Vitamin D3, long viewed as a simple micronutrient, is now recognised as a secosteroid hormone with endocrine, paracrine, and autocrine activity that extends well beyond skeletal physiology. Following cutaneous synthesis or dietary intake, it undergoes hepatic and renal activation to generate 1,25-dihydroxyvitamin D3, the metabolite that exerts genomic effects through the vitamin D receptor (VDR). VDR is highly expressed in the GI tract, regulating immunity, barrier function, and microbiome interactions.
Despite its physiological importance, vitamin D deficiency affects nearly one billion individuals globally, with far higher prevalence in India, with 70–90% across age groups.