When it comes to regulating or taxing foods in the interest of public health, there is a line. The line might not be where libertarian types say it is (basically, everywhere), but there is a line. Banning trans fats from restaurants or soda pop from schools might not cross it. Nor might imposing a tax on sugary drinks or forcing restaurants to disclose nutrition information on their menus.

But a year-old ban on new fast-food restaurants in a poor district of Los Angeles crosses that mark. Worse, a new study by the Rand Corp. concludes that the block is unlikely to do any good. If so, the situation is this: The government is imposing a restriction only on certain poor people, to the good of nobody, while enabling people with higher incomes to eat at whatever kind of restaurant they choose.

The City Council imposed the ban last year in South Los Angeles in the interest of curbing obesity. The law will be in place until at least March.

In the online edition of the journal Health Affairs (abstract only), Rand's Roland Strum and Deborah Cohen say the "premises for the ban were questionable." The National Institutes of Health financed the study.

The most noteworthy finding: There are actually fewer fast-food restaurants in South Los Angeles than in other districts of the city.

It's true that obesity rates are higher among the poor. But while the food industry—particularly the fast-food business—is surely responsible in part for this phenomenon, obesity is a complex problem that can't be solved simply by removing fast-food joints from certain neighborhoods.