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 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.
Ding
C
C
Menzies Research Institute, University of Tasmania, Hobart, Tas., Australia. changhai.ding@utas.edu.au
Parameswaran
V
V
Blizzard
L
L
Burgess
J
J
Jones
G
G
D050154:Adiposity / Q000502:physiology*; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D015992:Body Mass Index; D015331:Cohort Studies; D005260:Female; D006801:Humans; D015850:Interleukin-6 / Q000097:blood*; D020738:Leptin / Q000097:blood*; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D011863:Radioimmunoassay; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D049629:Waist-Hip Ratio
D015850:Interleukin-6; D020738:Leptin; D014807:Vitamin D; C104450:25-hydroxyvitamin D