Bone Abstracts

Although the optimal requirement of vitamin D for skeletal health in the general community remains uncertain, vitamin D deficiency impairs bone mineralisation, increases bone turnover, accelerates bone loss and increases fracture risk. Seasonal variation in the hip fracture incidenc, reported in several studies, supports a role for vitamin D deficiency in the epidemiology of hip fracture. We hypothesised that if the association is causal, then the amplitude of the seasonal variation and the hip fracture risk should vary by latitude.

We have examined the incidence of hip fracture in men and women aged 50 years or more from Sweden (latitudes 55° to 69°) between 1987 and 2009. In order to avoid double counting, only one fracture in a period of a year was counted per individual. The effects of season and latitude were examined by Poisson regression.

As expected hip fracture rates were higher in women than in men. Men contributed 104 822 hip fractures in 33 313 065 person-years of observation and women contributed 263 993 hip fractures in 38 387 660 person-years. After adjustment for age and seasonality, hip fracture incidence increased by 2.6% (95% CI: 2.3–2.8%) per degree increase in latitude for men and by 1.7% (95% CI: 1.5–1.9%) for women. The increases were even more marked when additionally adjusting for population density (as a surrogate of urban vs rural lifestyle). There was a marked seasonal variation of hip fracture. The highest risk was observed in February and the incidence was 37.6 and 23.5% lower in men and women respectively during the summer. Importantly, there were significant interactions of amplitude of the seasonal variation with latitude (P<0.001 for both men and women), indicating that seasonal variation during the year was more pronounced in the north of Sweden than in the south.

These associations strengthen the hypothesis that vitamin D status has an important impact in the causation of hip fracture.