Seasonality of vitamin D status in older people in Southern Germany: implications for assessment.

BACKGROUND: from a clinical and public health perspective, it is important to understand the influence of seasonality on the serum vitamin D level to adequately assess and interpret an individual measurement. Therefore, the aim of this study was to analyse the effects of seasonal conditions on 25-hydroxyvitamin D (25(OH)D) serum levels in a population-based cohort of older people. METHODS: between March 2009 and April 2010 the 25(OH)D serum level was assessed in 1,418 community-dwelling individuals living in Germany aged ≥65 years (56.7% men) with no subscribed vitamin D supplementation. Least-square means of monthly 25(OH)D serum levels with 95% confidence intervals (CI) were estimated, adjusted for gender, age and body mass index. Additionally, the proportion of vitamin deficiency (<20 ng/ml), insufficiency (20-<30 ng/ml) and sufficiency (30 ng/ml or higher) were estimated for each month. Finally, mean values of daily total global solar radiation and daylight were calculated for each month. RESULTS: the minimum 25(OH)D serum level was observed in March with 15.4 ng/ml (SD = 6.56 ng/ml) and the maximum in August with 25.6 ng/ml (SD = 6.59 ng/ml). Compared with daylight and global solar radiation the progression over the year was similar but delayed by ∼2 months. The proportion of vitamin D deficiency, insufficiency and sufficiency were 78.8, 19.2 and 1.9% in March and 16.1, 63.4 and 20.5% in August, respectively. CONCLUSION: vitamin D insufficiency was very common in this cohort and showed a strong seasonal effect with lowest values in March.