Here’s a study about the vitamin D-reducing effect of clothing:
Conclusion: This study analyzed the vitamin D levels of 1,111 healthy people in Isfahan, Iran. 50.8% were deficient, with 26.9% being severely deficient. Women were especially found to have low vitamin D levels due to the increased amount of clothing they’re required to wear under Islamic law. “Exposure to sun is limited due to the type of clothing required by current law.”
Conclusion: This experiment had people wear full-body outfits made from various types of clothing fabric (wool, cotton, and polyester) to see whether or not a person can synthesize vitamin D from sunlight while wearing clothes. The results were that ALL of the fabrics completely prevented any vitamin D production following 40 minutes of sun exposure. More clothing = less vitamin D. Get naked!