AUTHOR=Osredkar Joško , Vičič Vid , Hribar Maša , Benedik Evgen , Siuka Darko , Jerin Aleš , Čegovnik Primožič Urška , Fabjan Teja , Kumer Kristina , Pravst Igor , Žmitek Katja 

TITLE=Seasonal variation of total and bioavailable 25-hydroxyvitamin D [25(OH)D] in the healthy adult Slovenian population

JOURNAL=Acta Biochimica Polonica

VOLUME=Volume 71 - 2024

YEAR=2024

URL=https://www.frontierspartnerships.org/journals/acta-biochimica-polonica/articles/10.3389/abp.2024.13108

DOI=10.3389/abp.2024.13108

ISSN=1734-154X

ABSTRACT=Objective: Our study's objective was to compare, in a group of healthy persons, the total 25(OH)D fraction, the bioavailable vitamin fraction, and the free vitamin D fraction in spring and fall.Methods: In our study, we collected blood samples from healthy participants at the end of both summer and winter, measuring serum levels of albumin, DBP, and 25(OH)D. Utilizing this data, we calculated the percentage of free and bioavailable vitamin D. Our cohort comprised 87 participants, with a male-to-female ratio of 14:73, aged 35.95±12.55 years, spanning 19-70 years. We employed the chemiluminescent method to determine the vitamin 25(OH)D levels, the ELISA method was utilized to determine the levels of DBP, the albumin BCP Assay was conducted using the ADVIA biochemical analyser (Siemens) and an online calculator was used to determine the free and bioavailable 25(OH)D. Results: Our findings indicate significantly lower 25(OH)D levels in winter (44.13±17.82 nmol/l) compared to summer (74.97±22.75 nmol/l; p<0.001). For vitamin D binding protein there was no significant difference from summer (236.2±164.39 mg/L) to winter (239.86±141.9 mg/L; p=0.77), albumin levels were significantly higher in summer (49.37±4.15 g/L vs. 47.97±3.91 g/L, p=0.01). In the winter season a significantly lower calculated bioavailable 25(OH)D vitamin (7.45±5.66 nmol/L against 13.11±8.27 nmol/L; p < 0.001) was observed, and the free fraction also showed a significant decrease (17.3±12.9 pmol/L versus 29.7±19.1 pmol/L; p < 0.0001). We observed a moderately positive correlation between 25(OH)D and bioavailable percentage in winter (r = 0.680; p<0.001), contrasting with a lower positive association in summer (r = 0.343; p<0.001). Conclusions: Our data suggest a positive correlation between total and bioavailable 25(OH)D levels. Apart from the statistically significant variation in 25(OH)D between the two observation periods, there was an additional variation in the vitamin D free percentage. It summertime synthesis of vitamin D in the skin could contribute directly to the free fraction of vitamin D. Standardizing the measurement of free 25(OH)D and clinical studies will be needed to establish reference values before these methods can be implemented in clinical practice.