Eat your peas. Finish those lima beans. Clean your plate before dessert. These are the nightly naggings of good moms. But in the near future, the federal government might be taking their place.

You probably heard the pomp and circumstance behind first lady Michelle Obama’s announcement Tuesday that she’s enlisting in the childhood obesity wars. It’s a noble goal. But if initial reports are any indication, it appears Mrs. Obama is tossing aside her first lady title in favor of “First Nanny.”

The New York Times reported on Tuesday that upcoming legislation, sponsored by Sen. Blanche Lincoln (D-Ark.) and backed by the Obama administration, will ban so-called junk food—candy and sugar-sweetened beverages—from schools. From President Obama’s claim in September that a federal soda tax was “an idea worth exploring” to this latest sour idea, tactics of America’s killjoy “food police” seem to have crept into the administration’s preventive health philosophy.

The fundamental problem with food-focused tactics is that they act as blinders, blocking our view of the many other factors that contribute to obesity. Blaming one kind of food or drink for obesity is like saying a basketball game was won (or lost) during a single trip down the court.

Take soda and other sugary drinks. A University of Minnesota study published in October’s American Journal of Clinical Nutrition found no association between the consumption of sugared drinks and adolescent weight gain over a five-year period. In 2008, a large scientific review of past studies came to a similar conclusion.

In other words, calories are calories, whether they’re lurking in soft drinks, milk, cookies, or even applesauce.

Banning candy and snacks on school grounds tends to create an atmosphere of “prohibition.” Kids understand forbidden fruit. There are U.S. schools today where black markets provide children with contraband yummies when they get tired of skim milk, carrots, and celery. Snack food “speak easies” have literally sprung up to serve brownies and chips.

An Austin American-Statesman reporter toured the hallways of Austin High School following a snack food ban. The scene, he wrote, was “Willy-Wonka-meets-Casablanca.”

Michelle Obama also trotted out the oft-repeated claim that our children may be the first generation to live shorter lives than their parents. But the lead author of the CDC’s National Vital Statistics Report on life expectancy challenged this bit of pop-science folklore back in 2005, noting that “We’ve never seen anything like that. Life expectancy has gone up pretty steadily.”

To be fair, not everything in the first lady’s obesity campaign is stale and doomed to fail. She’s right to emphasize the need for kids to get physical activity (the other, less controversial half of the “obesity equation”). In its “Shape of the Nation Report,” the National Association for Sport and Physical Education finds that only eight percent of elementary schools and six percent of middle and high schools require daily phys-ed class. And the percentage of students participating daily dropped from 42 percent in 1991, to just 28 percent in 2003.

Unlike plans to confiscate easily replaced soda cans, this has real implications for kids’ long-term health. Cardiologists working with the British government reported in November that the amount of time children spend doing moderate and vigorous exercise is the factor most closely related to their fat mass. If anything, schools should devote more time to gym class and recess and make sure that kids are active.

Telling her own personal story last week, Michelle Obama described how she helped her daughters eat healthier and manage their weight after a doctor put the problem on her radar. It wasn’t a school lunch lady that weighed in. And it wasn’t a federal mandate that made her kids healthier. It was good old-fashioned parenting.

It’s too bad some politicians have a different prescription for the rest of America’s moms and dads. Legislation making its way through Congress would take power away from local school districts, teachers, and parents—those who really have responsibility for raising children. Instead, Big Nanny (the gloomy-caped cadre of Washington politicians who think they know what’s best for you) will make the call.

Banning snack foods may be the biggest political no-brainer since “hope and change.” But neither the heavy hand of government nor a steady diet of scaremongering will make our kids healthier.