To see what happened, Morgan Spurlock decided to spend 30 days eating nothing but McDonalds. Something which I would assume would make you fat, but otherwise not be seriously bad for your health.
Within a few days of beginning his drive-through diet, Spurlock, 33, was vomiting out the window of his car, and doctors who examined him were shocked at how rapidly Spurlock's entire body deteriorated.
His liver became toxic, his cholesterol shot up from a low 165 to 230, his libido flagged and he suffered headaches and depression.
Spurlock charted his journey from fit to flab in a tongue-in-cheek documentary, which he has taken to the Sundance Film Festival with the hopes of getting a distribution deal.
"Super Size Me" explores the obesity epidemic that plagues America today - a sort of "Bowling for Columbine" for fast food.
Over the course of the film, Spurlock is regularly examined by a gastroenterologist, a cardiologist and SoHo-based general practitioner Dr. Daryl Isaacs.
"He was an extremely healthy person who got very sick eating this McDonald's diet," Dr. Isaacs told The Post.
"None of us imagined he could deteriorate this badly - he looked terrible. The liver test was the most shocking thing - it became very, very abnormal."