getting down to the plain and simple, because thats really the only thing I know with alli...the way I follow it is that you need to take in enough fat to get the nutrients you need AND to make sure that your body doesnt go into "starvation mode"
What starvation mode is, is your body thinking it isn't getting enough food so it wants to save up all the food it gets in case it doesnt get more for a while.
Think of it back when we were cave people hunting for our food. If the hunt goes bad for a few days in a row, we might not eat. "starvation mode" is a survival mechanism that allows us to store what food we do get when we are going through these bad times. So basically by eating less fat you are going to store more, your metabolism is going to slow down, and you can gain weight, because you arent actually starving like back then.
I hope that helps. and good luck with your weight loss. If any of this doesnt make sense, or you still dont understand, dont be afraid to ask, I'll come up with a better answer
~Star