92:: Is your fitness tracker lying to you? (How fitness trackers estimate calories & why they’re often wrong!)

In this investigative “Wellness Girl chat,” Dr. Kelsy Vick breaks down how fitness trackers estimate caloric expenditure—and why those numbers can be misleading for assessing workout effort, effectiveness, or weight loss. She defines calories (including the difference between food-label “calories” and kilocalories), explains total daily energy expenditure, and outlines what wearables typically model: basal metabolic rate (BMR) plus active energy expenditure, while often failing to capture thermic effect of food and non-exercise activity thermogenesis (NEAT). She reviews how BMR is estimated from static equations using age, sex, height, and weight (with limitations and common error ranges), then explains how active calories rely on proprietary algorithms driven largely by heart rate, supported by sensors like accelerometers, gyroscopes, barometers, GPS, and (for Oura) skin temperature—often switching sensor priorities based on the selected activity.The episode also connects heart rate to oxygen consumption and energy expenditure, explains VO2 and the lab-based VO2 max testing process, and shows how wearables use heart-rate-based VO2 relationships to estimate calories. Dr. Vic compares common devices: Apple Watch (Move vs Total Calories, activity classification, VO2 feature accuracy cited within 4% when using VO2 integration), Whoop (step-free strain model, continuous heart rate monitoring, algorithm shifts based on heart rate reserve), Oura Ring (finger-based sensing, emphasis on resting metrics, temperature and sleep inputs for BMR, MET-based intensity categories with user-reported effort), Garmin (GPS- and pace-informed VO2 estimates, cycling power-based calculations, with studies showing wide accuracy ranges and underestimation at low and very high intensities due to heart-rate lag and anaerobic work), and Fitbit (older models leaning more on accelerometer data, newer models incorporating heart rate).Dr. Vick summarizes research findings showing large real-world error in wearable calorie estimates, including a Stanford Benchmark study (most accurate device averaging 27% off; least accurate 93% off, with factors like skin color and BMI affecting results), a 2025 head-to-head comparison reporting varying under- and over-estimation across devices, and a systematic review where no brand consistently met acceptable accuracy limits. She notes that resting heart rate and step count are generally more accurate than calorie burn estimates, while sleep duration is moderately accurate and sleep staging is weak. The takeaway: calorie metrics may be useful for broad trends, but shouldn’t drive dietary or training decisions; she recommends emphasizing other wearable metrics such as heart rate, VO2/VO2 max, heart rate variability, and recovery instead.Resources:https://www.empirical.health/blog/apple-watch-calories-accuracy/https://www.apple.com/health/pdf/Heart_Rate_Calorimetry_Activity_on_Apple_Watch_November_2024.pdfhttps://pmc.ncbi.nlm.nih.gov/articles/PMC5738849/https://pmc.ncbi.nlm.nih.gov/articles/PMC9950693/https://blogs.sas.com/content/efs/2025/06/25/can-you-trust-your-smartwatch-a-deep-dive-into-calorie-burn-estimates/loving the pod? click the follow button, & we'd love if you could leave a review! thank you x 1000 :)sign up for the free Smart Girl Newspaper!insta:: @dr.kelsyvickdpt & @wellnessbigsispodyoutube:: @dr.kelsyvickdpttiktok:: @dr.kelsyvickdpt & @wellnessbigsispodsign up for our patreon for exclusive content & workouts!00:00 Calories Culture Backstory02:49 What Trackers Claim04:05 What Is A Calorie04:51 TDEE Basics BMR Active06:41 BMR Formula Limits08:28 Sensors And Algorithms11:55 Heart Rate And VO212:55 VO2 Lab Gold Standard15:35 Apple Watch Breakdown17:14 Whoop Strain Model18:57 Oura Ring And METs22:14 Garmin VO2 And GPS26:03 Fitbit And Accuracy Studies29:01 How To Use Calories31:08 Wrap Up And Takeaways