Each of the participants eat the diet of the other. At the end of the experiment, they have a better idea of how extreme their consumption looks to another person. Every episode I saw had the participants sharing an experience that gave them empathy for other that usually didn't exist before. At the follow-up after the end of the experiment, they were true friends who relied on each other for encouragement. I can think of only one show where a participant had gained weight. All of the others were successful in following their program.
Was on channels:
The Marsh King's Daughter 
The End of the Storm 
The Return of Sherlock Holmes 
The Conjuring