Testosterone, sex hormone-binding globulin, and body composition in young adult African American and Caucasian men

This study examined the diurnal variation in circulating total and free testosterone and sex hormone-binding globulin (SHBG) levels in young adult African American and Caucasian men in order to investigate whether there are differences in the secretion of these plasma hormones in populations at different risks of developing prostate cancer as they age. A significant and similar diurnal rhythm for total and free testosterone was found for both groups. Serum levels of total testosterone were 29.4% and 23.9% lower at 8:00 PM than at 8:00 AM in African American and Caucasian men, respectively. Significantly higher serum levels of total testosterone ( P[lt ] .01) and SHBG ( P [lt ] .02) were found in the African American than in the Caucasian men in both the morning and evening, whereas free testosterone levels were similar in both groups. The higher SHBG levels appear to have an environmental/metabolic basis in that the waist circumference, waist-to-hip ratio, and fasting insulin concentration were lower ( P [lt ] .05) in African Americans than in Caucasians. In summary, these data indicate that racial differences in central adiposity in men are established in early adulthood and influence circulating SHBG and thereby testosterone levels. In light of the findings by others that SHBG increases cyclic adenosine monophosphate (cAMP) production in the prostate and that cAMP-dependent protein kinase A is a coactivator of the androgen receptor, these studies provide a possible mechanism by which circulating androgens may contribute to the increased risk for prostate cancer among African American men.