Your fancy fitness-tracking gizmo probably isn’t measuring your calorie expenditure very accurately. A new Japanese study in JAMA Internal Medicine compared a dozen such devices, including the popular Fitbit and Jawbone, with two “gold standard” laboratory methods of measuring energy expenditure. The fitness trackers ranged from 278 calories lower to 204 calories higher than what was measured by a metabolic chamber. Compared to the doubly-labeled water method, however, which can more completely measure calories burned through daily life, the gizmos all undercounted – between 69 and 590 calories lower per day.
The market for wearable fitness trackers doubled last year to $1.5 billion. But experts caution that the gadgets are better at simply counting steps and (if properly calibrated) tracking distance traveled than the complex task of measuring energy expenditure. If these fitness trackers motivate you to move more, however, they could still be a good buy.