It absolutely is. Someone made a documentary called "Fat Head" aimed at eating nothing but McDonalds for 30 days in order to LOSE weight. By the end of it, the guy actually lost weight. All he did was eat reasonably. They asked Spurlock's people if they could release his food journal (like he claimed to record in Super Size Me), but they refused. What most likely happened: Spurlock wasn't getting notable results early, so he just tried to eat as much food as humanely possible. The doctor in Super SIze Me claimed he was eating 5000 calories a day. That is EXTREMELY hard to do, and I can confirm this from personal experience.Can't read the article due to the paywall but I always thought that doc was a load of horse manure.
During a college biology class, we were studying caloric intake. My professor gave us an assignment to plan an entire days worth of meals reaching 2000 calories based off of McDonald's nutritional menu. I tried to order what I would typically eat at McDonalds, and I was falling short by about 500 calories. I had to add extra stuff I would never get (like milkshakes) just to reach 2000.
That being said, even though Super Size Me is inaccurate, it's still a fun documentary, and I'd still probably watch it again. The same could be said about The King of Kong.