EFFECT OF PREFRACTURE VERSUS POSTFRACTURE DIETARY ASSESSMENT ON HIP FRACTURE RISK ESTIMATES

Background. Dietary factors are presumed to have influence on bone mas s and hence fracture susceptibility. Most information in this respect is based on retrospective assessment of previous dietary habits. In a population-based case-control study nested within a cohort, we collect ed dietary information both before and after a first hip fracture. Thu s it was possible to study reported changes in dietary habits, intenti onal as well as unintentional, among hip fracture patients after a fir st hip fracture and to compare postfracture with prefracture dietary i nformation. Methods. More than 65 000 women born 1914-1948 in two coun ties in central Sweden completed a food frequency questionnaire regard ing their usual current dietary habits, before attending a mammographi c screening between the years 1987 and 1990. Subsequently 123 of them sustained a first hip fracture and were defined as cases in the presen t study. For every case, one control, individually matched by age and county of residence, was selected from the cohort. A second identical food frequency questionnaire was mailed to both cases and controls on average 2 years after the hip fracture event. In total 98 case/control pairs could be included in the analysis. The association between diet and hip fracture was evaluated and the results from the two dietary a ssessments were contrasted. Women who themselves claimed that they had not changed their diet in recent years were analysed separately. Resu lts. The hip fracture cases, compared with the controls, had reduced t heir reported dietary intake of dairy products after the fracture. App arently this was not intentional since this effect was more pronounced among those cases who claimed that their diet was unchanged. The chan ges were most apparent among the younger cases with a more recent hip fracture and with a body mass index above the median. Half of the case s, more than twice the frequency in controls, who were initially class ified as having high intake of dairy products were classified as havin g low intake (<800 mg calcium/day) after the hip fracture. This also l owered, in fact reversed, the relative risk estimates of hip fracture both for intake of dairy products and calcium. Crude odds ratios of hi ghest quartile of intake versus lowest, changed from 3.0 to 0.6 for da iry products and from 2.6 to 0.9 for calcium. No other foods or nutrie nts displayed such notable differences between the two surveys. Conclu sion. We conclude that the use of current and retrospective dietary in formation after a hip fracture can lead to a differential misclassific ation in dietary studies and to biased estimates of hip fracture risk as compared with prospectively collected dietary information.