Here’s a study about the vitamin D-reducing effect of clothing:

Role of Vitamin D Deficiency and Lack of Sun Exposure in the Incidence of Premenopausal Breast Cancer: A Case Control Study in Sabzevar, Iran (2014)

Conclusion: Sun exposure habits and vitamin D levels were evaluated and compared in 60 premenopausal breast cancer patients and 116 matched, healthy controls in northern Iran. Researchers found that 95% of women in both groups were vitamin D-deficient, but women who reported ever consuming any vitamin D supplements were less likely to develop breast cancer. 98% of the women reported receiving ZERO sun exposure due to how Iranian women dress, which has them completely covered up all the time. Iranian women are also known to start developing breast cancer a decade earlier compared to women in other countries. Could their sun-starved way of dressing be killing them?

 

Prevalence of Vitamin D Deficiency Among Adult Population of Isfahan City, Iran (2011)

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.”

 

Vitamin D From Dietary Intake and Sunlight Exposure and the Risk of Hormone-Receptor-Defined Breast Cancer (2007)

Conclusion: Canada study where researchers compared vitamin D intake and lifetime sun exposure habits between 759 breast cancer patients and 1,135 healthy, matched controls. The study found that women with a higher vitamin D intake and increased lifetime sun exposure enjoy a significantly lower risk of estrogen-receptor positive and progesterone-receptor positive breast cancers, and a slightly lower risk for estrogen and progesterone negative breast cancers. For sun exposure specifically, they found that increased frequency of outdoor activities and ever having an outdoor job between the ages of 10 and 29 was associated with lower breast cancer risk, suggesting that breast cancer could possibly be due to developmental defects caused by early life vitamin D deficiency. The study also found that women who reported exposing more naked skin to the sun enjoyed a lower breast cancer risk. Early life sunburns, while not good for skin cancer risk, are actually associated with lower breast cancer risk.

 

Clothing Prevents Ultraviolet-B Radiation-Dependent Photosynthesis of Vitamin D3 (1992)

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!

 

Also see:

Negative Health Effects of Sunscreen

 

 

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