Fitness trackers can do a lot of good: They help you reach your daily goals (how awesome does it feel when you hit 10,000 steps?), alert you when you've been sitting for too long at the office, and even monitor your heart rate. But there's one thing they're not incredibly good at: keeping track of how much energy (aka calories) you're actually burning throughout the day.
In a small March 2016 study published in JAMA, researchers asked 19 healthy participants to wear 12 different popular fitness trackers — from Fitbit to Jawbone — at the same time on their waists, chests, and wrists. Then they put the trackers to the test using two proven methods of monitoring energy expenditure.
For the first test, participants spent 24 hours in a sealed room performing normal daily activities — the only change was that the oxygen and carbon dioxide levels in the room were being controlled. For the second method, participants started each day of a 15-day test period with a drink that allowed researchers to calculate how many calories they burned by testing the participants' urine. Then they went about their normal day-to-day routines wearing their fitness trackers.
The result? In the first method, researchers found the trackers were off by quite a bit. They either underestimated the calories burned by as much as 278 calories, or overestimated by as much as 204 calories. In the second method, the amount of calories burned on the trackers was between 69 and 590 calories lower than what the urine tests indicated.
This probably doesn't shock anyone who owns a tracker; the calories burned measurement can seem pretty arbitrary on any given day. But this does serve as a good reminder to not take tracker numbers — especially calories burned — too seriously. Fitness trackers are merely tools that help us embrace healthy habits — they're not the be-all and end-all of your value as a person. Stay active, fill your body with the nutrients you need, and you'll be just fine.