They would probably pass out. You would exhaust your protein stores very quickly, so doing any weight training would be a waste. Without protein, you can't build and repair the muscle (which is how you gain strength).
You would also exhaust your carb stores, so the only thing left to burn is fat and protein for energy. And since protein breaks down easier than fat, that would be the first thing to go. So, yes, you would lose weight, but you would be losing a lot of muscle, which can set back weeks or months of weight training.
Your body would also go into starvation mode, so any food that you do put into you, after a day or however long you want to torture your body, would immediately be stored as fat. Your body will think you are going to starve it again.
So, would I recommend this ... obviously not. It doesn't make any sense. There are no long term gains from it. In fact, it is actually long term negative.
