My main dissatisfaction with Vitamin D studies is the fact that they usually try to isolate it as a self-contained factor that you can simply raise or lower and

My main dissatisfaction with Vitamin D studies is the fact that they usually try to isolate it as a self-contained factor that you can simply raise or lower and observe effects, while neglecting the complex system it is normally a part of. How about instead treating it as a simple block of that complex system? What I mean is that Vitamin D typically correlates with sun exposure and a healthy outdoorsy lifestyle (i.e. the system). I would think that if you want to understand how its presence affects an individual, you would try raising/lowering it specifically on subjects whose lifestyle guarantees the presence of that system.