patterns relatively close to the time of diagnosis, could actually provide belier measures of the relevant exposure than cohort studies, in which the diet information is more remote. The validity of this explanation for the discrepant results could be tested by comparing the findings for prostate cancer cases occurring at various time intervals in a prospective cohort. Epidemiological investigations of dietary vitamin A or its precursors have also produced equivocal results (3). Dietary studies have reported both increases and decreases in risk of prostate cancer associated with this nutrient. Serum studies ofvitamin A have produced inconsistent results (4-6) that may be related to the different sources of vitamin A or its precursors in the diet. Although a protective effect of vitamin D against prostate cancer has been hypothesized (7), no reports on this subject have appeared previously. In this issue of Cancer Epidemiology, Biomarkers and Prevention, Corder et a!. (8) describe the first such study. They found that low serum levels of the vitamin D metabolite, 1 ,25-D,2 were associated with an increased risk of prostate cancer, but only in older men. The study participants were members of the Kaiser Permanente Medical Care Plan in Oakland and San Francisco, California. A nested casecontrol study design was used, in which stored prediagnostic serum from 181 prostate cancer cases and from 1 81 matched controls was tested. The authors concluded that, in men 57 years of age or older, 1 ,25-D was an important predictor of risk for palpable and anaplastic prostate tumors, but not for incidental tumors.