Association of diabetes with prostate cancer risk in the multiethnic cohort.

Among men of European ancestry, diabetics have a lower risk of prostate cancer than do nondiabetics. The biologic basis of this association is unknown. The authors have examined whether the association is robust across populations in a population-based prospective study. The analysis included 5,941 prostate cancer cases identified over a 12-year period (1993-2005) among 86,303 European-American, African-American, Latino, Japanese-American, and Native Hawaiian men from the Multiethnic Cohort. The association between diabetes and prostate-specific antigen (PSA) levels (n = 2,874) and PSA screening frequencies (n = 46,970) was also examined. Diabetics had significantly lower risk of prostate cancer than did nondiabetics (relative risk = 0.81, 95% confidence interval (CI): 0.74, 0.87; P < 0.001), with relative risks ranging from 0.65 (95% CI: 0.50, 0.84; P = 0.001) among European Americans to 0.89 (95% CI: 0.77, 1.03; P = 0.13) among African Americans. Mean PSA levels were significantly lower in diabetics than in nondiabetics (mean PSA levels, 1.07 and 1.28, respectively; P = 0.003) as were PSA screening frequencies (44.7% vs. 48.6%; P < 0.001); however, this difference could explain only a small portion ( approximately 20%) of the inverse association between these diseases. Diabetes is a protective factor for prostate cancer across populations, suggesting shared risk factors that influence a common mechanism.