Abstract
Cell culture experiments suggest that vitamin D may inhibit renal carcinogenesis, but human studies of circulating 25-hydroxyvitamin D [25(OH)D], the accepted measure of vitamin D status, and kidney cancer have been null. Limited research has examined the role of circulating vitamin D-binding protein (DBP) in the association between 25(OH)D and disease risk, and it is unclear whether free 25(OH)D in circulation is a better measure of effective exposure, or if DBP may independently impact outcomes. We conducted a nested case-control analysis within the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study to examine whether circulating DBP concentration was prospectively associated with risk of renal cell carcinoma, and whether it modified the association with 25(OH)D. Renal cell carcinoma cases (n = 262) were matched 1:1 to controls on age (±1 year) and date of blood collection (± 30 days). We estimated odds ratios (ORs) and 95% confidence intervals (CIs) of renal cell carcinoma risk by quartiles of 25(OH)D, DBP and the molar ratio of 25(OH)D:DBP, a proxy for free circulating 25(OH)D. Men with higher DBP concentrations were at significantly decreased risk of kidney cancer (Q4 vs. Q1: OR = 0.17, 95% CI = 0.08-0.33; p-trend < 0.0001), a finding unchanged by adjustment for 25(OH)D. Although we observed no association with total 25(OH)D, we found slightly increased risk with higher levels of estimated free 25(OH)D [Q4 vs. Q1 of the 25(OH)D:DBP ratio, OR = 1.61, 95% CI = 0.95-2.73; p-trend = 0.09]. The strong protective association observed between higher circulating DBP concentration and kidney cancer risk requires replication but suggests a vitamin D-independent influence of DBP.