Final answer:
Vitamin D is produced in the skin and activated in the liver and kidneys to form calcitriol, which is crucial for calcium absorption and bone health. Deficiencies in vitamin D can cause bone diseases such as rickets and osteomalacia.
Step-by-step explanation:
The epidermal layer of human skin synthesizes vitamin D under the influence of UV radiation from sunlight. A precursor molecule, 7-dehydrocholesterol, found in the skin, is converted to cholecalciferol (vitamin D3). This is then processed in the liver into 25-hydroxycholecalciferol (calcidiol), followed by conversion in the kidneys into the bioactive form, calcitriol (1,25-dihydroxycholecalciferol).
Calcitriol plays a critical role in maintaining proper serum levels of calcium and phosphorus, which support a variety of body functions such as bone health, muscle contractions, and cellular processes. Deficiencies in vitamin D can lead to conditions like rickets in children and osteomalacia in adults, emphasizing the necessity of this nutrient.