Smartwatches show substantial error at tracking calories burned especially for certain kinds of people

Sleek smartwatches are all the rage these days.Nearly 4 in 10 US adults wear them to track their workouts.

For every step and heartbeat measured, fitness trackers are becoming more than tiny wrist computers, blending style and stats.However, one of the most commonly tracked metrics — calories burned — may not be completely accurate, especially for one group of people.A study from Florida International University’s Medical Photonics Laboratory tracked 58 men and women of various ages and body weights who simultaneously wore smartwatches from Apple, Garmin, Samsung and Fitbit.Participants alternated between two minutes of fast pedaling and five minutes of rest on a recumbent bicycle for 10 minutes, while researchers monitored their energy expenditure with a metabolic analyzer face mask.Researchers then compared how many calories recorded by the watches with measurements from the masks, which are considered the most reliable way to measure calorie expenditure.All four watches recorded substantial errors in calorie tracking, ranging from 15% to 25% compared with the lab measurements.The smartwatch estimates were least accurate in participants with body fat percentage above 35%.Three of the brands overestimated how many calories the wearers burned, with Apple having the smallest average error, while Garmin and Samsung produced larger overestimates.While Fitbit’s measurement errors weren’t consistently positive or negative compared with the lab readings, a different problem arose.The wearable produced several implausible estimates, including multiple measurements that exceeded 450% of the metabolic system’s readings.Despite the errors across all four smartwatches, the researchers weren’t surprised by the findings, as previous studies have found similar inaccuracies.What stood out most was the strong relationship between calorie-tracking error and body fat percentage.As the body fat percentage of each wearer increased, the watches’ measurements of calorie...