High Body Fat Impacts Smartwatch Calorie Accuracy

A study involving Florida International University researchers found that popular smartwatches may inaccurately measure calories burned during exercise for individuals with higher body fat percentages. The devices' estimations become less reliable as body fat increases, a detail relevant to Cutler Bay residents who enjoy active lifestyles, such as walks in Palmetto Bay Park or bike rides along Old Cutler Road.

Researchers at FIU’s Medical Photonics Laboratory evaluated four smartwatches: the Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch5, and Fitbit Sense 2. They recruited 58 adults from Miami with diverse body fat percentages and skin tones for standardized cycling sessions. Participants alternated between 10 minutes of moderate and vigorous intensity exercise while wearing the devices.

To establish accuracy, researchers used a metabolic analyzer, the most reliable method for measuring calorie expenditure, which measures inhaled and exhaled gases. This allowed a direct comparison between the watches' calorie estimates and actual energy expenditure. The study used recumbent cycling to control exercise intensity and minimize wrist movement, which can interfere with heart rate sensors.

All four tested smartwatch brands showed substantial errors, with median errors ranging from 15% to 25% compared to the metabolic analyzer. Apple, Garmin, and Samsung generally overestimated calorie burn, with Apple showing the smallest average error. Fitbit produced several implausible readings, some exceeding 450% of the measured value.

The study found a strong correlation between a wearer's body fat percentage and measurement inaccuracy. As body fat percentage rose, so did the error rate, with some watches showing errors of 50% to 100% or more for participants with higher body fat.

While the study did not find a significant effect of skin tone on accuracy under tested conditions, the relationship between body fat and measurement error was consistent across all brands, though the magnitude varied. Researchers suggest proprietary algorithms and individual differences in body composition and physiological responses to exercise likely play a role. Users, particularly those with higher body fat percentages, should view smartwatch calorie burn data with caution.