As schools around the country wrap up their first month back in session, parents soon may start receiving the first reports on how their children are shaping up — literally.

Public schools in 19 states now track students’ body mass index numbers and report the fat metrics back to parents. There may, indeed, be more schools tracking students’ BMI than there are schools teaching kids the arithmetic needed to do such calculations on their own. To some, this would seem like the prudent step to take given that childhood obesity has nearly tripled since 1990. The Centers for Disease Control and Prevention says about one in five American children is obese, and one in three is overweight or obese.

There are only a couple of problems with this increasingly popular nanny-state tactic: There’s not much evidence that BMI-tracking reduces obesity, and it may harm the very children it’s meant to help.

Parents object

There is no shortage of parents opposed to the idea, as they note the bureaucratic nature of the procedure only adds to children’s insecurity about their size and can expose them to unwelcome scrutiny from their classmates. As Shannon Park, a New York City mother of two daughters, told ABC News earlier this month:

Their bodies are changing … And then they get this number that says, ‘Oh, you know, you’re not the right number.’ It’s just a horrible way to start womanhood.

That is not just idle talk. There’s a real chance that government-run BMI-tracking can contribute to eating disorders, which is one reason the Washington-based Eating Disorders Coalition opposes such policies. And the tactic can actually worsen another problem: society’s pervasive bias against obese people.

Researchers have found widespread conscious bias against fat people and a preference for individuals who are thin, with study subjects agreeing with statements such as “People like this make me feel uncomfortable” and “I’d like to be friends with someone like this.” Implicit, or unconscious, anti-fat attitudes are even more common.

For big people, big stigma

As I reported Sept. 2 in American Medical News, even doctors fall prey to this bias in ways that can hurt their ability to build rapport with the patients who are most in need of their advice. Meanwhile, recent research suggests that such anti-fat attitudes can send overweight people into an all-you-can-eat spiral of self-hatred. A November 2012 PLoS One study that tracked more than 6,000 Americans found that — even after adjusting for demographic factors — the people who experienced some kind of weight-based discrimination were two and a half times likelier to pack on the pounds and become obese during the four-year study period.

More to the point, such anti-fat attitudes are common among students, with one study finding that teens often use words such as “sickening,” “gross” and “disgusting” when describing obese people. It is easy to imagine how a mandatory, school-wide process of putting students’ size into a single number can exacerbate the problem. As if shopping for “husky”-sized jeans with mom wasn’t embarrassment enough.

Even the American Academy of Pediatrics — often among the first organizations to support various nanny-state interventions — says BMI tracking should be done in the doctor’s office and that “discussions to raise parental awareness should be conducted in a nonjudgmental, blame-free manner so that unintended negative impact on the child’s self-concept is avoided.” It is safe to say that kids do not usually regard letters sent home from school as being filled with “blame-free” tidings of happy news.

Slim evidence

But it’s all good, because these programs have a great track record of success, right? After all, the headlines last week blared some infrequent good news on the childhood obesity front. Well, there are signs that kids are starting to move more and eat less, but there’s minimal evidence to show that BMI-tracking is responsible. The CDC notes:

Little is known about the outcomes of BMI measurement programs, including effects on weight-related knowledge, attitudes and behaviors of youth and their families. As a result, no consensus exists on the utility of BMI screening programs for young people.

The widely respected Institute of Medicine has backed the idea of annual BMI-reporting in public schools, but another body that hews even more closely to the scientific data — the U.S. Preventive Services Task Force — has said there’s not enough evidence to support the concept.

A more promising — and certainly less intrusive — course of action on obesity would be to reduce the barriers to healthy living, by making it cheaper and easier for families to find fresh fruits, vegetables and lean meats. That shopping, heaven forbid, may even be done at a dreaded Walmart superstore, where groceries cost 20% to 33% less than at traditional grocers.