Just saw a neat article about a scientific study based on the movie Supersize Me. If you haven't seen it, it's a documentary where the subject eats nothing but McDonalds for one month. By the end he is depressed, fat and has no libido. It was generally hailed as a condemnation of the fast food industry.
A Sweedish scientist saw the movie as was bothered by how unscientific it was, so he replicated it with eighteen graduate students. The results were mixed, some ballooned in weight like the Supersize me guy did, others actually lost weight. The experiment was somewhat different, the subjects weren't limited to McDonalds, but did have to eat junk food. They could eat burgers, fried chicken, chocolate, that sort of thing. They had to eat at least 6600 calories per day.
Also, most people found the diet created an aversion therapy effect. They didn't want to eat fast food anymore after the experiment.
Link