Activity trackers many of us use on a daily basis may not be as accurate as we want to think they are. Actually, their heart rate measurements are correct, according to a team of researchers at the Stanford University School of Medicine, but things are not that bright when it comes to measuring energy expenditure.

The research included 31 women and 29 men who wore seven devices — including the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2 — while walking or running on treadmills or using stationary bicycles. Each volunteer’s heart was measured with a medical-grade electrocardiograph, while metabolic rate was estimated with an instrument for measuring the oxygen and carbon dioxide in breath — which is a good proxy for metabolism and energy expenditure. Results from the wearable devices were then compared to the measurements from the two “gold standard” instruments.

Six of the devices measured heart rate with an error rate of less than 5 percent. Some devices were more accurate than others, with factors such as skin color and body mass index affecting the measurements.

However, none of the seven devices measured energy expenditure accurately, with even the most accurate device missing the number by an average of 27 percent. And the least accurate was off by 93 percent.

“People are basing life decisions on the data provided by these devices,” said Euan Ashley, DPhil, FRCP, professor of cardiovascular medicine, of genetics and of biomedical data science at Stanford. “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.”

The take-home message, he said, is that a user can pretty much rely on a fitness tracker’s heart rate measurements. But basing the number of doughnuts you eat on how many calories your device says you burned is a really bad idea, he said.

A paper with the researchers’ findings was published online in the Journal of Personalized Medicine. Ashley is the senior author, with other lead authors including a graduate student Anna Shcherbina, visiting assistant professor Mikael Mattsson, PhD, and senior research scientist Daryl Waggott.

The work is a part of Stanford Medicine’s efforts on precision health, the goal of which is to anticipate and prevent disease in the healthy and precisely diagnose and treat disease in the ill.