Unlike other vitamins, we can absorb Vitamin D from our diets or synthesize it directly through exposure to sunlight.
Drinking milk actually helps us get a daily allowance of calcium in two ways. Milk naturally contains calcium, and is frequently fortified with Vitamin D. The presence of the vitamin assists in the uptake of calcium which supports strong healthy bones.
Vitamin D differs from other vitamins because the body can either synthesize it upon exposure to sunlight or absorb it from the diet. Dietary sources of the vitamin include fatty fish, egg yolks, and liver, but since these are not on the short list of popular kids’ foods. Products such as milk, yogurt, and breakfast cereals add Vitamin D to ensure an adequate supply. A deficiency of Vitamin D contributes to rickets, a softening and weakening of bones due to inadequate supply of calcium and phosphate. This condition poses a specific threat to children because of their rapid bone growth. Vitamin D also protects adults from loss of bone density due to osteoporosis.
A fifteen-minute exposure of sunlight three times a week on a fair-skinned person’s face and hands goes a long way toward assisting the body make Vitamin D, but sunscreen and windows block light required for the synthesis. Additionally, in the winter time we often do not get that much direct sunlight. People with darker skin tone need greater sunlight exposure to produce adequate Vitamin D, although no correlation with a higher concentration of melanin in their skin exists. Although Vitamin D is fat soluble, which means that the body can accumulate and store it, the body has a feedback mechanism so that too much exposure to sun does not result in an overdose of the vitamin. Concerns about sun-damaged skin leading to skin cancer, however, suggest that large amounts of unprotected sun exposure can potentially cause more harm than good. Lastly, extensive sun exposure still fails to compensate for a shortage of Vitamin D in the diet.
More information is available at http://www.nlm.nih.gov/medlineplus/ency/article/002405.htm