Hijab does not cause vitamin D deficiency
Question

                What should hijabi women do 4 deficiency of Vitamin D and calcium which is becoz of less exposure to sun?

What can we provide if someone argues with us that they dont wana wear hijab due to this reason?.
Praise be to Allah.
      What is required of women is to cover the head and the 
      entire body in front of non-mahram men, because Allah, may He be exalted, 
      says (interpretation of the meaning):

    “O Prophet! Tell your wives and your daughters and the 
    women of the believers to draw their cloaks (veils) all over their bodies 
    (i.e. screen themselves completely except the eyes or one eye to see the 
    way). That will be better, that they should be known (as free respectable 
    women) so as not to be annoyed. And Allaah is Ever Oft‑Forgiving, Most 
    Merciful”

    [al-Ahzaab 33:59]. 

    And because of other evidence which has been discussed 
    previously in the answer to question
    11774. 

    Hijab is a definite obligation for every Muslim woman. 
    Deficiency in calcium and vitamin D is something that can be treated 
    medically and by eating food that contains those nutrients, and by exposure 
    to the sun in a place where there are no non-mahram men, such as exposure to 
    the sun through windows, on the roof of the house, in remote parks, and so 
    on. 

    Allah, may He be exalted, has prescribed and enjoined hijab, 
    and He knows best about His creation and what is best for them. Sharee‘ah 
    does not enjoin anything that causes obvious harm or considerable problems. 

    Following a medical study in which a number of doctors took 
    part, Dr. Muhammad al-Shaakir (consultant in bone disease at the specialist 
    King Faisal Hospital in Riyadh) rejected what has been said about women’s 
    Islamic clothing playing a part in vitamin D deficiency and said: This 
    theory has no connection to truth. 

    Specialists think that one of the means of protection against 
    bone disease is exposure to the sun so as to enable the skin to produce 
    vitamin D. The Muslim woman’s hijab does not prevent her being exposed to 
    the rays of the sun, because minimal daily exposure is enough to produce 
    sufficient amounts of this vitamin. 

    End quote from the [Saudi] Ministry of Health website.


    http://www.moh.gov.sa/des/sections.php?CatParent=13&CatID=42

    It should be said to women who do not wear hijab for this 
    reason: Fear Allah, may He be exalted, and beware of His Punishment; adhere 
    to that which He has enjoined upon you, and stop making false excuses, for 
    nothing is hidden from Allah; Allaah knows the fraud of the eyes, and all 
    that the hearts conceal (cf. Ghaafir 40:19). Seek remedies and treatments in 
    that which Allah has not forbidden, and you will find it, in sha Allah. 

    May Allah help us all to do that which He loves and which 
    pleases Him. 

    And Allah knows best.