Vitamin D, Fracture Risk and Season of Blood Draw
Abstract
Background: Although both season and fat mass affect serum 25-hydroxyvitamin D (S‑25OHD), these factors are often ignored when assessing vitamin D status. We aimed to investigate the impact of season and fat mass on the association of S‑25OHD with fracture risk.
Methods: We enrolled 5003 women, mean age 68 years, with body composition scans and blood collection in a population-based cohort. Fracture information was obtained from Swedish clinical databases. Proportional hazards regression, stratified by season and fat mass, was used to determine hazard ratios (HR) of fracture according to S-25OHD. Our secondary exposures were serum 1,25‑dihydroxycholecalciferol, the most active vitamin D metabolite and plasma parathyroid hormone (P-PTH).
Findings: During an average of 9.2 years of follow-up, 1080 women had a fracture. Compared with women with S‑25OHD >60 nmol/L during sunny months (May through October), those with S‑25OHD <30 nmol/L had a multivariable-adjusted fracture HR of 2.03 (95% CI 1.25-3.30) and those with S‑25OHD 30-40 nmol/L a HR of 1.56 (95% CI 1.10-2.22). In contrast, S‑25OHD drawn during dark months were unrelated to fracture risk. The increased risk with low sunny season S‑25OHD was seen only among women with body mass index ≥25 kg/m2 or fat mass index ≥9.8 kg/m2. High fat mass and low S-25OHD were independently related to lower S‑1,25‑dihydroxycholecalciferol, which itself predicted fracture risk. P-PTH was unrelated to fracture risk.
Interpretation: Older…
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