Seasonal variations of 25(OH)D vitamin in Latvia are age and gender-related – implications for testing schedule

Considerable seasonal deviations of 25(OH)D, typical for countries with seasonal climate, are important for proper testing and correction regimens. We present an extensive retrospective analysis of seasonal 25(OH)D status in Latvian population. Anonymized test results from two leading Latvian laboratories were assessed – E.Gulbja Laboratory (EGL, 849739 tests, 2012-2019) and Central Laboratory (CL, 410427 tests, 2014-2019). 25(OH)D level, rate of deficiency <12 ng/mL, testing date and clients’ age and gender were analyzed by IBM SPSS v25 software (Kolmogorov-Smirnov and Kruskal-Wallis tests). Testing methods and target populations of the laboratories were different, so results are not directly comparable. Still, trends were remarkably similar. In both data sets 25(OH)D level was highest in summer (SU), intermediate in autumn (AU) and lowest in winter (WN)/spring (SP), deficiency rate mirrored the dynamics. Maximal seasonal difference was 3.8 ng/mL of 25(OH)D level and 6.8% of deficiency rate in EGL and, respectively, 4.7 ng/mL and 10.5% in CL, all differences highly significant. Seasonality was more pronounced in men, with WN/SP levels significantly lower than in women and SU levels equal. Variations in both sets were similar in all age groups, except children and adolescents (no gender differences) and elderly women (negligible seasonal variations). Maximal number of tests in both laboratories had been performed in AU (27.6% in EGL and 27.3% in CL), minimal – in SU, this distribution was seen in all age groups. The study that covered a major part of 25(OH)D testing in Latvia demonstrated significant variations, characteristic for seasonal climate and diet. These variations were more pronounced in younger people and in men, thus defining target populations for additional correction. Higher testing activity in autumn that probably reflects clients’ preferences, seems to be clinically unfounded.