Final answer:
To activate vitamin D, UV radiation first converts cholesterol in the skin to cholecalciferol, which is then processed in the liver and kidneys to become calcitriol, the active form of vitamin D necessary for calcium absorption and bone health.
Step-by-step explanation:
In order to convert vitamin D to its active form in the skin, it must first be synthesized from cholesterol in the presence of UV radiation as cholecalciferol (vitamin D3). Then, cholecalciferol is converted in the liver to calcidiol (25(OH)D), and subsequently in the kidneys, calcidiol undergoes a final transformation to calcitriol (1,25-dihydroxycholecalciferol), which is the biologically active form of vitamin D essential for the absorption of calcium. This process is vital for bone health and has systemic effects on the body, from bone growth to immune function.