There are no foods that fatten or lose weight as such, there are foods with greater or lesser energy concentration. Fluctuations of up to 3 kg or so in a day are normal.
Only if you evaluate body composition with an examination by a nutritionist, before and after following a diet for a month, can you realize what happens to the body. People who suffer from bulimia nervosa with laxative abuse know this: they enter an obsessive spiral of laxativeabuse and dehydration that poses serious health risks.