V i t a m i n D
Vitamin D is different from other essential vitamins because our own body can manufacture it with sunlight exposure. The main function of vitamin D is to regulate the absorption of calcium and phosphorus in our bones and aid in cell to cell communication throughout the body.
Vitamin D is responsible for enhancing intestinal absorption of calcium and phosphate. In humans, the most important compounds in this group are vitamin D3 (also known as cholecalciferol) and vitamin D2 (ergocalciferol).
Vitamin D for humans is obtained from sun exposure, food and supplements (as only few food contain it). It is biologically inert and has to undergo chemical reactions to become active in the body. The synthesis of vitamin D3 in the skin, from cholesterol, is happening, when exposure to the sun is adequate.
That's why it is often called the "sunshine" vitamin. Vitamin D deficiency is clearly linked to an increased risk of viral infections, playing a role in the high rates of influenza during the winter season.
Vitamin D is carried in the bloodstream to the liver, where it is converted into a prohormone (calcidiol), which may then be converted in the kidneys into calcitriol, the biologically active form of vitamin D. Following the final converting step in the kidney, calcitriol is released into the circulation.
Calcitriol circulates as a hormone in the blood, regulating the concentration of calcium and phosphate in the bloodstream and promoting the healthy growth and remodeling of bone. Calcitriol also affects neuromuscular and immune function (reduction of inflammation). Many genes encoding proteins that regulate cell proliferation and differentiation are modulated in part by vitamin D.
Vitamin D3 is a multi-talented nutrient and deficiency has been linked to a host of unwanted conditions such as high blood pressure, uberculosis, fibromyalgia, diabetes, multiple sclerosis, rheumatoid arthritis, insulin resistance during pregnancy and increased prevalence of early age-related macular degeneration.
15 µg RDA - 100 µg UL
RDA (recommanded dietary allowances) per day 
UL (tolerable upper intake level) per day 
 Food and Drug Administration (FDA)
 Dietary Reference Intakes: Tolerable Upper Intake Levels, Food and Nutrition Board, Institute of Medicine, National Academies, 1997