If you generate Vitamin D by being exposed to sunlight, then it makes sense that if you block those sun rays that you may not be making enough Vitamin D.
On the NY Times website, C. CLAIBORNE RAY, has written an article about this very topic. The article explains that you may in fact be blocking the UV rays that generate Vitamin D, but these harmful rays contribute to skin cancer and premature aging. But, one can get Vitamin D from eating foods like fatty fish and vitamin fortified milk and taking Vitamin D supplements!
How do you know if you are YOU vitamin D deficient?