A perfectly healthy guy eats exclusively at McDonald’s for a month and suffers serious physical–and mental–consequences in this guerilla-style documentary inspired by such films as Michael Moore’s “Roger and Me” and “Bowling for Columbine.” The fellow is Morgan Spurlock, an agreeably ordinary twentysomething New Yorker who narrates his own story in glib, amusing fashion against a backdrop of footage of him chowing down, visiting with his horrified doctors, arguing with his vegan girlfriend and trying futilely to arrange an interview with a company representative. (The title comes from one of the “rules” he establishes for his experiment: whenever a restaurant clerk asks whether he wants to supersize his order, he’ll have to say yes. He further restricts his physical activity to that of the average American, cutting back on walking and any form of exercise.) Spurlock also uses the experience as a springboard for a broader discussion of the problem of obesity in America, an indictment of the fast-food industry as a whole, and a consideration of the legal cases that have been filed against businesses that sell fattening meals. He also inserts material showing how switching to healthier food can have a broadly beneficial effect, recording the change among students when their cafeteria hires a quality-conscious firm to take over its lunch program while elsewhere kids are still provided with the stuff they crave that does them little good.

But though its subject is obviously a serious one, as the current concern about the epidemic of obesity in America clearly demonstrates, “Super Size Me” isn’t an angry film–as Moore’s, for all their humor, definitely are. The tone remains resolutely light-hearted, as for instance when Spurlock is unable to keep down his first fatty meal, and even when Spurlock’s doctors, trainers and nutritionists express alarm at what’s happening to his body. His efforts to get through to a McDonald’s spokesperson, moreover, have a dutiful quality to them, as though Spurlock felt obligated to mimic Moore’s attempt to corner Roger Smith without any expectation of success. So do his interviews with nutrition and legal experts. In fact, the talks Spurlock seems to enjoy most are those he conducts with fast-food junkies, including a fellow named Don Gorske who happily discourses on his habit of eating at least one Big Mac a day. (But he doesn’t order the fries.)

At nearly a hundred minutes, the premise of “Super Size Me” wears a bit thin by the time it’s over. And there’s a certain smugness Spulock’s revelation at the end of the picture that McDonald’s has since discontinued its super-sizing procedure, and his snide observation that the company claims that the film had nothing to do with their decision. But as a whole the picture is an engaging documentary helped by the fact that Spurlock and his compatriots don’t take themselves too seriously and don’t act condescendingly toward the folks who blithely (and unthinkingly) continue to consume McDonald’s product.

Incidentally, if you decide to go see “Super Size Me”–not a bad idea–you might want to skip that stop at the concession stand on the way into the auditorium, or at least make do with the bottled water.