Vitamin D, Calcium, and Dairy Intakes and Stress Fractures Among Female Adolescents

Objective: To identify whether calcium, vitamin D, and/or dairy intakes are prospectively associated with stress fracture risk among female adolescents. Design: Prospective cohort study. Setting: Adolescent girls living throughout the United States. Participants: A total of 6712 girls aged 9 to 15 years at baseline in the Growing Up Today Study, an ongoing prospective cohort study. Main Exposures: Dairy, calcium, and vitamin D intakes assessed by food frequency questionnaire every 12 to 24 months between 1996 and 2001. Main Outcome Measure: Incident stress fracture that occurred between 1997 and 2004 as reported by mothers of the participants in 2004. Cox proportional hazards models were used to examine associations. Results: During 7 years of follow-up, 3.9% of the girls developed a stress fracture. Dairy and calcium intakes were unrelated to risk of developing a stress fracture. However, vitamin D intake was inversely related to stress fracture risk. The multivariable-adjusted hazard ratio of stress fracture for the highest vs the lowest quintile of vitamin D intake was 0.49 (95% CI, 0.24-1.01; P-trend=.07). We conducted a stratified analysis to estimate the association between vitamin D intake and stress fracture risk among girls participating in at least 1 h/d of high-impact activity, among whom 90.0% of the stress fractures occurred, and found that higher vitamin D intake predicted significantly lower risk of stress fracture (P-trend=.04). Conclusions: Vitamin D intake is associated with lower stress fracture risk among adolescent girls who engage in high levels of high-impact activity. Neither calcium intake nor dairy intake was prospectively associated with stress fracture risk.