Vitamin D is fat soluble and is acquired through sunlight or diet. Ultraviolet sun rays act on the oils of the skin to produce vitamin D, which is then absorbed into the body. When taken orally, vitamin D is absorbed with fats through the intestinal walls. The production of vitamin D stops after a suntan has been established. The RDA for adults is 200-400 IU, but studies have shown that ones total lifestyle dictates a range of 2000 IU - 5000 IU daily. For example Smog reduces the amount of Vitamin D available for city dwellers. Night workers require more vitamin D. Dark complexion people living in northern climates need an increase in vitamin D. If you are older than 65 years or are overweight, you may be at risk of too little vitamin D.