Vitamin D has co-factors that the body needs in order to utilize vitamin D properly. They are:
- vitamin K2
- a tiny amount of vitamin A
Magnesium is the most important of these co-factors. In fact, it is common for rising vitamin D levels to exacerbate an underlying magnesium deficiency. If one is having problems supplementing with vitamin D, a magnesium deficiency could be the reason why.
For more about vitamin D's cofactors, please go here.
I would add fat and protein to this list of nutrients. These may not have been included because these are presumably assumed to be present; but this is often not the case, given the heavy emphasis on high carbohydrate, low fat diets for health (Yeah - right!).