Association between serum 25-hidroxyvitamin D concentrations and ultraviolet index in Portuguese older adults: a cross-sectional study

BACKGROUND The older population is a risk group for hypovitaminosis D. The Ultraviolet Index (UVI) can be an indicator of potential for cutaneous synthesis of vitamin D but physiological and other environmental factors also influence vitamin D synthesis and status. Knowledge about vitamin D status in Portuguese older adults is limited. This study aims to explore the association between Ultraviolet Index and serum 25-hidroxyvitamin D3 [25(OH)D] levels accounting for other potential influential factors. METHODS A cross-sectional study was conducted between December 2015 and June 2016, in 1497 Portuguese older adults (≥ 65 years) within Nutrition UP 65 project. For each participant, serum 25(OH)D was determined and the mean UVI (mUVI) in the respective residence district was calculated for the previous 30 days. Stepwise linear regression analyses were conducted for the following periods of blood collection: between December and June, December and March and April and June. Standardized regression coefficients (Sβ) and 95% confidence intervals were calculated. RESULTS The median 25(OH)D concentration was 35.9 nmol/L. The UVI was independently and positively associated with 25(OH)D in the models for December-June (Sβ = 0.244, 95% CI: 0.198; 0.291, P < 0.001) and April-June (Sβ = 0.295, 95% CI: 0.299; 0.362, P < 0.001) and independently and negatively associated in December-March period (Sβ = -0.149, 95% CI: -0.211; -0.087, P < 0.001). CONCLUSIONS In this sample with high vitamin D deficiency frequency, the UVI was a predictor of 25(OH)D levels but the direction of the association varied according to the blood collection period. Our results suggest that accounting for the time of year in future research regarding vitamin status and related public health recommendations may be relevant.