Yeah, I was surprised the doctor was so surprised.
As far as the movie's premise, and whether or not in fact the guy could legitimately gain 20 pounds in a month.... Well, no doubt the movie's premise was ridiculous. Of course, it's idiotic to only eat McDonald's for a month. Or Pizza Hut or Taco Bell, or most other popular food places. Still, the premise worked as a good vehicle for the movie's plot, although I do think the guy took unnecessary health risks doing so.
I really didn't have any reason to doubt the guy really did eat all McDonald's and only McDonald's, or to doubt the weight gain of 20 pounds. Maybe some other FReepers who know more about health and diet can challenge this or inform me otherwise.
-- Joe
He also adopted a sedentary lifestyle, wearing a pedometer to make sure he didn't walk more than the average american.
I didn't see the movie but I believe it is possible to gain 20 pounds specifically at McDonalds. Everything on the menu has, not just fat, but saturated trans-fats.