Ugh, I really wish "Supersize me" never existed...
"Hurrrrr, lets see what happens if I only eat greasy fast food for a month DURRRRRRRRRR" -.-
NO SHIT it's gonna be bad for you! the same thing would happen if you had only chocolate cake for a month or if you had only beef jerky for a month... heck, it wouldn't even be healthy for you if you had only apples for a month.
Humans need a variety of foods, McDonalds is FINE! as long as you aren't a complete dumbass and over-consume, same with EVERY OTHER food on the fucking planet.
*sigh* apologies for the somewhat heated rant... I REALLY hate the whole "Zomg food that isn't explicitly 'good' for you is evil!" bullshit thats been flying around recently -.- esspecially when these 'good' foods are sometimes worse for your body then the 'bad' foods, (Bacon is arguably better for you then bread for example, yet bread is 'good' and bacon is 'bad' >.<)