Vitamin D deficiency as a risk factor for osteoporotic fractures

The evidence on the association between vitamin D deficiency and fracture incidence is contradictory. Therefore, the objective of this study was to examine whether low serum 25-hydroxyvitamin D (25(OH)D) levels are associated with osteoporotic fractures. The study was conducted among 1311 community-dwelling older men and women of the Longitudinal Aging Study Amsterdam (LASA), an ongoing multidisciplinary cohort study. Serum 25(OH)D was determined using a competitive protein binding assay. Fractures were assessed during six years of follow-up. The data were analyzed using Cox proportional hazards model. In total, 11.3% of the persons had a serum 25(OH)D below 10 ng/ml, 48.4% had a value below 20 ng/ml, and 82.4% had a value below 30 ng/ml. Furthermore, 115 persons (8.5%) had one or more osteoporotic fractures. Different cut points of serum 25(OH)D were examined with a cut point of 12 ng/ml giving the best discrimination between persons with and without fractures (17.5% of the persons fell below this cut point). The lowest percentage of fractures (5.6%) was found above 30 ng/ml. Because an interaction effect with age was found (p = 0.04), further analyses were conducted separately for persons aged 65-75 years (n = 656) and for persons aged 75-89 years (n = 664) at baseline. After adjustment for age, sex, season of blood collection, body mass index, number of chronic diseases, serum creatinine, cognition, smoking and alcohol use, serum 25(OH)D below or equal to 12 ng/ml was associated with an increased fracture risk in the youngest age group (HR = 3.1; 95% CI: 1.4-6.9), but not in the oldest age group (HR = 1.3; 95% CI: 0.7-2.2). For commonly used cut points of serum 25(OH)D (< 10 ng/ml, 10-19.9 ng/ml, 20-29.9 ng/ml, ≥ 30 ng/ml), no statistically significant associations were found after adjustment for confounding. Serum 25(OH)D levels below or equal to 12 ng/ml were associated with an increased fracture risk in persons aged 65-75 years. The relatively low cut point of serum 25(OH)D in our population is possibly caused by high calcium intake in the Netherlands.