This hormone is vital for strong bones and muscles. But do our indoor work lives mean many of us are not getting the sunlight that our bodies need to produce it?



Presenter Justin Rowlatt heads to a hospital in India where Dr Ambrish Mithal explains why vitamin D deficiency - including rickets in children - is still commonplace in such a sunny country.



Back in the UK, endocrinologist Maralyn Druce explains why vitamin D is indeed a hormone. Plus the BBC's Laurence Knight reports on one indoor profession where deficiency poses a serious risk of injury - ballet dancing.



(Picture: Elisha Willis as Swanilda and Joseph Caley as Franz in the Birmingham Royal Ballet's production of Coppelia; Credit: Andrew Ross/Birmingham Royal Ballet)