The most important role of vitamin D is to preserve normal blood levels of the minerals calcium and phosphorus. With assisting minerals such as phosphorus, boron, magnesium and zinc, Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. Without vitamin D, bones can become thin, brittle or soft, and more breakable. Bone-strengthening vitamin D thereby prevents childhood rickets and adult-onset osteomalacia.

Do Americans get enough Vitamin D? Over the past few years, a growing number of scientists have come to believe we don't. The latest research even goes so far as to say that vitamin D deficiency is an unrecognized epidemic in the US.

A recent study in Boston found vitamin D deficiency in almost a quarter of adolescents tested during routine checkups who were otherwise completely healthy. Several studies have shown muscle problems and osteoporosis in adults to be strongly associated with a lack of vitamin D and pediatricians have also observed an increase in children and babies with rickets, a bone disorder caused by vitamin D deficiency. Rickets was a major health concern for children in the US until the introduction of Vitamin D fortified milk in the 1930s, but now the problem seems to be returning.

A growing body of research also suggests associations between vitamin D deficiency and a slew of more serious illnesses including multiple sclerosis and other auto-immune diseases, some cancers, cardiovascular disease, high blood pressure, rheumatoid arthritis and diabetes. Incidences of some of these illnesses are higher in populations who live further from the equator, suggesting a connection to sun exposure levels. Although these associations are more tentative, they're strong enough that scientists are pushing for a sharp increase in the recommended dose of daily vitamin D, and are also encouraging a small amount of sun exposure for the general population.

Understandably, the suggestion that people should be getting more sun has upset some dermatologists and skin cancer experts who would prefer people to have complete protection from UV rays. However, it seems possible to practice safe sun exposure by baring your face, arms and legs without sunscreen for 5-15 minutes a day, a few times a week. As commonly recommended, try to get this sun before 10:00 am and after 3:00 pm. Most researchers believe this will generate sufficient vitamin D without increasing the risk of skin cancer or wrinkling. After those few minutes though, blocking up with SPF 15 or higher is definitely recommended.

Sometimes actually catching enough rays can be difficult, especially during the winter months. There are several things that negatively affect the sun's UV ray exposure and hence decrease vitamin D. Season, latitude, time of day, cloud cover, smog, color of skin, clothing cover, indoor work, glass, and sunscreens (SPF 8 or higher) can all diminish your chances of getting enough vitamin D.