The picture accompanying this story actually depicts a woman in a niqab, not a hijab.
From the Evening Standard (thanks to Magnus):
Muslim women who wear the hijab are at risk of serious illness because they do not get enough sun, doctors have warned.
They said an alarming number of women who cover their skin are suffering bone deficiencies over a lack of vitamin D.
Most of the body’s vitamin D – which prevents rickets – is obtained through sunlight acting on the skin. Only a little comes from food.
Doctors told a London conference today that people with dark pigment are at risk because of “cultural reasons” and because they are less efficient at producing the vitamin.
The bone disorder rickets has now broken out in young Muslim children as babies are not getting enough calcium from mothers’ breast milk.
The National Health Service is launching a campaign aimed at Muslim women, particularly Bangladeshi, Pakistani and Somalis, to encourage them to increase their vitamin D intake.