Are activity trackers accurate? Don't count on it
People who wear activity trackers to count calories 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," he added.
Dr 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 per cent.
The exception was the Samsung Gear S2, which had an error rate of 5.1 per cent.
But for calculating energy expenditure - or calories burned - all of the devices had error rates above 25 per cent.
The Fitbit Surge had the lowest average error rate for calories burned at about 27 per cent.
The PulseOn had the highest at about 93 per cent, 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.
Data tended to be less accurate for men, people with higher body weights and darker skin tones, and while walking.
In a statement to Reuters, Fitbit said its trackers show an estimated total number of calories.
"Fitbit uses a scientifically validated estimate of (basal metabolic rate) based on height, weight, age and gender information that users provide when setting up their Fitbit account."
Added Mr Mark Gorelick, chief science officer at Mio Global: "While the Mio Alpha 2 was designed for the individuals focused on heart rate zone training, and not for all-day activity tracking, we agree that more accurate calorie estimation is important for the industry as a whole, since most individuals are monitoring calorie deficits for weight loss."
Mr Markku Lankinen, head of operations for PulseOn Oy, said the researchers may not have adjusted the device specifically for each participant.
"With the PulseOn device, you would need to apply these user parameters in the application before exercising, and this seems not to have been done," he said. "This causes the (energy expenditure) estimates to be badly off."
Apple, Microsoft and Samsung did not provide comments for publication.
All Basis Peak watches were recalled in 2016 due to overheating, according to its website.