Fitness Trackers Aren’t Very Accurate
There’s nothing wrong with keeping track of your workout, or trying to beat your record for a particular activity. Just don’t take your results as written in stone.
Researchers at National Institute of Health and Nutrition in Tokyo matched 12 devices like the Fitbit Flex and Jawbone Up24 against two proven (if stark) methods of monitoring energy expenditure -- locking people in a room to assess every calorie consumed and burned, or asking people at home to drink specially treated water that makes it possible to detect energy output with a urine test.
In the first experiment, measurements from the fitness trackers were well off from the lab results in a typical day – they underestimated energy expenditure by as much as 278 calories or overestimated it by up to 204 calories. All in all, half of the fitness trackers underestimated energy expenditure and half overestimated it.
With the second experiment, the devices ranged from 69 to 590 calories lower than the urine tests. Every fitness tracker underestimated energy expenditure in this experiment.
The results are troubling because when fitness trackers overestimate exercise, people who need more exercise to maintain or lose weight might get too little activity, increasing their risk for obesity and other chronic health problems.