Cauley JA et al. (2008) Serum 25-hydroxyvitamin D concentrations and risk for hip fractures. Ann Intern Med 149: 242–250

Cauley et al. conducted a nested case–control study to investigate the relationship between serum 25-hydroxyvitamin D (25OHD) concentrations and risk of hip fracture in postmenopausal women. Previous studies had produced contradictory findings in relation to whether serum 25OHD levels affect fracture risk.

Participants were women aged 50–79 years enrolled in the Women's Health Initiative Observational Study who did not have a history of hip fracture and who were not receiving bone-active agents or estrogens (n = 39,795). At study entry, participants provided information about their fracture risk factors and blood samples were obtained for the measurement of serum 25OHD levels. After a median follow-up of 7.1 years, 404 of the women had a hip fracture, of whom 400 were randomly selected as cases; controls were matched for age, ethnicity and date of blood sample collection.

Compared with women who had serum 25OHD concentrations in the highest quartile (70.7–121.5 nmol/l), those with serum 25OHD concentrations in the lowest quartile (9.2–47.5 nmol/l) had a significantly increased risk of hip fracture. This association was independent of frailty, falls, physical functioning, renal function and sex-steroid hormone levels; however, higher levels of the bone resorption marker C-terminal telopeptide of type I collagen in cases than controls suggested a possible mechanism by which serum 25OHD level influenced fracture risk.

The authors suggest that measurement of serum 25OHD level in postmenopausal women might help to identify those at high risk of fracture.