Seasonal changes and serum 25-hydroxyvitamin D levels among community-dwelling elders who live in Boston, Massachusetts and Stockholm, Sweden
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BACKGROUND: The prevalence of Vitamin D deficiency is roughly 40% in the world and is increasing every year. Populations 65 years and older show a higher prevalence of vitamin D deficiency, because the aging process decreases the capacity of the skin to produce vitamin D. Some studies have reported that the prevalence of vitamin D deficiency is higher in the winter, however the effect of seasonal change on serum vitamin D level remains controversial in some specific populations. Moreover, this association remains uncertain in the elderly population because there is no study that specifically targets individuals over the age of 65. This study investigated the effect of seasonal changes and serum 25-hydroxyvitamin D among individuals 65 years and older residing in the Boston, Massachusetts and Stockholm, Sweden. METHODS: Cross-sectional and longitudinal cohort designs were both adapted to examine an existing data from VIVE2 parent study; the data was collected from 2012 to 2014. Data from the subjects who had finished this 6-month trial were analyzed for this study. Serum 25(OH)D levels, BMI, sex, study sites and age were collected and analyzed by univariate regression analysis and t-test. Serum 25(OH)D and confounders were included in multivariate analysis. Study sites were analyzed by effect modification model. RESULTS: In total, the prevalence of vitamin D deficiency (serum 25(OH)D levels less than 20 ng/ml) was 70%, while the mean serum 25(OH)D level was 20 ng/ml in summer and 16.4 ng/ml in winter. The average of seasonal serum 25(OH)D level changes were 6 ng/ml and 3 ng/ml in Stockholm, Sweden and Boston, MA, respectively. In addition, the prevalence of vitamin D deficiency increased 80% during winter (95CI: 1.1 – 2.9). There was no significant different in serum 25(OH)D levels among elderly populations between low latitude study site Boston, MA and high latitude site Stockholm, Sweden. There was no significant relation found in BMI, age and sex with serum 25(OH)D levels in the study. The seasonal serum 25(OH)D level changes was significantly different in the cross-sectional study design but not in the longitudinal study. CONCLUSION: Serum 25(OH)D levels were higher in the summer than in the winter among the elderly population resided in Boston, MA and Stockholm, Sweden.