In the 2004 documentary "Super Size Me", Morgan Spurlock ate nothing but McDonald's food for a month.  He felt terrible at the end of it, but it was for art. 

Fast food has a lot of calories, most people could subsist just fine for 30 days eating just one such meal a day - but even one McDonald's meal every single day in conjunction with two other meals can still lead to weight loss, says Don Thompson.

He lost 20 lbs. in the last year despite eating McDonald's food every day - it's part of his job because he is CEO of McDonald's. Speaking at the At the Sanford Bernstein analyst conference on Wednesday, he revealed that his weight loss was due to getting his "butt up" and "working out again", his McDonald's food didn't have to go.

McDonald’s CEO: I lost weight while eating fast food 'every, single day' - Associated Press