Is there any thing you could subsitute in this diet to get the same effect?

For the movie "The Machinist", Christian Bale was disgustingly thin. In order to prepare for the role, he basically fasted on coffee, and either one apple or one can of tuna each day, for four months, and lost over sixty pounds. I was just curious as to why he chose those foods, especially because the mercury in tuna could cause serious damage if eaten too much. Is it because those are the only foods to produce such fast weight loss without technically starving? Or is there some other foods that he could have eaten for the same amount of time in order to acheive the same results?