Background: Classically, vitamin D has been implicated in bone health by promoting calcium absorption in the gut and maintenance of serum calcium and phosphate concentrations, as well as by its action on bone growth and reorganization through the action of osteoblasts and osteoclasts cells. However, in the last 2 decades, novel actions of vitamin D have been discovered. The present report summarizes both classic and novel actions of vitamin D. Summary: 1,25(OH)₂ vitamin D, the active metabolite of vitamin D, also known as calcitriol, regulates not only calcium and phosphate homeostasis but also cell proliferation and differentiation, and has a key a role to play in the responses of the immune and nervous systems. Current effects of vitamin D include xenobiotic detoxification, oxidative stress reduction, neuroprotective functions, antimicrobial defense, immunoregulation, anti-inflammatory/anticancer actions, and cardiovascular benefits. The mechanism of action of calcitriol is mediated by the vitamin D receptor, a subfamily of nuclear receptors that act as transcription factors into the target cells after forming a heterodimer with the retinoid X receptor. This kind of receptors has been found in virtually all cell types, which may explain its multiple actions on different tissues. Key Messages: In addition to classic actions related to mineral homeostasis, vitamin D has novel actions in cell proliferation and differentiation, regulation of the innate and adaptative immune systems, preventive effects on cardiovascular and neurodegenerative diseases, and even antiaging effects.

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