hi OP,
Low vitD can be caused by many things.
Primary amongst them.
1) We just are not exposed to enough of it . Either through sunlight on skin , or through food. I explained in those above threads that we are tropical animals , where UVB radiation was available every day. Our physiology evolved in a high Vit D climate, now we hardly get any from sunlight. You need to be outside in strong sun , sometime between the hours of 11 and 3 pm , exposing large areas of skin. The period of 6 months , Mid-Oct to Mid- April , simply does not have UVB in sunlight , here at our latitude of 50+ degrees. It only takes 20 mins to saturate any particular area of skin , in strong sunlight. When sunbathing , we can make 10,000 to 20,000 IU per day. BUT there is no need to burn . definitely NOT advisable. It is very unlikely that your daughter gets enough sunlight , under those above conditions. IF she did , she most likely would not be deficient in Vitamin D . The reason you might think that she only needs light on face and hands , perhaps , is because a few years ago , it was thought that our requirement was for only 200 IU per day. Which was widely promulgated, and widely believed. You could get that , from a 10 min walk to a bus stop. We now know much better. That is incorrect. It is not nearly enough.
Foods contain very little vitamin D , natural foods like fruit and veg have none whatsoever ( unless artificially fortified . ) That's why vegetarians and vegans are at risk. Years ago , pig lard had lots of Vit D , nowadays we use veg oils , which have none. Wild oily fish has some , but farmed fish is much depleted. e. g. farmed salmon only 1/4 the amount. Vit D3 is a mammalian hormone. Technically it is a seco-steroid pro hormone. We need vitamin D quite frequently , to go 6 months over winter with not much , is not a good idea.
2) It is possible that you might be getting some Vitamin D through food , but not absorbing much of it. For instance , if the subject has IBS , Coeliacs , Crohns, bile or other gut problems.
3 ) Genetic Problems , For example, we may not be able to utilize what we have , for instance the CYP2R1 gene may be less effective in some people. VitD receptors on cells may be less effective in some people.
4 ) ..... the list is rather long . It includes , lack of co-factors , such as magnesium , that might be a reason, etc etc.
I'm glad you want to read up on the subject. Here is a site which gathers together a great number of the published medical/research papers on that and closely related subjects. <a class="break-all" href="https://vitamindwiki.com/VitaminDWiki" rel="nofollow" target="_blank">vitamindwiki.com/VitaminDWiki</a>
Here is another site , which has myriad references , details of research etc. www.grassrootshealth.net
IF there is any specific area that you want to know more about , then just get back to me, and I should be able to find it. PubMed is another great resource , it has many of the worldwide publications , gathered together in one American database ( National Centre for Biotechnology Information) 30 million refs. of Biomed literature.
hope that is useful,
BTBH.
.