We've all heard the warnings about the dangers of continued overexposure to the sun and many of us have heeded them. Continued overexposure and frequent episodes of sunburn as a child and young adult and can break down the elastic proteins in the skin, causing wrinkling, and increase the risk of basal- and squamous-cell carcinomas later in life. It can also put us at greater risk for the most deadly form of skin cancer, melanoma (although the connection still begs the question why most melanomas occur in unexposed skin).These recent scares send us rushing for the sunscreen every time we leave the house. But the attention to the risk of sun worshiping may have led us to overadhering to the aforementioned warnings. With the current cursing of the sun there is also greater awareness of the essential role of vitamin D in overall health. As it turns out, D is not just something that prevents rickets in children.The indication that the sun was intimately related to health occurred during the Industrial Revolution, when people flocked to crowded, polluted cities to live in small dwellings, especially in the northern latitudes of Europe and the eastern coast of the United States. Children growing up with limited exposure to the sun suffered from growth retardation, bony projections along the ribcage, and either bow legs or knocked knees.By 1900, it was estimated that 80 percent of the children in Boston suffered from rickets. Making the connection between the sun and rickets led to the fortification of foods with vitamin D, and by the 1930s rickets was virtually eradicated. (After World War II, vitamin D fortification was poorly regulated, and many children received toxic amounts of the vitamin, which led to the banning of fortification of many products in Europe.)During exposure to the sun, a derivative of cholesterol called 7-dehydro cholesterol - present in the plasma membranes of skin cells - absorbs ultraviolet radiation (UVB). This produces a rearrangement 7-dehydro cholesterol to previtamin D3, which undergoes further rearrangement to vitamin D3, which enters the circulation. The liver converts vitamin D3 to 25(OH)D3, which goes to the kidney for further conversion to the active form, 1,25(OH)2D3. Whatever affects the amount of UVB photons penetrating the skin influences the amount of active vitamin produced. Latitude, time of year, pollution, sunscreen, skin color, and age all affect vitamin D production.Today, we take vitamin D for granted, assuming that it's plentiful in the diet and that short of rickets, it's not very important. This could not be further from the truth. There are very few foods naturally rich in vitamin D. Humans have always relied on sun exposure the supply vitamin D. There is no risk of toxicity from sun exposure because the excess inactive forms are converted to inert substances.How important is vitamin D outside of bone metabolism? In the 1940s, Dr. Frank Apperley reported in Cancer Research that death rates for various cancers increased progressively as you moved away from the equator. In 1980, researchers noted a similar pattern in colon, breast and prostrate cancers. There appears to be the same latitude association with multiple sclerosis, cardiovascular disease and hypertension. Do we eat more fruits and vegetables at the lower latitudes? Or is the vitamin D connection to health stronger than we had thought?A primary function of vitamin D is to maintain serum calcium concentrations within the normal range. However, it plays a number of other vital roles. As it turns out, vitamin D interacts with certain nuclear receptors similar to other steroid hormones. These vitamin D receptors are present in many different cells, like those of the small intestines, colon, bone, activated T and B lymphocytes, β-islet cells in the pancreas, most organs, brain, heart, skin, gonads, prostate and breast.In other words, D is needed everywhere. The concern for a growing vitamin D deficiency is not only that it can increase the risk of osteomalacia, osteoporosis and hip fractures, but that it can wreak havoc with general health.O.K., so is the choice skin cancer or a host of other diseases? As for sunshine, about 15 to 20 minutes a day in the sun can take care of the bare minimum needs, that is, the amount required to prevent rickets. Even the most heliophobic health nuts wouldn't blanch at a 15- to 30-minute walk in the open on a sunny day.But what about the days that's not possible? Should we load up on tons of vitamin D? Again, the "M" word - moderation - enters into the health picture. Excessive supplemental vitamin D can be toxic, producing hypercalcemia.The easiest solution all around is dietary intake. There is no risk of vitamin D toxicity from sun exposure because the excess is converted to inert substances. Dairy foods, various fortified milk beverages or other D-fortified drinks, in reasonable portions, all add to the protection and the value of vitamin D.Editor's note: This subject will be covered in greater depth in an upcoming article.