Physiology and patho­physiology of 1,25-dihydroxycholecalciferol or calcitriol: hormone and/or vitamin D?

Here we review the history of vitamin D discovery, its mechanisms of action, and their clinical implications. This molecule, long considered a simple vitamin, actually acts as a steroid hormone involved in the regulation of more than a thousand genes. We describe its essential roles: calcium and phosphorus metabolism, immunity, cardiovascular health, reproduction, energy metabolism, and cancer prevention. We describe the many situations of deficiency (lack of sunlight, obesity, medication, chronic diseases) and their pathological consequences: rickets, osteoporosis, diabetes, autoimmune diseases, cancers, etc. We also warn against excessive supplementation, particularly in the Coimbra Protocol, and emphasize the need for an individualized and reasoned approach to vitamin D as a true hormone-vitamin.