The primary source of vitamin D for the human body is sunlight. When skin is exposed to ultraviolet B (UVB) rays from the sun, it synthesizes vitamin D3, which is crucial for calcium absorption and maintaining bone health. Additionally, vitamin D can be obtained through certain foods, like fatty fish, fortified dairy products, and supplements. Insufficient vitamin D levels can lead to health issues such as osteoporosis and weakened immune function, making sunlight exposure vital for overall health.