Wearables overcount calories

A calorie number on a watch looks precise because it ends in a single digit, but Stanford researchers found the calorie part was the least reliable thing on the screen. In a 2017 study of 60 people wearing seven popular devices, none of the trackers measured energy expenditure accurately. (med.stanford.edu) The researchers tested Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and Samsung Gear S2 while volunteers sat, walked, ran, and cycled. They compared each device with laboratory tools including indirect calorimetry, which measures calories by tracking oxygen use and carbon dioxide output like a car test that checks fuel burn from the exhaust. (mdpi.com) Heart rate was mostly fine. Stanford said six of the seven devices stayed within 5 percent error for heart rate, with the Apple Watch performing best in that part of the test. (med.stanford.edu, cnbc.com) Calories were a different story. Stanford reported that the most accurate device was off by 27 percent on average, and the least accurate was off by 93 percent, which means a workout shown as 300 calories could really be closer to 155. (med.stanford.edu, acsh.org) That gap exists because calorie burn is not a direct sensor reading in the way pulse is. The paper says energy expenditure depends on variables including age, weight, height, fitness level, and the efficiency of a person’s movement, so companies are estimating with formulas rather than measuring the number itself. (mdpi.com) Efficiency is the part most people never see. Two runners can cover the same mile at the same speed, but the more trained runner often uses less energy, like two cars driving 60 miles per hour while one engine burns less gas. (mdpi.com) Stanford’s team also found the errors were not evenly spread across people. The paper says error increased with higher body mass index, darker skin tone, and male sex for some heart-rate readings, which shows how a generic consumer model can miss real biological differences. (mdpi.com) Treadmills have a similar weakness when they flash a calorie total on the console. Those machines usually estimate from speed, incline, time, and a few profile details, so the number is still a model, not a lab measurement of what your body actually used. (mdpi.com) The practical mistake is treating exercise calories like money you definitely earned back. If a tracker overstates a session by 200 calories and you eat those 200 calories back four times a week, that adds up to about 800 extra calories a week. (google.com)) The safer use for wearables is trend tracking, not exact accounting. A watch can still tell you that today’s run was harder than yesterday’s, or that your resting heart rate is changing over time, even if the calorie number is too noisy to build meals around. (med.stanford.edu) Stanford’s lead author Euan Ashley said the team wanted companies to publish validation data the way medical devices do, because people increasingly share wearable data with doctors. Until that becomes standard, the calorie number on your wrist is better treated like a rough guess than a receipt. (med.stanford.edu)