Fitness trackers are exploding in popularity, tracking everything from heart rate to steps taken. But a new study from the National Institute of Health and Nutrition in Tokyo shows that this technology isn't always accurate at tracking energy expenditure. Researchers tested 12 fitness trackers against current methods of detecting energy expenditure. The results suggest that these trackers can both under and overestimate energy expenditure, sometimes by hundreds of calories. Some devices underestimated calories burned by almost 600 calories while others overestimated by around 200. Researchers say it is possible the trackers are inaccurate due to people taking them off during the day. For people who use fitness trackers to make health decisions based on fitness level, both over and underestimates can be harmful. Those trying to lose weight could be actually burning too few calories. Those watching their activity levels due to heart problems may be too active. There’s not much research on trackers yet, and devices could vary from brand to brand. Either way, people using fitness trackers to make health decisions should be cautious about relying too much on device data.