Used to prevent and treat vitamin D deficiency, which can lead to weak bones (rickets in children, osteomalacia in adults). Commonly added to fortified foods like milk, cereals, and plant-based beverages to support bone health by aiding calcium and phosphorus absorption. Also used in dietary supplements and prescribed in higher doses for individuals with low sun exposure or certain medical conditions affecting nutrient absorption. Plays a role in supporting immune function and may help reduce inflammation.