A common source of vitamin D is fatty fish, such as salmon, mackerel, and sardines. These fish contain high levels of vitamin D3, which is crucial for maintaining bone health and supporting immune function. Vitamin D can also be synthesized in the skin through exposure to sunlight, making it unique among vitamins. For those who may not get enough Sun exposure or have dietary restrictions, fortified foods such as milk, orange juice, and cereals, as well as supplements, can also be good sources of vitamin D to help maintain adequate levels.