Fitness trackers produce inaccurate data on calories burned, scientists claim

A team of researchers at the Stanford University School of Medicine has found that the majority of fitness tracking devices do not report accurate data on energy expenditure, or calories burned.

An evaluation of seven devices, including Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2, in a diverse group of 60 volunteers showed some devices were more accurate than others, and factors such as skin color and body mass index affected the measurements.

The volunteers wore the seven devices while walking or running on treadmills or using stationary bicycles. Each volunteer’s heart was measured with a medical-grade electrocardiograph. Metabolic rate was estimated with an instrument for measuring the oxygen and carbon dioxide in breath.

The research, published in the Journal of Personalised Medicine, showed that six out of seven devices measured heart rate within five per cent. In contrast, none of the seven devices measured energy expenditure accurately, the study found. Even the most accurate device was off by an average of 27 percent. And the least accurate was off by 93 percent.

Euan Ashley, DPhil, FRCP, professor of cardiovascular medicine, of genetics and of biomedical data science at Stanford, said: “People are basing life decisions on the data provided by these devices. But 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 data from a patient’s wearable device.”