@MrsPramm - The thing is, what people get wrong is that they think that feeling full is about having a lot of food in the stomach. It's actually about keeping blood sugar at good levels in the blood. So, even if your father had managed to delay digestion, it wouldn't have made him less hungry and being thirsty actually makes you more likely to overeat.
One of the things people do wrong is cut fat out of their diets, when fat is what makes you feel full. Not because it sits in the stomach for longer, but because it takes so long to digest, that it keeps your energy levels even for longer.