This is not true. 50% is too much even if you take into account the whole fat tissue (which is not what you lose when losing weight) and even there for extremely lean subjects with very little fat in the adipose cells. (Maybe anorectic / cacectic subjects are different in this regard for whole-body-average). You've probably read the conclusions of a confused self-taught "expert" who is making hypotheses based on exaggerated figures of water in fat _tissue_ vs. the fat that is lost when losing weight.

As an interesting sidenote: fat people have relatively a lot less water in their fat tissue than lean people.

You can also think about the fact that when you drop triglycerides in water, it will not solute at all. Also, human body creates fatty membranes (cell walls) to absolutely _prevent_ the influx / efflux of water.