Credit goes to Eat, Move, Improve for this extensive link on Vitamin D.
Here are a few tastes of what the article includes.
"85-90% of people are deficient to severely deficient in Vitamin D. This is because we get very little from our diets except for fish and in fortified dairy now. This is exacerbated by having jobs that are indoor. When we do go outside we use sunscreen which blocks UVB radiation that allows Vitamin D production to occur in our skin."
"This is particularly alarming because of the health benefits of Vitamin D on our bodies. Those with darker skin are often more deficient than others because it takes more sunlight to produce the same amount of Vitamin D as other ethnicities...I hate to be one to claim that there is any “panacea” for modern societies ills, but Vitamin D deficiency sure fits a lot of the symptoms of many of the ills of our society (besides such things as processed food and overeating). Let’s examine what Vitamin D deficiency is implicated in."