The higher your body fat percentage, the less accurate your smartwatch is at measuring calories you burn when exercising – new study

Smartwatches are wearable devices used by nearly 100 million adults in the U.S. (about 4 in 10) and approximately 560 million people worldwide to monitor health and fitness. These devices typically include sensors like accelerometers, which track movement, and light-based sensors to estimate heart rate. They combine this data with user information such as age, sex, height, and weight, then use an algorithm to estimate calories burned during exercise. While these features are widely used, research shows that the calorie estimates provided by smartwatches are often inaccurate. A study by Florida International University’s Medical Photonics Laboratory found that the accuracy of these estimates varies significantly depending on the wearer’s body fat percentage.

Smartwatches estimate calories burned by combining data from movement sensors (accelerometers) and heart rate sensors. The heart rate sensor works by shining light onto the wrist and measuring how much is absorbed by blood, as blood absorbs light differently with each heartbeat. The device then uses this data, along with user details like age and weight, to calculate energy expenditure through a proprietary algorithm. However, calorie measurement is complex because it relies on indirect estimates rather than direct measurement. Factors like body composition, fitness level, movement efficiency, and physiological responses to exercise can all affect the accuracy of these estimates.

Researchers at Florida International University conducted a study with 58 Hispanic adults in Miami to test the accuracy of four popular smartwatches: Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch5, and Fitbit Sense 2. Participants cycled at moderate and vigorous intensities for 10 minutes while wearing two watches on each wrist. The researchers measured their actual calorie expenditure using a metabolic analyzer, a device considered the gold standard for this purpose. The study controlled exercise intensity and reduced wrist movement to minimize interference with heart rate measurements. The goal was to compare the smartwatches’ calorie estimates against the metabolic analyzer’s readings.

The study found that all four smartwatches had significant errors in measuring calories burned, with a median error of about 15% to 25%. Apple, Garmin, and Samsung systematically overestimated calorie expenditure, with Apple having the smallest average error and Garmin and Samsung showing larger overestimates. Fitbit’s performance was inconsistent, with some implausible readings exceeding 450% of the metabolic analyzer’s measurements. The most surprising finding was that the error increased as the wearer’s body fat percentage rose. For participants with higher body fat percentages, errors ranged from about 50% to over 100%, meaning a 100-calorie expenditure could be estimated as 150 to 200 calories. Skin tone did not significantly affect the accuracy of the measurements in this study.

The study highlights that people with higher body fat percentages, who may be most interested in using smartwatches for weight loss, receive the least accurate calorie estimates. For example, someone with a body fat percentage of 35% or higher exercising 4 hours per week could see their smartwatch overestimate calories burned by 1,200 to 2,400 calories per week. This could lead to consuming nearly an entire day’s worth of calories in error, thinking they had burned them through exercise. The researchers caution against relying solely on smartwatch calorie estimates for weight management, as these numbers are approximations rather than exact measurements.

While smartwatches have limitations in measuring calories burned, they can still provide useful information such as heart rate, workout duration, distance, and pace. The researchers recommend treating the calorie number as an estimate rather than an exact figure. For weight loss goals, users should be conservative when using these numbers to determine food intake or evaluate workout success. Tracking patterns over time, combined with other metrics like heart rate and body weight changes, may be more reliable than relying on a single calorie estimate. The study also notes that smartwatch technology is improving, with ongoing research focused on enhancing accuracy across different body compositions.