The body does make its own vitamin D-3. Vitamin D-3, also called cholecalciferol, is made in the skin of human beings from a compound called 7-dehydrocholesterol.
Vitamin D-3 forms when 7-dehydrocholesterol is exposed to enough ultraviolet B light. The amount of vitamin D-3 that forms depends on the person's age, skin color, clothing and whether or not she's wearing sunscreen. The time of year and the latitude also determine how much vitamin D-3 a person makes. Although it's possible to get too little vitamin D-3 from UVB light, spending a long time in the light doesn't result in vitamin D toxicity.