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One of a smartwatch’s main selling points can be its ability to track the calories burned while exercising. But similar to the health tech industry’s “10,000 steps” selling point, new research strongly suggests a wearable’s workout assessment may overestimate your efforts—and in some cases, the readings are off by as much as 25 percent.

“You can’t treat the calories burned number it gives you as an accurate number, because it’s not,” Florida International University kinesiologist Jason Kostrna said during a recent profile.

Kostrna and his colleagues compared four of the most popular fitness wearables on the market: the Apple Watch Series 8, Samsung Galaxy Watch 5, Fitbit Sense 2, and Garmin Forerunner 955 against clinical-grade measurement equipment. They then asked 58 adult volunteers to use a stationary bike during a controlled workout session, while alternating between moderate and intense activity. Their results were simultaneously recorded by one of the commercially available wearables, as well as a COSMED K5 metabolic analyzer. For reference, COSMED fitness equipment can be seen during James Bond’s physiology assessment scene in the 2012 film Skyfall.

According to results recently published in the journal PLOS One, smartwatch calorie overestimations ranged between 15 and 25 percent. Apple was the most comparatively accurate, while Garmin and Samsung offered the most generous results. Meanwhile, the researchers needed to discard Fitbit from their trial entirely because the device repeatedly glitched during experiments. In some instances the Fitbit recorded a single calorie burned during an entire workout, or even failed to generate any assessment.

To put the larger findings in perspective: if your wearable claims you burned 500 calories at the gym, that number may be much closer to 350 calories. This may not seem too serious, but the errors can easily add up over time.

“You could be hundreds of calories off each week and easily end up in a calorie surplus when you think you’re in a calorie deficit,” said Kostrna.

Tests also consistently indicated that smartwatches become less accurate for wearers with higher body fat percentages. The exact reason is still unclear, but Kostrna’s team suspects the issue may stem from how each device’s algorithms are calculated. This information is often highly guarded from public scrutiny by the tech companies themselves.

“When the companies are developing those algorithms, we don’t know where the test data is coming from,” Kostrna explained. ” That’s why I think it’s beneficial for the companies to see this type of data, so they can refine their products over time.”

However, this doesn’t mean the devices are completely useless. Their caloric measurements should just be taken as generous estimates. Researchers suggest treating a reading as a best case scenario after each workout, while people with higher body fat index may want to view the actual numbers even more conservatively. At the same time, the study’s authors warn against solutions like more aggressive dieting, since the body still requires healthy foods to recharge and improve. In any case, Kostrna stressed that calorie monitoring should only be one part of a person’s larger fitness goals.

“Focus on how you’re feeling, and use data to get a little better over time,” he suggested.

The post Your smartwatch may be overestimating calories burned by 25 percent appeared first on Popular Science.