• Vitamin D is necessary to help the body maintain normal levels of calcium and phosphorus in the bloodstream. It also helps the body absorb calcium, which is necessary for the production and maintenance of strong bones. Research suggests that vitamin D may provide other health benefits, including protection from osteoporosis, cancer and high blood pressure.
  
  Research also suggests that vitamin D has a role in the prevention and treatment of type 1 and type 2 diabetes. This evidence comes from in vitro, epidemiological and animal studies, however, and is not considered as reliable as randomized clinical trials.
  
  

Exposure

• Most individuals can get the necessary amount of vitamin D3 simply by exposure to sunlight. Seasons, cloud cover, pollution and other factors can impede the production of vitamin D3. Researchers suggest 5 to 30 minutes of exposure to the sun at least twice a week between 10 a.m. and 3 p.m. without sunscreen. The required UV level must be 3 or higher for proper production of the nutrient in the body. It is, of course, prudent to limit an individual's exposure to the sun since UV radiation in sunlight is a known carcinogen.
  
  

Sources

• An important source of vitamin D is food. Few foods are known as natural sources of vitamin D, however. The best-known sources for naturally occurring vitamin D include salmon, tuna and mackerel, as well as fish oils. Smaller amounts of vitamin D can be found naturally in beef liver, egg yolks and cheese. In these foods, the primary form is vitamin D3. Vitamin D2 can be found in varying amounts in some varieties of mushrooms. Other foods fortified with vitamin D include processed breakfast cereals, yogurt, margarine and orange juice.
  
  

Types

• While vitamin D occurs in several forms, two are significant to humans: ercocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Plants synthesize vitamin D2, while vitamin D3 is synthesized in humans through exposure to sunlight.
  
  

Deficiencies

• A skeletal deformity known as rickets is one of two classic vitamin D deficiency diseases. Rickets typically strikes children. Adults can suffer from the other classic vitamin D deficiency disease, osteomalacia, which is characterized by muscle weakness and weak bones. Osteomalacia typically strikes obese individuals, the elderly, anyone whose exposure to the sun is limited and infants who are exclusively breastfed.