Britain’s “fat” surveillance of schoolchildren has once again backfired. The Sun reported last week that a perfectly healthy-sized girl is refusing to eat after the government told her parents that she’s “overweight.” The key problem is the British health authorities’ use of a measure called the “Body Mass Index” (BMI) to classify kids as fat, underweight, or healthy. Parents are receiving warning letters if their kids fall on the "overweight" end of the spectrum. Our main criticism of the BMI is its inaccuracy as an obesity-measuring tool, and a number of similar gaffes this summer make us wonder how much longer it will be in widespread use. Fortuitously, The New York Times analyzed some of the BMI scale’s limitations yesterday:

For children and the elderly, body mass values can be especially misleading because the relationship of lean body mass to height changes as they get older …

If you fall into the “healthy weight” or “underweight” range, you can easily be lulled into a false sense of security. But thinness is not necessarily healthy—recall the 97-pound weakling from the Charles Atlas ads of yore. A low B.M.I. could be indicative of malnutrition, anorexia, cancer or a wasting disease. On the other hand, if you are an athlete or body builder, your B.M.I. could mistakenly put you in the range for overweight or obese.

Those worried about a high BMI should be sure to get both cardiovascular and weight-resistance workouts, the Times notes. And what better complement to that advice than new research showing that a genetic predisposition to obesity can be largely offset by getting a healthy dose of physical activity? As epidemiologists struggle to accurately classify data, and governments labor to accurately classify kids, the obesity solution seems to be as obvious as ever: Eat in moderation, and move your body often.