Abstract. Ding C, Parameswaran V, Blizzard L, Burgess J, Jones G (Menzies Research Institute, University of Tasmania, Hobart, Tas.; Monash University, Melbourne, Vic.; Royal Hobart Hospital, Hobart, Tas., Australia). Not a simple fat soluble vitamin: changes in serum 25‐(OH)D levels are predicted by adiposity and adipocytokines in older adults. J Intern Med 2010; 268: 501–510.
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 and inflammatory 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 serum level of leptin was 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 (β: −0.09/unit, 95% CI: −0.17, −0.03), IL‐6 (β: −0.68/quartile, 95% CI: −1.35, −0.02) and total cholesterol/high‐density lipoprotein (HDL) ratio (β: −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. Body fat is not simply a passive 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 on change in 25‐(OH)D.