Not a simple fat-soluble vitamin: changes in serum 25-(OH)D levels are predicted by adiposity and adipocytokines in older adults

Objective. To determine the associations between body adiposity and change in serum 25-(OH)D levels over 2.6 years, and if these associations are mediated by metabolic andinflammatory factors in older adults. Methods. This is a longitudinal study of 859 randomly selected subjects (mean 62 years, range 51–80, 49% women). Serum 25-hydroxyvitamin D [25-(OH)D] was assessed by radioimmunoassay at baseline and 2.6 years later. Baseline serumlevel of leptinwas assessed by radioimmunoassay and interleukin (IL)-6 by a chemiluminescent immunoassay in the first 183 subjects. Results. In multivariable analyses, body mass index, trunk fat percentage and waist-to-hip ratio were significant predictors of increased incident vitamin D deficiency [a 25-(OH)D < 50 nmol L)1 at follow-up when ‡50 nmol L)1 at baseline] and decreased recovery of vitamin D deficiency [a 25-(OH)D ‡ 50 nmol L)1 at follow-up when < 50 nmol L)1 at baseline]. Change in 25-(OH)D levels per annum was also independently predicted by baseline leptin (b: )0.09⁄unit, 95% CI: )0.17, )0.03), IL-6 (b: )0.68⁄ quartile, 95% CI: )1.35, )0.02) and total cholesterol ⁄ high-density lipoprotein (HDL) ratio (b: )0.51, 95% CI: )0.88, )0.14). The associations between body adiposity measures and change in 25- (OH)D completely disappeared after adjustment for leptin, diminished after adjustment for IL-6, but remained unchanged after adjustment for total cholesterol ⁄HDL ratio. All associations were independent of season and sun exposure. Conclusions.Bodyfat isnot simplyapassive reservoir for 25-(OH)D. In addition to season and sun exposure, 25-(OH)D levels appear to be determined by metabolic and, to a lesser extent, inflammatory factors, and these appear to mediate the effects of adiposity onchange in25-(OH)D.