How inaccurate is your fitness tracker in counting calories? Scientists weigh in

A Woman running in the streets of Los Angeles. Mike Medby / Mood Board / Rex Features

You wear them on your wrist all day to track how many steps you’re taking, how many calories you’re burning and even how many hours you’re sleeping. But how accurate are fitness trackers?

A new study that stacked 12 devices against other ways of monitoring these markers suggests that fitness trackers are way off when logging calories burned – to the tune of up to 278 calories.

Japanese researchers looked at eight different devices that are popular in the country, including the Fitbit Flex and the Jawbone UP24, along with wearable options made by Epson and Garmin.

Nine men and 10 women between 21 and 50 wore the devices in two separate tests.

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The researchers wanted to see how the technology held up next to other ways of recording caloric burn. In their first experiment, the volunteers were locked in a metabolic chamber, which is a room that’s able to accurately monitor calories taken in and calories burned off. They received three meals, could work on a treadmill and carry out other tasks like housework, watch TV, or write on a desk.

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Turns out half of the fitness trackers would overestimate calories burned by about 204, while others underestimated by up to 278 compared to what the chamber documented.

In their second study, they tracked calories burned using a urine sample. Across the board, the fitness trackers underestimated energy expenditure.

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The latest findings, published in the Journal of the American Medical Association, aren’t the first to call out inaccuracies.

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A 2015 study, conducted out of North Carolina’s RTI International, zeroed in on Fitbit and Jawbone (Nike was included initially, until the company said it was moving out of the wearable tech sector).

Dr. Robert Furburg, a senior clinical informaticist at RTI, said he and his team pored over thousands of studies on fitness trackers and narrowed their review down to 23. They wanted to figure out the validity and reliability of trackers’ functions, such as steps, calories burned and sleep monitoring compared to lab-designed equipment that costs thousands of dollars.

Trackers were best at accurately measuring steps as a pedometer.

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It makes sense that steps were the only accurate measurement. These devices don’t look at calories burned and calories consumed – they infer this information based on how many steps you’ve taken and how much distance you’ve logged, he said.

“Steps taken is frequently the only actual variable being measured on the device and how these other numbers are reached is a mystery to us as consumers,” Furburg said. If 10,000 steps is roughly 5.5 miles, that would be roughly 1,300 calories, he said as an example.

“Calories burned can be estimated based on your age in years, height, weight, gender and how many steps you’ve taken. All of that is plugged into an ugly equation that spits out a number. That’s where we find the greatest inaccuracies, taking steps away from the primary source,” he told Global News.

These errors could lead to issues with weight loss – if you think you’re working out enough to create a calorie deficit, you could be way off. You might indulge in more snacks too, if you think you’ve worked off 200 more calories than what’s reality.

Underestimating exercise is an issue, too, according to Dr. Adam Schoenfeld, a University of California scientist who authored an editorial accompanying the study.

“It could be quite dangerous if someone with heart disease had inaccurate recordings of their activity and exercise that was being used to make medical decisions,” Schoenfeld told Global News via email.

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“It is currently quite challenging to tell which fitness trackers are accurate and reliable and which are not since there aren’t much data available. These studies demonstrate that even the most popular applications and devices may be inaccurate or highly variable,” he said.

A Fitbit spokesperson says that the company performed its own internal studies to test its products.

“FItBit trackers are designed to provide meaningful data to our users to help them reach their health and fitness goals, and are not intended to be scientific or medical devices,” the spokesperson said in an email to Global News.

“The success of Fitbit products comes from empowering people to see their overall health and fitness trends over time – it’s these trends that matter most in achieving their goals,” the statement said.

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