From Scientific American:
The more conservatively the women dressed, the lower their vitamin D; those who wore the hijab but ate vitamin D-rich foods such as milk or oily fish had higher levels, though not as high as the women who didnâ€™t adhere to hijab. A measure of 30 nanograms per milliliter of blood is considered sufficient; the most conservatively dressed women in the study had levels as low as 4.5, but even those who didnâ€™t wear the hijab and got some vitamin D in their diets had an average level of 8.5 â€” “and that’s still low,” says co-author Raymond Hobbs, a senior staff physician Henry Ford Hospital in Detroit.
“We’re not trying to get anyone to take off their hijab,” Hobbs tells ScientificAmerican.com, but “to do things to prevent problems that might arise” from the tradition. The vitamin was once thought to be necessary only to prevent rickets (soft bones) in childhood and osteoporosis later on, but now, vitamin D deficiency is associated with diabetes, cancer, Crohn’s disease, multiple sclerosis, heart disease and infections.