Vitamin D, in fact a hormone, is said to be soluble in fat. Therefore, it has been cautioned, that too large an intake or production of D3 or more precisely Cholecalciferol can be dangerous since it could be stored in fat. Solubility is not that easy, especially when considering biological systems. First of, anything is soluble in anything to a varying degree. When saying D3 is fat soluble, we also need to consider that there are three different molecules that we mean when we say D3 (see vitamin D for a disambiguation).