Fitness trackers may give you an idea of how many calories you burned during a workout or an entire day, but don’t expect that readings are accurate.
Researchers at the Stanford University School of Medicine studied seven popular fitness trackers and found that “none of [them] measured energy expenditure accurately,” according to a press release. That is also true for the Apple Watch, the Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and the Samsung Gear S2.
The team discovered that even the most accurate of these devices was still wondering “with an average of 27 percent.” The least accurate was off by an impressive bad 93 percent.
“People base life decisions on the information provided by these devices,” the study’s senior author Euan Ashley, a professor of cardiovascular medicine, genetics, and biomedical data science at the university of Stanford, said in a statement. But this might not be a wise decision.
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Ashley and his team invoked sixty volunteers — 31 women and 29 men — who wore the seven devices while walking or running on the treadmill or using stationary bikes. They measured each volunteer’s heart rate with medical-grade electrocardiograph and measured their metabolic rate with an instrument that is used for measuring the oxygen and carbon dioxide in the breath. The researchers are then compared with the results of the portable devices with the measurements of the two “gold standard” instruments.
The team found that six of the seven devices measured heart rate within 5 percent.” It was a very different story for their measurements of the calories you’ve burned.
“The heart rate measurements that have been carried out much better than we had expected’, said astrid, “but the energy expenditure of the measures were way off the mark. The extent of how bad they were surprised me.”
Thus, according to the release, “the base of the number of donuts do you eat on how many calories your device says you burned is really a bad idea.”
A paper reporting of the researchers findings was published Wednesday in the Journal of Personalized Medicine.
This article originally appeared on PCMag.com.