http://www.naturalnews.com/029195_magne ... min_D.html
Vitamin D, just like all other nutrients, works in harmony with several other nutrients to perform its many functions. Most importantly, vitamin D requires and 'uses up' magnesium to convert from supplements or sun into its active form in the blood. As such, it is a big mistake to simply take large doses of vitamin D without taking the need for magnesium into consideration.