We’ve long held that using the “Body Mass Index” (BMI) as a tool for measuring obesity is wrongheaded. BMI is a simplistic measure that only combines height and weight—ignoring muscle mass—which leads to erroneous classifications. Even the Centers for Disease Control and Prevention has cast doubt on its usefulness. So it wasn’t surprising to read yesterday that the British National Health Service (NHS), which measures schoolchildren’s BMI and sends “warning letters” to parents of supposedly “overweight” kids, has made a number of big fat screw-ups.

It turns out that the NHS has been sending warning letters to parents of kids who aren’t fat at all—including the mom and dad of a 5-year-old boy battling cancer. One letter even caused an already-thin boy to stop eating. Yikes.

Unfortunately, here in the US there’s a similar movement to increase the use of BMI reporting, especially among youngsters. This is ill-advised and bound to hurt the already fragile self-esteem of many kids. There are plenty of smart ways to fight obesity—especially since American children aren’t getting enough exercise. But using a tool that labels Tom Brady and George Clooney officially “fat” is not a good place to start.