This is the most comprehensive information about Vitamin D I have yet to find. As you get towards the bottom ( long) it explains why doses are being given above 2000 IU a day.

It is very interesting because it cites how steroids affect calcium and Vitamin D, and how other medications alter absorption.

http://ods.od.nih.gov/factsheets/vitamind.asp

For more info google: vitamin + D + absorption

Thanks for the article from the Journal of Clinical Nutrition!