If you think your fitness tracker on your wristis giving you a good excuse to eat that sweet treat,you may want to think again.A recent study from the Stanford University School of Medicinesays fitness trackers are not accurate at tracking calories.The study says, however, that the trackers are accurate at measuring heart rate.The Stanford researchers examined measurementsrecorded when people were at rest and while they exercised."People are basing life decisions on the dataprovided by these devices," said Euan Ashley, one of the study investigators.The study looked at seven fitness trackers worn on the wrist:They were Apple Watch, Basis Peak (version 1),Fitbit Surge and Microsoft Band (version 1).The others three were Mio Alpha 2, PulseOn, and Samsung Gear S2.Sixty participants of both sexes wore up to four trackers on their wrists as they sat,and as they exercised on a treadmill and a stationary bike.The participants were a mix of sizes and fitness levels.The trackers measured the heart rates and energy used, or calorie burn,in activities from sitting to a slow walk to a run.They also tested people cycling at different speeds.Professional medical instruments measured participants'heart and breathing rates as well as temperature and blood pressure.The researchers compared the fitness tracker measurementsto those of the "gold standard medical instruments. "The final report on the study was published last week.It found that none of the trackers met what researchers consideredan acceptable error rate for energy use measurement.The acceptable error rate is five percent or less.The researchers wrote that the Fitbit Surgewas the most accurate tracker of energy use.It had a 27.4 percent average error rate.The study found the PulseOn to be the least accurate tracker,with a 92.6 percent error rate.The error rate was higher for males than for females with all fitness trackers tested.The average error rate for all the trackers studied under all forms of exercisewas four percent higher for males than for females.The good news is the tested fitness trackers were accurate at measuring heart rate.All but one of the trackers met the acceptable error rate limit in heart rate measurement.Of the trackers studied, the Apple Watch was the most accurate at measuring heart rate,with an average error rate of two percent.The Samsung Gear S2 was the least accuratewith an average error rate of close to seven percent.For both the measurement of energy used and heart rate,the researchers said "the Apple Watch had the most favorable error profilewhile the Samsung Gear S2 had the least."The Stanford researchers are continuing their investigation of fitness trackers.Next, they will test participants wearing fitness trackers during their daily livesand while exercising outside a laboratory.The full study is available online at The Journal of Personalized Medicine.