The main dietary source of vitamin D is fatty fish, such as salmon, mackerel, and sardines. These fish not only provide essential fatty acids but also contain significant amounts of vitamin D, which is vital for bone health and immune function. Additionally, fortified foods like milk, orange juice, and cereals can also be good sources of vitamin D. The body can synthesize vitamin D when skin is exposed to sunlight, which is why it’s often termed the "sunshine vitamin." However, factors like geographic location and skin pigmentation can affect synthesis, making dietary sources important.