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People who wear activity trackers to count the calories they’re burning are probably not getting accurate estimates, suggests a new study.

Researchers who tested seven popular activity trackers found that while heart rate measurements were generally accurate, none of the devices provided a reliable calorie count.

“At this point with this level of error, I would be wary of using that estimate to alter a calorie-controlled diet,” said senior author Dr. Euan Ashley, of Stanford University in California.

Patients “have been bringing data from these devices to us and some of us were using these devices ourselves,” Ashley told Reuters Health.

Because so little is known about the data’s accuracy, “We realized that we had to do our own study,” he said.

Ashley’s team recruited 60 healthy adults to test seven popular wrist-worn activity trackers: the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, MIO Alpha 2, PulseOn and Samsung Gear S2.

Participants wore up to four devices at a time, and they also wore laboratory devices to measure heart rate and calories burned while sitting, walking, running and cycling.

All of the devices but one had an average heart rate error rate below 5 percent. The exception was the Samsung Gear S2, which had an error rate of 5.1 percent.

But for calculating energy expenditure – or calories burned – all of the devices had error rates above 25 percent. The Fitbit Surge had the lowest average error rate for calories burned at about 27 percent. The PulseOn had the highest at about 93 percent, according to a report in the Journal of Personalized Medicine.

Overall, the Apple Watch had the lowest error rates while the Samsung Gear S2 had the highest.

The researchers were surprised at the unreliability of the calorie counts.

The devices “were literally all over the map with error rates,” Ashley said.

Data tended to be less accurate for men, people with higher body weights and darker skin tones, and while…

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