Fitbits are way off the mark when calculating weight loss, study finds
Trendy fitness trackers like Fitbits do not work on overweight people and are "way off the mark" when calculating weight loss, according to new research.
Scientists set out to measure the accuracy of wristband activity trackers - including Fitbit and Apple Watch - worn by millions of people to monitor their own exercise and health.
They found that if the device measures heart rate, it is probably doing a good job.
But if it measures energy expenditure, it's probably out by a "significant" amount, according to the study by researchers at Stanford University School of Medicine in the US. An evaluation of seven devices in a group of 60 volunteers showed that six of the devices measured heart rate with an error rate of less than five per cent.
The team evaluated the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2.
Some devices were more accurate than others, and factors such as skin colour and body mass index affected the measurements, according to the researchers.
But the study found that none of the seven devices measured energy expenditure accurately.
Even the most accurate device was out by an average of 27 per cent while the least accurate was off by 93 per cent, according to the findings published by the Journal of Personalised Medicine.
The study's senior author Professor Euan Ashley said: "People are basing life decisions on the data provided by these devices."
But he said consumer devices aren't held to the same standards as medical-grade devices, and it's hard for doctors to know what to make of heart-rate data and other figures from a patient's wearable device.
Prof Ashley said: "Manufacturers may test the accuracy of activity devices extensively, but it's difficult for consumers to know how accurate such information is or the process that the manufacturers used in testing the devices."
He and his colleagues set out to independently evaluate activity trackers that met criteria such as measuring both heart rate and energy expenditure and being commercially available.
Co lead author graduate student Anna Shcherbina said: "For a lay user, in a non-medical setting, we want to keep that error under 10 per cent."
The volunteers wore the seven devices while walking or running on treadmills or using exercise bikes. Each volunteer's heart was measured with a medical-grade electrocardiograph (ECG).
Metabolic rate was estimated with an instrument for measuring the oxygen and carbon dioxide in breath. Results from the wearable devices were then compared to the measurements from the two "gold standard" instruments.
Prof Ashley said: "The heart rate measurements performed far better than we expected, but the energy expenditure measures were way off the mark.
"The magnitude of just how bad they were surprised me."
He said the message from the findings is that a user can "pretty much rely on" a fitness tracker's heart rate measurements.
But he said basing the number of doughnuts you eat on how many calories your device says you burned is a "really bad idea."
The researchers couldn't be sure why energy-expenditure measures were so far off. They said each device uses its own proprietary algorithm for calculating energy expenditure.
Ms Shcherbina said it's likely the algorithms are making assumptions that don't fit individuals very well, adding: "All we can do is see how the devices perform against the gold-standard clinical measures.
"My take on this is that it's very hard to train an algorithm that would be accurate across a wide variety of people because energy expenditure is variable based on someone's fitness level, height and weight, et cetera."
She said heart rate is measured directly, whereas energy expenditure must be measured indirectly through proxy calculations.