Should you trust your Fitbit or Jawbone when it comes to counting calories?

Your fitness tracker may be lying to you when it comes to counting calories. A new study has revealed that while trackers made by Fitbit and Jawbone slightly underestimate calorie burn for sedentary activities, they can be way off during strenuous exercise – sometimes overestimating by up to 40%.

To put this into perspective, an average adult burns about 2,000 calories a day. If your fitness tracker overestimates calories by a third, it is telling you you burned an extra 600 calories today. That equates to a Big Mac!

To determine the accuracy of some of the most popular fitness trackers on the market, researchers from Ball Sate University tested four devices against a portable metabolic analyzer. Two wrist-worn trackers – the Fitbit Flex and Jawbone UP24, and two hip-worn trackers, the Fitbit Zip and Fitbit One. The tests were conducted on 30 adults of different ages and fitness levels, who were asked to perform 10 activities of varying difficulty.

The study, which will be published in Medicine & Science in Sports & Exercise later this summer, found that readings were all accurate to within 8% when it comes to sedentary activities, but the more strenuous the activity the larger the inaccuracy.

For exercises that simulated household work (vacuuming, gardening and sweeping), all the trackers except the Fitbit Flex underestimated calorie output by close to a third. And during strenuous exercise (walking, jogging and climbing stairs), none of the trackers were accurate, with overestimates ranging from 16% to 40%. Cycling was the worst activity recorded in terms of accuracy.

“When you look at the literature, there is really not a lot of information on how well these wearable technologies work [or] how well they track variables that they are displaying on their apps,” says Alex Montoye, a co-author of the study and an assistant professor of clinical exercise physiology at Ball State University.

“I was a little bit skeptical, just because I know how hard it is to measure activity even with the research-grade monitors.”

A Fitbit spokesman claimed their trackers are ‘not intended to be scientific or medical devices’ and are instead designed to ‘provide meaningful data’. A spokesman for Jawbone called the UP24 a ‘relatively old product’ which used ‘older technology’.

FitBit and Jawbone were chosen because in 2013, when the group was in the advance planning stage of the study, the companies “occupied about 85% of the wearable technology market,” Montoye explains.

We would tend to agree with Jawbone’s statement. All four of these trackers are now several years old and none contain a heart rate sensor. They estimate resting caloric burn using information about your weight, height and gender and rely on added sensors to estimate how many calories you’re burning when you’re active. There is an advantage to using devices with a heart rate monitor, as these fitness trackers can detect changes in exercise intensity. In general, the more parameters you can have, the more accurate your equation should be.

However, these findings aren’t the first to call out inaccuracies. We wrote recently about a study published in JAMA Internal Medicine, which showed that when it comes to measuring energy expenditure, many leading fitness trackers can be way off. The paper found the margin of error to be about 200 calories per day, with some over-calculating and others under-calculating. Last year, a study led by researchers from the UNC Gillings School of Global Public Health and RTI International, also zeroed in on Fitbit and Jawbone and found that trackers were best at accurately measuring steps as a pedometer and not that great at measuring calories and sleep.

The news of inaccurate readings comes at a time when Fitbit is dealing with a class action lawsuit, filed by a group of three dissatisfied Fitbit customers. The company’s heart-monitoring technology was called “wildly inaccurate” and Fitbit was accused of false advertising. The lawsuit alleges that readings “don’t count every beat” and sometimes show dangerously low BPM numbers, which may pose major health risks to certain users.

In the past three years, the fitness tracker market has exploded worldwide, dominated by big names such as Fitbit, Jawbone and Garmin. And while these devices may be great for counting steps, distance and tracking exercise habits, they tend to be less effective when it comes to maintaining healthy eating habits. Which means, you probably shouldn’t truly rely on your activity tracker for accurate calorie information. Or for that matter your heart rate during high intensity activity.

Fitness trackers work well for resting caloric burn and sedentary activities, but the more active we become typically the less accurate the resulting data will be. If you are interested in a ballpark figure, that’s fine. But don’t let inaccurate information about calories derail your progress.

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