The Effect of Latitude on the Risk and Seasonal Variation in Hip Fracture in Sweden

Abstract

Although the optimal requirement of vitamin D for skeletal health in the general community is controversial, vitamin D deficiency impairs bone mineralization and increases bone turnover via secondary hyperparathyroidism, thus accelerating bone loss and increasing fracture risk. Support for a role of vitamin D deficiency in the epidemiology of hip fracture is found in the seasonal variation of hip fracture incidence that is reported in several studies. If the association were causal, then the incidence and amplitude of the seasonal variation in hip fracture risk should vary by latitude. We addressed this hypothesis by examining 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 reduce double counting, only one fracture in a period of a year was counted per individual. Men contributed 104,888 fractures in 33,313,065 person years and women 264,362 fractures in 38,387,660 person years. The effects of season and latitude were examined by Poisson regression. As expected, hip fracture rates were higher in women than in men. After adjustment for age, season and population density, hip fracture incidence increased by 3.0% (95% CI: 2.7-3.2%) per degree increase in latitude for men and by 1.9% (95% CI: 1.8-2.1%) for women. There was a marked seasonal variation of hip fracture with the highest risk in February and lower by 37.5% in men and by 23.5% women during the summer. 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. The associations found with latitude and season is consistent with a role of vitamin D in hip fracture causation.

Older Swedish Adults with High Self-Perceived Health Show Optimal 25-Hydroxyvitamin D Levels Whereas Vitamin D Status Is Low in Patients with High Disease Burden

“…Variations in solar UVB doses and 25(OH)D concentrations have been proposed to explain this variation in hip fracture incidence [10]. In the current study, 25(OH)D levels did not differ significantly among hip fracture patients in the summer months compared with those in the winter months.…”

“…Older adults usually spend less time in the sun than younger and have a lower capacity for their skin to produce vitamin D. Sweden has an aging population, and one of the highest reported incidences of hip fractures. There is also a seasonal variation for hip fracture, with the highest risk in winter, and the lowest in summer [10]. The economic burden of incidental fragility fractures in Sweden is very high [11].…”

The role of vitamin D in osteoporosis

“…Recently, two Swedish studies show that there is a seasonal effect on the risk of fractures, which is highest in winter, when there are lower serum and plasma levels of 25-hydroxyvitamin D [26,27]. This supports the results of a meta-analysis which confirms the strong association between 25-hydroxyvitamin D concentrations and cause specific and all cause mortality [28].…”

Geovariation in Fracture Risk among Patients Receiving Hemodialysis

“…One possibility is the confluence of sunlight exposure, latitude, and nutritional vitamin D [25(OH)D] levels. Latitude, a likely proxy for sunlight exposure, has been associated with fracture risk in ecologic studies of the general population (36,40,41). Individuals in more solar-rich environments generally have higher levels of 25(OH)D, which itself is associated with lower fracture risk (42).…”