The national discourse about health and obesity has never been a particularly cordial conversation.

In 2008, it hit a tendentious peak when a ban on new fast-food restaurants in South Los Angeles brought the term "food apartheid" to the table. The ordinance, which was implemented in a part of the city that is both disproportionately poor and obese, came as a response to the idea that there are two different systems for accessing food in Los Angeles, one with more limited options in an economically depressed part of the city that is predominantly black and Latino, and the other with more variety in more affluent neighborhoods.

The pushback was equally charged. "Opening a McDonald's in South-Central L.A. is not government-enforced racial discrimination," William Saletan argued in 2008. "But telling McDonald's it can open franchises only in the white part of town—what do you call that?"

Fast-food outlets have previously been banned for environmental or aesthetic reasons. In Calistoga, California, for example, chains were forbidden in the spirit of preserving the hamlet's small-town charm. But the South Los Angeles ban was unprecedented in that it was the first to connect a policy to the obesity epidemic. The ordinance didn't shutter existing restaurants, but it did block construction of new stand-alone fast-food restaurants in an area with 700,000 residents. (That's a population that, if separated from the rest of Los Angeles, would still make one of the U.S.'s 20 largest cities.) The effort also dovetailed with an initiative to encourage supermarkets and stores with presumably healthier fare to move in.

On Friday, the ban got a dose of bad news: A study released by the RAND Corporation revealed that the ordinance had "failed to reduce fast-food consumption or reduce obesity rates in the targeted neighborhood." In fact, obesity rates in the area had grown at a faster clip than elsewhere in the city. As NBC News reported, the percentage of people in South Los Angeles who were overweight or obese in 2007 was 63 percent. By 2011, that figure was 75 percent.

"We never believed it was going to be an overnight situation where all of a sudden the community was going to be healthy," one of the co-authors of the zoning restriction told the Los Angeles Times.

But the problem wasn't just the limited timeframe. As Roland Sturm, one of the co-authors of the study noted, the 32-square-mile area covered by the fast-food ban was relatively light on the very stand-alone franchises that had been prohibited. In a phone call, Sturm characterized the South Los Angeles restriction as "symbolic" and "not at all effective."

But this particular failure illuminates a number of the reasons why the conversation about diet, health, obesity, and fast food is such a speculative and discordant one.

The U.S. Department of Agriculture defines food deserts as places "without ready access to fresh, healthy, and affordable food," be they small towns or urban areas. "Instead of supermarkets and grocery stores, these communities may have no food access or are served only by fast food restaurants and convenience stores that offer few healthy, affordable food options." According to the RAND study, even though 10 percent of the total food outlets in South Los Angeles opened after the regulation passed, "there was no evidence that the composition of those establishments has changed as a result of the ordinance."

If a Jack in the Box doesn't open in a food desert, there's no guarantee a supermarket or grocery store will arrive in its stead. And, even if one does, access to a supermarket doesn't always ensure a healthier neighborhood.

Sturm, who has already looked askance at the impact of food deserts on obesity in children and teens, argues that large supermarkets “are especially good at purveying soda and candy at low prices." He is backed up by a recent study by Drexel University, which found that supermarkets built in underserved areas ranked among the least impactful policy changes affecting the nutritional quality of foods.

Sturm added that in the United Kingdom and Australia, "large supermarkets are the bad guys because they are seen as trying to get rid of small green grocers." The issue is larger than access to food. Smaller retailers certainly could be part of the solution here, but there are many reasons (commercial and logistical) why they aren't.

Additionally, we shouldn't expect diets to shift dramatically "until we start changing the relative price of food," Barry Popkin, a professor at the University of North Carolina, told NPR. Sturm's research on a South African program that offered subsidies for healthier food buttresses that notion. It's pretty intuitive. When given the rebate, participants didn't fully switch to healthier food, but they certainly bought more of it.

Ultimately, the philosophy behind South Los Angeles's fast-food ordinance mirrors the efforts to regulate school lunches in many ways. Eliminating access to less nutritious food makes sense if it is to be replaced by something healthier, affordable, and appealing. "Substitution is always an issue," Sturm notes.

But more troubling yet is that the subtext here is that policy should be dictated for poor people the way it is for children. Unless the tenor of the conversation about obesity gets a little less paternalistic, it's going to be a challenge to get anyone to the table.