Impact of 25-hydroxyvitamin D, free and bioavailable fractions of vitamin D, and vitamin D binding protein levels on metabolic syndrome components

Introduction: Various forms of vitamin D and factors involved in their metabolism can play a role in the etiopathogenesis of metabolic disorders. This paper aims to define the relationship between concentration of the hydroxylated form of vitamin D (25(OH) D), the fraction of free and bioavailable vitamin D, and of vitamin D binding protein (VDBP) levels on the one hand and the prevalence of metabolic syndrome components on the other. Material and methods: The studies were conducted on 79 people, including 52 with metabolic syndrome (MetS+) and 27 without it (MetS-). Biochemical measurements (lipid profile, glycemia, 25(OH) D, VDBP, albumin, calcium, parathyroid hormone) were performed, concentration of free and bioavailable vitamin D was mathematically calculated, and anthropometric and blood pressure measurements were taken. Results: The mean +/- SD concentration of 25(OH) D among MetS+ individuals (41.90 +/- 13.12 nmol/l) was lower (p < 0.0001) than among the MetS- group (66.09 +/- 18.02 nmol/l). Differences between groups were observed in relation to medians/means of concentrations of free and bioavailable vitamin D (p < 0.0001) but not in the case of VDBP. In the entire study population, 25(OH) D correlated with all metabolic syndrome components, whereas its free and bioavailable fraction correlated with particular components of the syndrome. In the MetS+ group, VDBP concentration negatively correlated with body mass index (p = 0.037) and levels of diastolic pressure (p = 0.022). In the case of the MetS- group, the free fraction of vitamin D negatively correlated with triglyceridemia (p = 0.049). Conclusions: The evaluation of various forms of vitamin D and VDBP in different population groups seems to have significant clinical value in evaluating the prevalence of metabolic disorders.