Predictors of 5-Year Change in Plasma 25-hydroxyvitamin D (25(OH)D) Concentrations in Postmenopausal Women

Objective: Determine the predictors of 5-year change in (∆) 25(OH)D concentrations. Methods: Postmenopausal women in the Osteoporosis and Periodontal Disease Study (N=668) had 25(OH)D assessments at baseline (1997-2000) and 5-years (2002-2005). Baseline and ∆ dietary, lifestyle and health-related factors were tested as predictors using linear regression. Results: The mean increase in 25(OH)D was 7.7 nmol/L (SD=0.7, P<0.001). Predictors explained 31% of the variance in ∆ 25(OH)D and included baseline 25(OH)D, baseline and ∆ vitamin D supplement intake and physical activity, ∆ season of blood draw, ∆ body mass index, ∆ whole body T-score, and baseline hormone use. Baseline 25(OH)D and ∆ vitamin D supplement intake explained the most variance (25%) in 25(OH)D. In exploratory analyses, there was a borderline significant interaction between baseline 25(OH)D and ∆ vitamin D after adjustment for predictors of ∆ 25(OH)D (P=0.063). The greatest increase in 25(OH)D (M=22.9 nmol/L, SD=16.8) occurred in women in the lowest tertile of baseline 25(OH)D who increased supplement intake from baseline to follow-up (M=400.6 IU/d, SD=240.7). Conclusions: Baseline 25(OH)D and ∆ vitamin D supplement intake were most predictive of increasing 25(OH)D. Increasing 25(OH)D through supplementation depends on initial 25(OH)D