Vitamin D is a lipid-soluble prohormone that is vital for the maintenance of bone and muscle health by promoting the absorption and metabolism of calcium and phosphate.1 In addition to food sources such as fatty fish, eggs, fortified milk and cod liver oil, the human body uses ultraviolet B (UVB) radiation from sunlight to synthesise a significant portion of vitamin D requirements.2 There are two forms of vitamin D: vitamin D2 (ergocalciferol) and D3 (cholecalciferol). The skin synthesises vitamin D3 after sun exposure and it may be obtained from animal sources, while vitamin D2 is the synthetic form that is often found in fortified food and is derived from plants.
The primary role of vitamin D has been considered to be the absorption of calcium from the intestine (i.e., calcium homeostasis in the body) and is necessary for skeletal health (bone mineralization, remodelling, and maintenance; Fig. 1). Over the years, it has become increasingly clear that vitamin D not only has a function in bones, but it also significantly affects cell proliferation and differentiation.
Vitamin D is a global regulator of gene expression and signal transduction in virtually every tissue. In epithelial cells vitamin D, by binding with the vitamin D receptor (VDR), contributes to maintenance of the quiescent, differentiated phenotype and promotes pathways that defend cells against endogenous and exogenous stresses.
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