The INSIDER Summary:
- Lots of people use fitness trackers to measure the number of calories they burn.
- A new study says they’re not very accurate for this purpose.
- Researchers studied 7 trackers and found they were off by 27% to 93% when calculating calories burned.
- But the trackers were much more accurate when measuring heart rate.
Fitness trackers can be helpful tools when you’re trying to lose weight or stay active. But a new study finds that they’re surprisingly bad at counting the number of calories you burn, the Guardian reports.
Researchers at Stanford University chose seven different trackers to study: the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and Samsung Gear S2. Then they recruited a group of 60 volunteers, who wore the devices while walking or running on treadmills or using stationary bikes.
Each person’s heart rate was measured using an electrocardiograph. And each person’s calorie burn was measured using a device that detects the amount of oxygen and carbon dioxide in the breath. The results were published Wednesday in the Journal of Personalized Medicine.
The trackers excelled in their first test: They measured heart rate with an error rate of less than 5%. But measuring the number of calories burned was a different story entirely.
The most accurate device, the Fitbit Surge, was off by an average of 27%. The worst-performing device, the PulseOn, was off by an astounding 93%. That means that, if the device says you burned 100 calories, it’s possible you could have burned as few as 7.
But the errors also went both ways.
“The trend we’re seeing is that most of these devices are underestimating calories when people are sitting still or when they’re walking at a moderate pace or cycling, and then the devices are overestimating your energy expenditure when you’re running or doing some vigorous exercise,” lead study author Anna Shcherbina told INSIDER. “So the direction [of the error] depends on the intensity and the types of activity that the user is performing.”
The study authors couldn’t say exactly why the trackers did so poorly in this regard. But it’s possible that the devices predict calorie burn using algorithms that aren’t accurate across a wide variety of individuals.
“Energy expenditure is variable based on someone’s fitness level, height and weight,” Shcherbina said in a Stanford University press release. In other words: One size does not fit all.
Next, the researchers want to test the accuracy of fitness trackers while people wear them in their real lives — not just while they’re exercising in lab. For now, the study’s results are good reminder to take your tracker’s calorie calculations with a grain of salt.
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