Morgan Spurlock has Michael Moore's provocateur mentality, sharp sense of humor and mischievous good nature, without Moore's obnoxiousness or unsightly slovenliness.

It is that last point, in fact, that is the focus of "Super Size Me," Spurlock's funny, surprising and thoroughly effective documentary about America's unhealthy love affair with fast food.

The star of the film is Spurlock himself, a likable West Virginia native now living in New York with his vegan girlfriend. The film is an experiment: What would happen if you ate nothing but McDonald's food, three meals a day, for an entire month?

Spurlock does just this. His rules are that he must eat three meals a day, he can ONLY eat food from the McDonald's menu (luckily, many of them sell bottled water), he must super-size when asked, and he must eat everything on the menu at least once.

The camera follows him through his 30-day journey, during which he busies himself with research. He interviews the son of Robbins (as in Baskin-Robbins), who is now a health expert after having grown up on ice cream and feeling sick all the time. He investigates junk food in public schools, including the USDA-provided school lunches, which Spurlock discovers are nearly as unhealthy as letting kids choose items from vending machines. He goes to Texas, home to six of the nation's 20 fattest cities.

Oh, and he makes repeated phone calls to McDonald's headquarters to get their side of the story for the film. His calls are not returned.

The most entertaining highlights, though, are of Spurlock eating, and trying to eat, and wishing he could stop eating, McDonald's food. We track his progress, his weight gain, his inability to walk up stairs easily, and his girlfriend reports on the sexual side effects. He learns the food has an addictive effect on him, and his team of doctors -- who checked him out beforehand and pronounced him in fantastic health -- are positively STUNNED at the effects his insane diet produces. "I knew it would be unhealthy," one of them says. "But this...."

What could have been just an anti-fast food screed is instead a remarkably watchable, high-spirited, persuasive film that is as entertaining as it is alarming. It's packed with faceless statistics about the fattening of America, but it's packed with human interest, too, as Spurlock becomes a real figure for us to root for and worry over.

No one loves fast food more than I do. I expected the film to have the opposite of its intended effect, i.e., that it would make me hungry to go eat some McDonald's food. Instead, I came out of wanting to skip fast food forever. The feeling wore off in a day or so, of course, but that it changed my attitude even for a while is a testament to the power of good filmmaking.

Grade: A

Rated PG-13, some profanity, some sexual dialogue, brief partial nudity

1 hr., 38 min.