Prospective observational study of physical functioning, physical activity, and time outdoors and the risk of hip fracture: a population-based cohort study of 158,057 older adults in the 45 and up study.

Low levels of physical activity or sun exposure and limitations to physical functioning (or disability) have been identified as possible risk factors for hip fracture. However, these factors are closely related, and data on their independent and joint association with risk of hip fracture are limited. A total of 158,057 individuals aged ≥45 years sampled from the general population of New South Wales, Australia, from the prospective 45 and Up Study completed a baseline postal questionnaire in 2006 to 2009 including data on physical activity (Active Australia questionnaire); sun exposure (usual time outdoors); and physical functioning (Medical Outcomes Score-Physical Functioning; scored 0 to 100). Incident first hip fractures were ascertained by linkage to administrative hospital data (n = 293; average follow-up 2.3 years). The relative risk (RR) of hip fracture was estimated using Cox proportional hazards. Poorer physical functioning, lower physical activity, and less time outdoors were positively related to each other at baseline and individually associated with significantly increased hip fracture risk. However, physical activity and time outdoors were not significantly related to hip fracture risk after adjustment for baseline physical functioning or when analysis was restricted to those with no or mild baseline physical limitation. In contrast, physical functioning remained strongly related to hip fracture risk after adjustment for the other two factors; compared with the group without limitation (100), the RR of hip fracture among those with mild (75-95), moderate (50-70), severe (25-45), and greatest (0-20) level of physical limitation was 1.38 (95% confidence interval [CI] 0.88-2.14), 2.14 (1.29-3.53), 3.87 (2.31-6.44), and 5.61 (3.33-9.42), respectively. The findings suggest that limitation in physical functioning, but not physical activity or time outdoors, is strongly related to hip fracture risk. The apparent increased risk of hip fracture previously described for low physical activity or sun exposure may be, at least in part due to uncontrolled confounding.