Conventional wisdom among producers, editors, and agents is that even the most interesting topic is nothing without a narrative. Taking that advice, New England-bred documentarians Aaron Woolf, Ian Cheney, and Curt Ellis decided to investigate the current American diet by transplanting themselves to Iowa, renting an acre of farmland, and spending a year cultivating corn. That’s a narrative, all right, but it’s still not much of a story. Even hard-core urbanites could probably guess most of what King Corn tells about growing and harvesting a cash crop. (To summarize: fertilizer, pesticides, tractors, and farm-support payments.) Fortunately for director Woolf and on-screen guides Cheney and Ellis, the topic picks up the slack, and even fans of such writers as Michael Pollan (The Omnivore’s Dilemma), who makes an appearance, will probably be surprised by the pervasiveness of corn’s influence. The easygoing Cheney and Ellis, whose style suggests a less acerbic Morgan Spurlock, begin by getting their hair analyzed. It reveals that the two men, unapologetic burger fans, are nourished primarily by corn. That is, if you can call today’s genetically engineered corn nourishing: Formulated to resist certain pesticides, Iowa’s corn crop is an industrial product used principally for ethanol, animal feed, and high-fructose corn syrup, which has transformed American soft drinks from mere empty calories into dietary time bombs. Corn byproducts now feed most cattle, whose meat is much higher in saturated fat than it was back when the animals grazed on grass. Cheney and Ellis both boast great-grandfathers who once lived in the same small Iowa town, and they spend some time getting to know the locals. They also talk to experts, including Nixon-era Agriculture Secretary Earl Butz, who’s credited with creating the contemporary farm-support system. Yet the film’s highlights all concern the properties of industrial corn and the product’s effects on a population with epidemic growth in obesity and diabetes. If the corn the filmmakers grew tasted good, that’s a piece of the narrative King Corn doesn’t expand on.