@Scarecrow thanks for clarifying the conversion.
Sure, glass reduces UV, but the reality is that lots of people eating a normal diet do not get out in the sun much and at least in the UK clinical vitamin D deficiency is largely restricted to dark skinned people with specific diets and elderly immobile individuals. Presumably others who do not get out much either have the windows open enough in the summer or get enough through the glass.
Thank you for taking the time to share your clinical experience with us. And I want to make it clear that I am not implying that
@wolfita 's levels are unhealthy. As I wrote above, I follow Chris Masterjohn in his suspicions both of the current higher recommendation for 25(OH)D and of drawing too many conclusions from 25(OH)D levels.
I am not yet convinced, however, that sunlight through windows could possibly be responsible for the relatively low level of vitamin D deficiency in the UK. Here is a
well-researched article that analyses UV exposure through glass. The author states that "glass effectively blocks all of UVB radiation."
It seems much more likely to me that most people who "don't get out much" get their vitamin D from some combination of vitamins, fortified foods (breakfast cereals, orange juice, milk, et) infrequent sun exposure and in some cases open windows in the summer than by getting the virtually non-existent UVB through windows.
@Gondwanaland : I looked at the google results you linked to. I see articles indicating that low vitamin D is associated with insulin resistance, and that supplementing vitamin D reduces insulin resistance, but I am not seeing something that says that insulin resistance lowers vitamin D (admittedly my search was not exhaustive). Do you have a link to an article like that?
@wolfita : You didn't say, so I thought I'd ask again: Do you eat any foods fortified with vitamin D?
For what it's worth, I think you have a good question. And provided you don't eat vitamin-D-fortified foods, don't open your windows, and don't take calcium or boron supplements, I don't have a good answer for you.