Dietary intake and sunlight-driven synthesis are the two main sources of vitamin D for the human body. Under optimal sunlight exposure skin cells can synthesize vitamin D3, a vitamin D precursor. Briefly, the UVB component of the sunlight reacts with provitamin D3, a cholesterol-based steroid in the cells of the stratum basale and stratum spinosum converting it to previtamin D3, which later transforms into cholecalciferol or vitamin D3. It then enters the circulation, attaches to vitamin D-binding protein in the serum, and reaches the liver, where it is converted into calcidiol. It is a biologically inactive primary circulating form of the vitamin, which ultimately reaches the kidneys and undergoes conversion to calcitriol, the active form of vitamin D. Calcitriol stimulates calcium and phosphorus absorption from the gastrointestinal tract to raise their serum levels aiding skeletal health. Severe vitamin D deficiency can cause rickets in children, osteomalacia, and osteoporosis in adults. Seasons, location, and sunscreen use can also influence sun-induced vitamin D3 production.