You see, Morgan Spurlock is not the only person to have ever tested fast-food-only diets, or even McDonald's-only diets. After his movie came out, many people repeated his experiment themselves, including a number of scientific institutions that applied controls and conducted the research in a scientific manner. At least three other documentary movies were made, Bowling for Morgan, Portion Size Me, and Me and Mickey D, in which the filmmakers lived exclusively on McDonald's food for 30 days but (unlike Spurlock) did not force themselves to overeat when they were not hungry. All filmmakers lost weight during the period and suffered no ill effects; and the subjects in Portion Size Me, which was scientifically controlled, also had improved cholesterol.
Most famously, Swedish scientist Fredrik Nyström conducted an experiment with eighteen students; only he upped the ante — considerably. Rather than Spurlock's 5,000 calories per day, Nyström's subjects were required to consume a measured 6,000 calories per day. The food was controlled to ensure that most of the calories were from saturated fats. The subjects were not allowed to exercise during the 30 days, also unlike Spurlock, who made sure that he walked a normal distance every day. Considering these differences, Nyström's subjects should have been considerably worse off than Spurlock was, but they weren't. They did all gain 5-15% extra body weight, and complained of feeling tired; but none suffered any other negative effects. There were no mysterious psychological problems, no strange conditions that baffled the doctors. Nyström and his medical staff noted no dangerous changes at all. After his experiment, Nyström was asked his opinion of Spurlock's extreme reaction, especially his liver problems. Having never examined Spurlock, Nyström could only guess, but among two of his perfectly reasonable hypotheses were that Spurlock may have had pre-existing undiagnosed liver problems; or that his normally vegetarian diet may have rendered his liver poorly prepared to suddenly deal with a diet high in carbohydrates and saturated fat, a problem that anyone eating a normal diet would not experience. Any cynic can also easily propose a third possibility, that Spurlock was simply trying to make as dramatic, engaging, and commercial a movie as he could, which is the goal of every filmmaker.