Actual weight lose is mostly from the shunting of fluid which occurs when you suddenly deny yourself food. You will use up some of your glycogen stores, which contains lots of water. The day you re-commence normal eating, the glycogen stores, and the water, is replaced. The glycemic index is a tool used to describe how fast a source of sugar is absorbed into the bloodstream. The typical baseline uses white bread as 100.
High numbers mean the sugar is absorbed faster.
Post exercise, eat foods with triple digits first, add in foods with a lower index until you reach your real meal as described elsewhere. Stick to the starches if you want slow absorption. The longer you cook the food. The higher the glycemic index will be for that food. Thus, rice or spaghetti cooked for five minutes has a much lower index than if cooked for fifteen minutes.