At the heart of this issue is a single unit of measurement–the calorie–and some seemingly straightforward arithmetic. “To lose weight, you must use up more calories than you take in,” according to the Centers for Disease Control and Prevention. Dieters like Nash and Haelle could eat all their meals at McDonald’s and still lose weight, provided they burn enough calories, says Marion Nestle, professor of nutrition, food studies and public health at New York University. “Really, that’s all it takes.”
But Nash and Haelle do not find weight control so simple. And part of the problem goes way beyond individual self-control. The numbers logged in Nash’s Fitbit, or printed on the food labels that Haelle reads religiously, are at best good guesses. Worse yet, as scientists are increasingly finding, some of those calorie counts are flat-out wrong–off by more than enough, for instance, to wipe out the calories Haelle burns by running an extra mile on a treadmill. A calorie isn’t just a calorie. And our mistaken faith in the power of this seemingly simple measurement may be hindering the fight against obesity.