Thanks MrsD,

I will remain slightly confused, but I have a follow-up question about the best source of vitamin D, natural sunlight. Here, from webmd:

Just 6 days of casual sunlight exposure without sunscreen can make up for 49 days of no sunlight exposure. Body fat acts like a kind of storage battery for vitamin D. During periods of sunlight, vitamin D is stored in fatty fat and then released when sunlight is gone.

Gives the basic mechanics, but do you know whether the body produces vitamin D if the sunlight is filtered through glass. I know that glass stops uv rays, is uv the trigger for production?

Dave.