When it comes to keeping tabs on calories burned, your fitness tracker may be out of step, according to new research. The study was published in JAMA Internal Medicine, and pitted 12 different fitness trackers against two proven methods of calculating caloric expenditure in two experiments. The first experiment isolated 19 healthy participants, ages 21 to 50, for 24 hours in a metabolic chamber—a room designed to allow researchers to calculate burned calories through carbon dioxide production and oxygen consumption (calorimetry). While wearing all 12 trackers at once, the participants received three meals and engaged in a set of activities that included working at a desk, exercising on a treadmill, watching television, doing housework, and sleeping. Then, during the second experiment, researchers asked the same participants to drink specially treated water for 15 days while living normally. The treated water enabled researchers to measure calories burned through tests on urine samples collected on eight of the 15 days. Participants wore the 12 trackers during waking hours throughout the second experiment, except while bathing, charging the trackers’ batteries, and engaging in other activities in which wearing the tracker was not feasible. Five of the trackers were worn during sleep. Here is what researchers found:

• In the first experiment, half of the fitness trackers underestimated calories burned by as much as 278 calories and the other half overestimated calories burned by up to 204 calories, compared with the measurements taken by calorimetry.
• In the second experiment, the fitness trackers underestimated calories burned by between 69 and 590 calories compared with the urine test measurements. It’s possible that the participants’ occasional tracker removal caused some of these underestimates.

This research demonstrates the variability and limitations of fitness trackers, and highlights the importance of having other ways to evaluate your exercise program—especially if you need to meet specific health goals. Calorie counting aside, if you want to get a healthy dose of exercise, current guidelines recommend a minimum of 150 minutes of moderate exercise (such as brisk walking or tennis), 75 minutes of vigorous exercise (such as jogging or swimming laps), or a combination of the two spread out over at least three days every week.

Source: JAMA Internal Medicine