Vitamin D is a fat-soluble vitamin that is found in two forms - D2 and D3. Vitamin D2 or ergocalciferol is synthesized by plants and found in dietary sources such as fish, eggs and cod liver oil. Vitamin D3 or cholecalciferol is the primary source of vitamin D and comes from exposure to ultraviolet-B rays from the sun which synthesize vitamin D in the skin. Vitamin D is required for the absorption of calcium and helps maintain proper levels of calcium and phosphorous which together are the primary minerals required for bone growth and maintenance. Thus vitamin D plays a large role in the prevention of bone-related disorders such as osteoporosis and osteomalacia. However, inadequate vitamin D status has more recently been linked to diseases of the cardiovascular system, certain cancers and autoimmune diseases such as diabetes (type 1) and multiple sclerosis.

In the United States, the fortification of milk with vitamin D in the 1930's quickly put an end to rickets, a condition caused by a lack of vitamin D in children which led to fragile bones and debilitating deformities. However, many Americans still exhibit inadequate vitamin D levels resulting from lack of good food sources and inadequate sunlight exposure. Sunlight provides over 80 percent of vitamin D requirements and geographic location, skin color and the increased use of sunscreen and decreased exposure to sunlight due to skin cancer risk recommendations have all played a significant role in low vitamin D status. Dark skin pigmentation markedly reduces vitamin D synthesis as does the use of sunscreen where an SPF 15 decreases UV-B absorption by as much as 99 percent. The summer months may be the only time UV-B exposure is adequate enough for vitamin D synthesis for many individuals living north of southern Tennessee which is further supported by the increased risk of cancer, diabetes and hypertension among higher latitude populations.

Vitamin D deficiency is common among all ages with 36 percent of young adults, 50 percent of middle-aged and elderly adults and approximately 24 percent of adolescents lacking sufficient vitamin D levels. As mentioned, vitamin D deficiency may be linked to a variety of non-skeletal complications. Vitamin D receptors are found on a variety of cardiovascular system tissues and disturbances in calcium homeostasis contribute to an increased risk of high blood pressure or an enlarged heart. Insulin secretion is partly dependent on vitamin D and deficiencies can decrease secretion and increase insulin resistance leading to the development of diabetes. Thus, adequate vitamin D status may play a large role in preventative medicine.

Due to the recent findings of vitamin D's impact on chronic diseases, the Institute of Medicine (IOM) has recently increased the daily recommendations. Previous recommendations ranged from 200-400 IU daily but have now increased to 600 IU a day for all individuals between 12 months and 70 years old and 800 IU for individuals over 70. The upper intake levels safe for consumption have been set between 2,500 IU-4,000 IU daily. Since adequate vitamin D is difficult to obtain through dietary sources alone, most healthcare professionals recommend that individuals living in northern climates supplement with 1000 IU daily.