Normally you get adequate vitamin D, ie 400 International Units(IU), through daily exposure to the sun which stimulates its formation in the skin. Dietary sources make up any differences where necessary. Vitamin D Deficiency(VDD) occurs if the body does not receive 400IU daily. Usually the causes of vitamin D deficiencies are:inadequate sunlight, low dietary intake, poor fat absorption, liver diseases, kidney failure, in-born abnormality in metabolism and lack of organ responsiveness.
Attribution: Avallain