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Association of Serum Vitamin D Levels With Inflammatory Response Following Hip Fracture: The Baltimore Hip Studies

Divisions of ¹ Gerontology ² Biostatistics & Bioinformatics, Department of Epidemiology and Preventive Medicine, and the Department of ³ Microbiology and Immunology and the Center for Vascular and Inflammatory Diseases, University of Maryland School of Medicine, Baltimore. ⁴ Department of Physical Therapy, University of Delaware, Newark. ⁵ Division of Endocrinology, Diabetes, and Metabolism and Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania School of Medicine, Philadelphia. ⁶ Department of Orthopaedic Surgery, The Union Memorial Hospital, Baltimore, Maryland.

Address correspondence to Ram R. Miller, MDCM, MSc, University of Maryland, School of Medicine, Division of Gerontology, Department of Epidemiology and Preventive Medicine, 660 W. Redwood St., Suite 200, Baltimore, MD 21201. E-mail: rrmiller{at}epi.umaryland.edu

Background. Vitamin D, known for its role in calcium homeostasis, may also regulate immune function. Whether vitamin D deficiency at the time of hip fracture is associated with the inflammatory response postfracture is not known.

Methods. In a cohort from the Baltimore Hip Studies, women aged 65 years were evaluated at baseline and 2, 6, and 12 months after hip fracture repair. Serum at baseline was analyzed for 25-hydroxyvitamin D [25(OH)D], and serum from all time points was analyzed for interleukin-6 (IL-6). Participants were divided into two groups based on their baseline 25(OH)D levels. Vitamin D deficiency was defined as a 25(OH)D level of 15 ng/mL (< 37.5 nmol/L). We examined IL-6 level as a function of vitamin D status using generalized estimating equations, adjusting for covariates.

Results. Women deficient in vitamin D at baseline had higher IL-6 levels in the year postfracture (p =.02). On average, participants with low 25(OH)D levels had adjusted serum IL-6 levels that were 6.0 pg/mL (95% confidence interval [CI]: –6.7, 18.7 pg/mL), 11.9 pg/mL (95% CI: 3.5, 20.4), 13.1 pg/mL (95% CI: 4.6, 21.6), and 13.4 pg/mL (95% CI: 2.3, 24.5) higher at baseline, 2, 6, and 12 months after hip fracture, respectively.

Conclusions. Women with vitamin D deficiency at the time of hip fracture had higher serum IL-6 levels in the year after hip fracture. Whether the proinflammatory state of vitamin D deficiency explains the association of this deficiency with adverse outcomes in older adults warrants further study.