Among African American men ( n = 250 cases ) , higher vitamin D was associated with reduced risk of Gleason 7-10 cancer only : in the a posteriori contrast of quintiles 1-2 versus 3-5 , the HR was 0.55 ( 95 % CI , 0.31-0 .97 ; P = 0.037 ) , with no evidence of dose-response or a U-shaped association .