Vitamin D is not dietary if without sunshine, which means you have to get sunshine in order to have enough vitamin D. Vitamin D is not a real vitamin and it's actually a hormone that needs sunlight to absorb. Some regions' people have less exposure in sunlight have not enough Vitamin D and it's the same in winter in a lot of regions. While there is vitamin D in food, but it's so limited since people moved from the Equator to other places lacking in vitamin D absorption. Evidence indicates the synthesis of vitamin D from sun exposure is regulated by a negative feedback loop that prevents toxicity, but because of uncertainty about the cancer risk from sunlight, no recommendations are issued by the Institute of Medicine (US) for the amount of sun exposure required to meet vitamin D requirements. Accordingly, the Dietary Reference Intake for vitamin D assumes no synthesis occurs and all of a person's vitamin D is from food intake, although that will rarely occur in practice. As vitamin D is synthesized in adequate amounts by most mammals exposed to sunlight, it is not strictly a vitamin, and may be considered a hormone as its synthesis and activity occur in different locations. Vitamin D has a significant role in calcium homeostasis and metabolism. Its discovery was due to effort to find the dietary substance lacking in rickets.