The role of serum total and free 25-hydroxyvitamin D and PTH values in defining vitamin D status at the end of winter: a representative survey

We sought the lowest serum total 25-hydroxyvitamin D (t-25OHD) values in geographic areas with four seasons and investigated whether the calculation of serum free 25-hydroxyvitamin D (f-25OHD) could provide additional information on vitamin D status. This is a representative, cross-sectional study restricted to a sampling period at the end of winter, using a non-probability, stratified sample of the adult community-dwelling Hungarian population (n = 882). We measured t-25OHD, vitamin D binding protein (DBP), parathyroid hormone (PTH), and albumin levels. f-25OHD concentrations were calculated. We assessed environmental factors that could affect vitamin D levels and diseases possibly related to vitamin D deficiency. Mean t-25OHD values of the total population were 41.3 ± 20.6 nmol/L. t-25OHD levels were below 75, 50, and 30 nmol/L in 97, 77, and 34 % of participants not receiving vitamin D supplementation, respectively. t-25OHD values weakly positively correlated with DBP (r = 0.174; p = 0.000), strongly with f-25OHD (r = 0.70; p = 0.000). The association between t-25OHD and f-25OHD and between t-25OHD and PTH were non-linear (p squared term = 0.0004 and 0.004, respectively). t-25OHD levels were not affected by gender, age, place of residence; however, they were related to body mass index, sunbed sessions, and tropical travel. In contrast, f-25OHD levels were different in males and females but were not related to obesity. t- and f-25OHD were lower among people with cardiovascular diseases (p = 0.012). Nearly the entire Hungarian population is vitamin D insufficient at the end of winter. The use of t-25OHD could show a spurious association with obesity; however, it does not reflect the obvious sex difference.