Vitamin D3 (Cholecalciferol) is a fat-soluble vitamin and secosteroid hormone that plays a key role in calcium and phosphate homeostasis, bone health, and immune function. It is synthesized in the skin upon exposure to ultraviolet B (UVB) light or obtained through diet and supplements.

Vitamin D3 is believed to support calcium absorption in the intestines, regulate bone mineralization, modulate immune responses, and influence cellular growth and differentiation. It is commonly researched for its possible effects on bone density, skeletal health, immune system function, and overall metabolic and cardiovascular health.