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.