How Vitality Tracker estimates your energy expenditure: method note, version 1.0 (TDEE Engine v2)

Method note describing how Vitality Tracker estimates a person's total daily energy expenditure (TDEE Engine v2, the version in production). The estimate is the precision-weighted combination of two quantities. The first is an energy-balance measurement: the confounder-weighted mean of logged intake minus the energy content of the regression-estimated weight change, over a window bounded by Bayesian online change-point detection, with the energy density of weight change adapted to body composition after Forbes and Hall. The second is a physiological prior: resting metabolic rate from a fat-free-mass equation (Cunningham) where a DEXA-anchored lean mass exists, otherwise from an anthropometric equation (Müller or Mifflin–St Jeor), times a constrained activity multiplier (Pontzer). The output is a credible interval, shown before the point estimate. The note covers the inputs and their defaults when absent, the equilibrium fast path and the logging-consistency diagnostic, the handling of confounded days by mechanism, the observational metabolic-adaptation arm, what the app shows, the sources, the limitations (one subject's data, no reference method, hand-set parameters) and the versioning rule. Wearable calories-burned figures are not used. The estimate is a general-wellness description, not a measurement of metabolic rate.