Smartwatch Calorie Tracking Shows Significant Errors, Especially for Individuals with Higher Body Fat

Smartwatches, widely used by nearly 40% of U.S. adults for workout tracking, may not be as accurate as consumers believe when it comes to measuring calories burned. A study conducted by Florida International University’s Medical Photonics Laboratory revealed that four major smartwatch brands—Apple, Garmin, Samsung, and Fitbit—exhibited substantial errors in calorie tracking, ranging from 15% to 25% compared to measurements taken by a metabolic analyzer.

Study Methodology

The research involved 58 participants of varying ages and body weights. They engaged in a 10-minute exercise protocol on a recumbent bicycle, alternating between two minutes of fast pedaling and five minutes of rest. During this period, researchers used a metabolic analyzer mask to precisely measure energy expenditure. The calorie counts recorded by the smartwatches were then compared to these gold-standard measurements.

Key Findings on Inaccuracy

All four tested smartwatch brands showed significant discrepancies. While Apple's estimates had the smallest average error, Garmin and Samsung tended to overestimate calorie burn more considerably. Fitbit's measurements, though not consistently over- or underestimating, produced several highly implausible readings, some exceeding metabolic system readings by over 450%.

The study highlighted a strong correlation between body fat percentage and the accuracy of calorie tracking. As a participant's body fat percentage increased, the smartwatches' calorie burn estimates became progressively less accurate. In some cases, this inaccuracy could mean an overestimate of 50% to 100% or more compared to actual expenditure.

Potential Reasons for Inaccuracy

Researchers theorize that the algorithms used by smartwatches, which combine data from accelerometers and heart rate sensors with user demographics like age, sex, height, and weight, may be a contributing factor. These algorithms are often developed using data primarily from leaner individuals. Consequently, increased soft-tissue motion at the wrist and limited ranges for calorie data in individuals with higher body fat percentages could lead to estimation errors.

Implications for Users

While a difference of a few calories might seem minor, the cumulative effect over time can result in thousands of miscounted calories. Exercise scientist Jason Kostrna, a study author, advises users to treat calorie tracking data from smartwatches as an estimate rather than a precise measurement. He suggests being conservative when using this data to guide dietary intake or assess workout effectiveness, particularly for those aiming for weight loss.

Despite the inaccuracies in calorie tracking, Kostrna’s related research indicates that smartwatches generally provide accurate heart rate measurements. Additionally, wearables may still offer reliable data for other metrics, such as distance and pace during activities like walking, running, or biking.