He grossly overate for the entire 30 days. It is not necessarily a realistic portrait as most people don't eat 5-6,000 calories a day. He gorged on McDonald's. They've got a scene where he pulls in to the drive thru to order a shake. Only a shake; nothing else. He ate like a fat pig and started to look like one after a month. He also consumed an incredible quantity of fat which led to the toxification of his liver.
There's another chick doing the McDonald's only thing for a full 60 days at
http://www.cei.org/pages/debunk/debunk_the_junk.cfm I think she was overweight to begin with but actually lost weight.
Which is not to say I think for a moment that McD's is not toxic crap and addicting as hell. (I just can't stay away from it for long)