A study from Florida International University has revealed significant inaccuracies in how popular smartwatches calculate energy expenditure.. By comparing devices to a gold-standard metabolic analyzer, researchers found that most major brands provide inflated data during physical activity.
The 25 percent overshoot in Garmin and Samsung data
During a controlled 60-minute cycling test involving 58 adults, the Garmin Forerunner 955 and Samsung Galaxy Watch 5 demonstrated significant discrepancies in energy expenditure. According to the report, these devices overestimated calorie burn by up to 25 percent, averaging about 110 calories more than the actual expenditure measured by a hospital-grade metabolic analyzer. The test utilized a reclined stationary bike to ensure a controlled environment for measuring moderate and vigorous exercise.
This discrepancy poses a risk for those using wearables to manage weight. Lead researcher Jason Kostrna, an associate professor at Florida International University, noted that these small daily errors can accumulate into hundreds of calories per week. This could lead users to believe they are in a calorie deficit when they are actually in a surplus.
Apple Watch Series 8 maintains a 12 percent margin
While not perfect, the Apple Watch Series 8 emerged as the most reliable device among the four brands tested. The study, published in the journal PLOS One, found that the Apple Watch overshot calorie expenditure by roughly 12 percent. this device was compared against a mask-based system that measures oxygen consumption, which serves as the gold standard for calculating true calorie expenditure.
Despite its relative accuracy, the device still relies on indirect measurements. Dr. Cailbhe Doherty of University College Dublin's School of Public Health, Physiotherapy and Sport Science explained that smartwatches estimate burn using heart rate and movement, which is fundamentally different from the gas analysis used by metabolic masks.
Fitbit Sense 2 struggles with "glitchy" inconsistencies
The Fitbit Sense 2 was characterized by researchers as being consistently unreliable. Unlike the steady overestimation seen in Garmin or Samsung products, Fitbit readings fluctuated wildly, sometimes reporting as little as a single calorie for an entire workout. As the report states, these technical errors included both dramatic overestimations and implausible low numbers.
This erratic behavior makes the Fitbit device particularly difficult to use for anyone seeking consistent data trends. The study suggests that the device's inability to provide stable readings makes it a outlier compared to the more predictable, albeit inflated , numbers from Apple or Samsung.
The 35-calorie error linked to body fat percentages
Physical composition played a significant role in the accuracy of all tested devices, regardless of the brand. The research found that participants with higher body fat percentages experienced an additional estimation error of between 22 and 35 calories across every brand. This suggests that current algorithms may not sufficiently account for the physiological differences between users with varying muscle-to-fat ratios.
This finding implies that the utility of a smartwatch may vary significantly depending on the user's specific body type. For those attempting strict nutritional management, these compounding errors could lead to significant long-term miscalculations.
Can manufacturers bridge the gap with better algorithms?
While the study calls for improvements to device algorithms, it remains unclear how or when major tech companies will respond to these findings. The research highlights a fundamental gap between movement-based tracking and the "gold standard" of metabolic analysis. It is also unknown if the manufacturers of Apple, Samsung , Garmin, or Fitbit have plans to update their calorie-tracking logic in light of this data.
Furthermore, the study does not address whether the inaccuracies are a result of hardware limitations or software-based estimation models. Until more data is available, the researchers advise users to focus on personal trends over time rather than relying on precise daily totals.
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