Vitamin D levels in women and factors contributing to explain metabolic variations.

The elucidated metabolism of vitamin D in humans has been the support to explain the high involvement of this liposoluble vitamin in physiological functions. Clinical studies have associated levels of vitamin D metabolites with several disorders. Despite this knowledge, there is a controversy regarding the estimation of deficiency and the physiological and supraphysiological levels of vitamin D metabolites. The association between serum concentrations of vitamin D metabolites and several potentially influential factors (namely, age and anthropometric, seasonal, spatial and metabolic factors) is analyzed in this study. For this purpose, 558 women were recruited and interviewed in several Spanish provinces before blood sampling. Serum vitamin D and its metabolites were determined using an SPE-LC-MS/MS platform. The concentration range for vitamin D was 1.7-21.1 nmol/L and was influenced by body mass index (BMI), waist-to-hip ratio (WHR) and seasonal period. 25-hydroxyvitamin D levels were within 4.8-147.2 nmol/L and were related to WHR, season, latitude and calcium intake. The range of 24,25-dihydroxyvitamin D, 0.3-15.0 nmol/L, was associated to BMI, WHR, season, latitude and calcium intake. Finally, energy intake influenced the vitamin D 25-hydroxylase through the 25-hydroxyvitamin D/vitamin D ratio, which regulates the synthesis of the circulating form. According to these results, it is worth emphasizing the relevance of all these factors to explain the variability in serum levels of vitamin D and its metabolites. All these factors should be considered in future studies assessing the alteration of vitamin D metabolism.