CYP27B1 and vitamin D deficiency: Assuming that the aim of vitamin D supplementation is to achieve and to maintain the optimal concentrations of 25(OH)D, as a substrate for renal and extrarenal 1α-hydroxylation (CYP27B1), and in consequence—synthesis of calcitriol, recommendations including endocrine, paracrine and autocrine effects of 1,25(OH)2D seem to reflect a holistic view on vitamin D deficiency and human health.