Vitamin D is essential for bone health. Without enough vitamin D, your body can't absorb calcium well, leading to weaker bones and conditions like osteoporosis in adults and rickets in children. You can get vitamin D from sun exposure, certain foods (like fatty fish, egg yolks, and fortified dairy), and supplements. If you lack sun exposure or have a diet low in vitamin D, you might need a supplement. Adults generally need 600-800 IU daily, while older adults might need 800-1,000 IU. If you suspect a deficiency, your doctor can confirm it with a blood test and recommend appropriate supplements.