Factors to take into account when interpreting 25-hydroxy-vitamin D serum levels.

Background: Assessing vitamin D status, typically evaluated using serum or plasma 25-hydroxy vitamin D [25(OH)D] concentration, is complex because of various influencing factors.

Methods: Seasonality significantly affects intra-individual variability in 25(OH)D levels. This variation can be addressed by employing cosinor functions that are tailored to the geographical location of the patient to correct for seasonal effects. In addition to seasonality, genetic factors, such as DBP polymorphism and body composition, particularly adiposity, play crucial roles. Dialysis patients with DBP 2-2 phenotype exhibit higher vitamin D requirements. Genotyping/phenotyping of DBP allows for better tailored vitamin D supplementation. The lipid-soluble nature of vitamin D also interacts with plasma components such as serum triglycerides, which can influence vitamin D measurements. Adiposity, which is negatively correlated with vitamin D concentration, necessitates body mass-based mathematical adjustments for accurate vitamin D assessment in subjects with extreme BMI values.

Conclusions: Accordingly, vitamin D replacement therapy must be personalized, taking into account factors such as body size and seasonal variations, to effectively reach the target serum 25(OH)D concentrations.