Morgan Spurlock watches his weight while imbibing an obscenely large Coke in 'Super Size Me.' Super Size Me Starring and directed by Morgan Spurlock. Not rated.

Morgan Spurlock wanted to make a film that took on America's major food companies and their impact on our nation's obesity epidemic. To frame his movie, he came up with the ingenious (not to mention insanely self-destructive) gimmick of chronicling his own 30-day regimen, during which he ate nothing but McDonald's food.

You may think you don't need a documentary filmmaker to tell you that fast food is bad for you, but this movie starkly illustrates just how bad it is, combining the humorously confrontational style of Michael Moore's films with the horrible fascination of a Jackass stunt. Before he begins, three different physicians and a nutritionist give the 32-year-old Spurlock exhaustive physical exams and pronounce him in excellent health. An ominous sign crops up early in the diet, when he throws up after a Supersized lunch on Day 2, but he soldiers on. Predictably, his blood pressure and cholesterol levels surge, and his weight goes from 185 to 210. Less predictably, he becomes severely depressed and suffers pounding headaches, symptoms that go away only during the three times a day that he eats. He wakes up in the middle of the night with mysterious chest pains, something most people don't find reassuring. He gets winded climbing the stairs to his office. His body starts to smell like a cheeseburger. He becomes nearly impotent. (This is testified to by his girlfriend, a vegan chef, who goes into a tad too much detail about it.) The doctors monitoring him are amazed to find damage to his liver that they've seen only among binge drinkers. By Day 21, they and Spurlock's family members are so terrified that they're begging him to stop the McMadness.

It's appalling to watch this fit young man's body and mind undergo several years' worth of deterioration in a single month. It's also funny, because he's making himself gravely ill with food that most of us are intimately familiar with. Each time Spurlock shows himself chowing down on yet another fat-laden sandwich, you'll laugh to keep from crying. Of course, few people eat to his level of excess, but as he points out, quite a few McDonald's customers go there often, more than once a week. The company classifies such people as "heavy users" and "super heavy users," and the film posits (somewhat shakily) that regular customers are hooked on the food in much the same way as drug users are on their poison.

The diet aside, Spurlock digs up some juicy tidbits on the industry and our national eating habits. He points out that McDonald's "healthy" menu options are frauds; the chicken ranch salad has more calories than the Big Mac, and the fruit-and-yogurt parfaits pack more sugar than the ice cream sundaes. Health information on the food, which is supposed to be available and visible in all outlets, is frequently kept out of sight. Spurlock's particularly good at showing how Big Food has pervasively marketed itself to children, from direct marketing to putting up playgrounds at fast-food outlets to catering school-lunch programs that limit buying options to carbs, fat, and sugar. Tobacco companies have been publicly flogged for trying to instill brand loyalty in kids, but fast-food and junk-food conglomerates have received a free pass.

The movie also takes a welcome swipe at Subway spokesman Jared Fogle, who's seen telling an overweight schoolgirl that she needs to stay strong and follow his diet. (After Jared leaves, the girl laments that she doesn't have the money to eat at Subway twice a day.)

Spurlock's act is both likable and hard-hitting, and he talks to a number of experts in the field to bolster his claims, but his reportage isn't as unbiased as it could be. Near the film's start, he acknowledges that no one forces people to eat at McDonald's and asks, "Where does personal responsibility end and corporate responsibility begin?" He never answers his own question, though, because he's too busy savaging the corporations for foisting all this unhealthy stuff on us. In doing so, he seems to ignore some basic biology: Human beings are hardwired to binge on food when it's available in anticipation of lean times, and consuming fat and sugar hits pleasure receptors in the brain. Junk food makes you feel good, in other words. The only balance he provides is interviewing Don Gorske, a thin fellow who averages two Big Macs a day yet has a cholesterol level of 140. (Damn! What's in that guy's blood?) Of course, this film isn't nearly as rigorous journalistically as Eric Schlosser's book Fast Food Nation.

Nevertheless, Spurlock's own body becomes his most powerful piece of evidence against Big Food. Think about his weight gain -- 25 pounds in a month. If you came up with a diet plan that could make people lose weight at that rate, you'd either be rich or in jail for dealing in illegal substances. By indulging himself until he literally becomes sick, Spurlock makes Super Size Me an example of agitprop cinema at its most entertaining.