Associations of total and free 25OHD and 1,25(OH)2D with serum markers of inflammation in older men

Vitamin D is hypothesized to suppress inflammation. We tested total and free vitamin D metabolites and their association with inflammatory markers. Interleukin-6 levels were lower with higher 25-hydroxyvitamin D. 1,25-dihydroxyvitamin D and free 25OHD associations mirrored those of 25OHD. However, associations for the two metabolites diverged for tumor necrosis factor alpha (TNF-alpha) soluble receptors. Vitamin D is hypothesized to suppress inflammation, and circulating 25-hydroxyvitamin D (25OHD) and inflammatory markers are inversely correlated. However, total serum 25OHD may not be the best indicator of biologically active vitamin D. We tested serum total 25OHD, total 1,25(OH)(2)D, vitamin D binding protein (DBP), and estimated free 25OHD and free 1,25(OH)(2)D associations with inflammatory markers serum interleukin-6 (IL-6), TNF-alpha and their soluble receptors, interleukin-10 (IL-10), and C-reactive protein (CRP) as continuous outcomes and the presence of a parts per thousand yen2 inflammatory markers in the highest quartile as a dichotomous outcome, in a random subcohort of 679 men in the Osteoporotic Fractures in Men (MrOS) study. IL-6 was lower in men with higher 25OHD (-0.23 mu g/mL per standard deviation (SD) increase in 25OHD, 95 % confidence intervals (CI) -0.07 to -0.38 mu g/mL) and with higher 1,25(OH)(2)D (-0.20 mu g/mL, 95 % CI -0.0004 to -0.39 mu g/mL); free D associations were slightly stronger. 25OHD and DBP, but not 1,25(OH)(2)D, were independently associated with IL-6. TNF-alpha soluble receptors were inversely associated with 1,25(OH)(2)D but positively associated with 25OHD, and each had independent effects. The strongest association with a parts per thousand yen2 inflammatory markers in the highest quartile was for free 1,25(OH)(2)D (odds ratios (OR) 0.70, 95 % CI 0.54 to 0.89 per SD increase in free 1,25(OH)(2)D). Associations of 1,25(OH)(2)D and free 25OHD with IL-6 mirrored those of 25OHD, suggesting that 1,25(OH)(2)D and free D do not improve upon 25OHD in population-based IL-6 studies. However, associations for the two metabolites diverged for TNF-alpha soluble receptor, warranting examination of both metabolites in studies of TNF-alpha and its antagonists.