Vitamin D helps the body absorb calcium, one of the minerals needed for bone formation, and is used to prevent osteoporosis in adults and rickets in children. Because few foods contain vitamin D, it is added to milk, cereals, orange juice, yogurt and soy beverages, according to MedlinePlus.
Vitamin D is used to treat conditions involving the heart and blood vessels, such as high blood pressure and high cholesterol. It is also used to prevent and treat diabetes, obesity, muscle weakness, multiple sclerosis, rheumatoid arthritis, chronic obstructive pulmonary disease, asthma, bronchitis, premenstrual syndrome and tooth and gum disease, states WebMD. Some people use vitamin D to treat skin conditions such as scleroderma, psoriasis and lupus. It is also believed to boost the immune system and to prevent autoimmune diseases and certain types of cancer.
Vitamin D is stored in fatty tissue and released when needed. As much as 90 percent of the body's requirement of vitamin D is obtained by exposure to sunlight. Six days of exposure can replenish stores of vitamin D that have been depleted by 49 days without sunlight. People living in northern climates are most at risk for vitamin D deficiency, but many residents of sunny climates are deficient because of the time spent indoors and the use of sunscreens, says WebMD.