Introduction. Low serum vitamin D levels are associated with increased postural sway. Vitamin D varies seasonally. This study investigates whether postural sway varies seasonally and is associated with serum vitamin D and falls. Methods. In a longitudinal observational study, eighty-eight independently mobile community-dwelling older adults (69.7 7.6 years) were evaluated on five occasions over one year, measuring postural sway (force platform), vitamin D levels, fall incidence, and causes and adverse outcomes. Mixed-methods Poisson regression was used to determine associations between measures. Results. Postural sway did not vary over the year. Vitamin D levels varied seasonally ( ), peaking in summer. Incidence of falls ( ) and injurious falls ( ) were lower in spring, with the highest fall rate at the end of autumn. Postural sway was not related to vitamin D ( ) or fall rates, but it was associated with fall injuries (IRR 1.59 (CI 1.14 to 2.24, ). Conclusions. Postural sway remained stable across the year while vitamin D varied seasonally. Participants with high values for postural sway demonstrated higher rates of injurious falls. This study provides important evidence for clinicians and researchers providing interventions measuring balance outcomes across seasons. 1. Introduction Balance impairment is an important fall-risk factor [1], and increases in range of postural sway in the mediolateral direction in older adults are associated with increased fall-risk and rates [2]. Postural sway has been shown in older adults to be strongly related to other measures of balance [3]. Multivariate analysis reveals serum vitamin D levels as an independent variable associated with postural sway [4]. In individuals with suboptimal levels of vitamin D, balance and strength improve after supplementation [5], in particular postural sway [6]. Epidemiological studies have shown that vitamin D levels show seasonal variation [7, 8]. Lowest levels of serum vitamin D are recorded towards the end of winter, approximately four weeks after the shortest day of the year [8]. Overall, vitamin D supplementation did not reduce rate of falls (RaR 1.00, 95% CI 0.90 to 1.11; seven trials; 9324 participants) or risk of falling (RR 0.96, 95% CI 0.89 to 1.03; 13 trials; 26, 747 participants) but may do so in people with lower vitamin D levels before treatment [9]. Older adults are at risk for lower levels of serum vitamin D because of age-related changes in UVB absorption and skin capacity to synthesize vitamin D, reduction in activation in the kidneys, and reduced expression of
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