McDonald’s was recently sued for causing obesity, but the case was thrown out because of insufficient evidence that it was specifically McDonald’s food that was to blame. Morgan Spurlock decided to find out, and so began the thirty-day burger bender chronicled in Super Size Me.

Spurlock set himself some rules. For one month he would only consume food and drink available from McDonald’s, and he had to finish every meal. He would try everything on the menu at least once. And he would only ‘Supersize’ his meal if the server suggested it.

Spurlock begins the film in excellent health, but this quickly deteriorates. In five days he puts on an astonishing ten pounds. After two weeks he has liver damage that his doctor compares with Nicolas Cage drinking himself to death in Leaving Las Vegas.

On day twenty-one he wakes up sweating in the middle of the night, struggling to breathe. He asks his doctors to reassure him that he’ll be ok to finish the experiment, but they can’t confirm it.

McDonald’s makes a headline-grabbing target, but the film is actually about more general problems with America’s fast food culture. Spurlock meets doctors, lobbyists and health experts to investigate America’s obesity epidemic, and find out why it’s so pronounced among the young.

In the wrong hands this could have been a po-faced sermon, but Spurlock is the perfect guinea pig because he genuinely likes junk food. He approaches the project with relish, excited at the prospect of living "every eight year old’s dream."

Like Michael Moore, whose Bowling for Columbine is an obvious influence, Spurlock uses humour and visual gimmickry to keep his documentary entertaining. As a result this is one of the most enjoyable films of the year, although Spurlock shares Moore’s tendency to rely on sarcasm and inference, often to the detriment of his point.

Consequently you come away from the film determined to avoid junk food, but not quite clear why. Still, if it has a positive effect, maybe this is enough. Since the film’s launch in the US, McDonald’s has removed ‘Supersize’ portions from all its menus.

(Released by Samuel Goldwyn Films and rated "PG-13" for language, sex and drug references, and a graphic medical procedure.)