Recent research shows that smartwatches like the Apple Watch struggle to accurately measure calorie expenditure despite reliable heart rate data.
A 2024 study by the Stanford University School of Medicine tested devices including the Apple Watch on 60 volunteers. The most accurate tracker deviated by an average of 27 percent from medical-grade measurements, while the least accurate was off by 93 percent.
Dr. Euan Ashley, senior author of the study published in the Journal of Personalized Medicine, said the energy expenditure measures were way off the mark. Assistant Professor Anna Shcherbina noted that algorithms face challenges because energy expenditure varies with factors such as fitness level, height and weight.
A separate study published in Nature reached similar conclusions. It found that multiple sensors can compound errors and that skin tone and moisture levels also affect results.
Smartwatches estimate calorie burn using motion sensors, GPS, heart rate data and machine learning algorithms, but these methods rely on proxies rather than direct metabolic measurements.
