Treadmills and watches exaggerate burns
A viral thread flagged a Stanford study finding that treadmills and smartwatches can overestimate calorie burn by up to 93% because they don’t account for individual fitness and efficiency—so the advice is to track food intake rather than trust device calorie estimates. (x.com) That’s a useful reality check if you’re relying on wearable numbers for weight loss or precise calorie targets. (x.com)
A calorie number on a treadmill screen looks precise because it ends in digits, but the machine usually knows only your speed, incline, time, and a body weight you typed in once. It does not know whether you move like a beginner who wastes motion or a trained runner who does the same work with less energy. (med.stanford.edu) That missing piece is called energy expenditure, which is just the fuel your body burns to do a task. Scientists measure it with lab tools like oxygen testing, because oxygen use is a direct clue to how much energy your muscles are actually spending. (jamanetwork.com) Wrist wearables have the same problem. A watch can estimate pulse from light under the skin, but calories are a secondhand guess built from formulas, motion sensors, and population averages. (med.stanford.edu) Stanford researchers tested 7 popular wrist devices on 60 volunteers during treadmill walking, treadmill running, and stationary cycling. Six of the 7 measured heart rate within 5 percent, but none measured calorie burn accurately. (med.stanford.edu) Even the best device in the Stanford study missed calorie burn by an average of 27 percent. The worst missed by 93 percent, which means a screen showing 600 calories could be closer to 300 than most people think. (med.stanford.edu) The Stanford team also found that factors like skin color and body mass index changed measurement quality. That matters because a formula built from averages can drift fast when it meets a real person with different physiology. (med.stanford.edu) Another study in JAMA Internal Medicine compared 12 wearables with gold-standard lab methods, including a metabolic chamber and doubly labeled water, which tracks energy use in daily life with special isotopes. The researchers said there was still little evidence that consumer devices could validly estimate total energy expenditure. (jamanetwork.com) This is why “I earned this meal” can go sideways. If your watch overstates a workout by 200 or 300 calories and you eat to match the screen, the math for fat loss can disappear without you noticing. (med.stanford.edu) The more reliable use for these devices is trend tracking, not exact accounting. Step counts and workout time can show whether you moved more this week than last week, even if the calorie total is fuzzy. (jamanetwork.com) Stanford researchers looking at weight-loss tools found that people who tracked diet or activity more consistently with digital tools tended to lose more weight. The useful habit was regular self-monitoring, not blind trust in a single calorie number from a machine. (med.stanford.edu) So the practical move is simple: use the treadmill or watch for pace, distance, heart rate, and consistency, and treat calorie burn like a rough weather forecast. If body weight is the goal, food logs and scale trends usually tell you more truth than a glowing number on your wrist. (med.stanford.edu)
