Overview
- The Florida International University study published in PLOS One and reported Wednesday compared four wrist trackers to a COSMED K5 metabolic analyzer, a clinical gold standard that measures oxygen use to calculate true energy expenditure.
- Across 58 adults doing controlled recumbent cycling, the devices overestimated calories by about 15–25 percent, which translated to typical absolute errors of roughly 80–110 calories per workout.
- Device results varied: the Apple Watch Series 8 was closest to the analyzer, the Samsung Galaxy Watch 5 and Garmin Forerunner 955 tended to overestimate the most, and the Fitbit Sense 2 produced repeated implausible readings that required excluding many of its measurements.
- Accuracy worsened as participants’ body fat percentage increased, the team found, while no clear differences were seen across skin tones; researchers point to opaque, proprietary algorithms and sensor limits as likely causes.
- Researchers recommend using wearables for heart-rate data and long-term trends rather than single-session calorie totals and caution against making aggressive diet changes based solely on device estimates; follow-up studies on strength training are already underway.