- Read your watch's calories-burned figure as a trend on one device, if at all.
- Never eat to it.
- For anything that involves food, use VT's TDEE estimate, which is built from your weight and your log, and read its range before its number.
- If you are planning a deficit, let the Weight Loss Plan set it from the engine.
Why
A wrist device does not measure energy; it estimates it from heart rate, movement and your profile through a model its maker does not publish. In the laboratory test that VT's method note rests on, seven wrist devices were worn by volunteers through sitting, walking, running and cycling while their expenditure was measured by indirect calorimetry: heart rate came within about 5 percent, but the energy-expenditure error ran from 27 to 93 percent depending on the device, and none came within 20 percent (Shcherbina 2017). A plan that sets a deficit of a few hundred kilocalories a day cannot survive an input with an error of several hundred.
The failure it causes
"My watch says 912, so one more muffin" is the most reliable way to undo a plan, because the 912 is the least reliable number on the screen. The same logic runs the other way: a device that under-reads leads people to under-eat after training and to wonder why they are flat.
What VT does instead
The TDEE engine takes the energy-balance route, your weight against your log, and anchors it with a physiological estimate from your lean mass and training load; it excludes device calories entirely and says how sure it is (see The TDEE engine). Your training still counts: the session-RPE load you log moves the engine's activity term through a curve with a ceiling, following the finding that total expenditure plateaus as activity rises rather than adding up forever (Pontzer 2016).
What to do with the device figure
Keep it. As a trend on one device it says something about how much you moved this month against last, and the activity sheet shows it beside the fuel split, labeled as the device's. It is a fine way to notice a sedentary week and a poor way to decide what to eat.
Sources
- Shcherbina A, Mattsson CM, Waggott D, et al. Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. J Pers Med 2017;7:3. doi:10.3390/jpm7020003
- Pontzer H, et al. Curr Biol 2016;26:410–7. doi:10.1016/j.cub.2015.12.046
- Vitality Tracker, How Vitality Tracker estimates your energy expenditure, method note v1.0. doi:10.5281/zenodo.23224326.