Timescale Separation in Whole-Room Calorimetry and Direct Mass Balance Accounting: Decoupling Non-Stationary Dietary Adaptation Kinetics from Steady-State Flux Measurement

Whole-room indirect calorimeters and direct mass balance chambers are constrained by gas-analyzer baseline drift, calibration intervals, and confinement burden to continuous measurement windows of approximately 1 to 7 days. A fundamental methodological confound arises when these short-window instruments are used to evaluate dietary or pharmacological interventions with slow biological adaptation kinetics (ͳ_adapt ≈ 3–8 weeks). When an intervention is initiated upon or immediately prior to chamber entry, the calorimeter captures a non-stationary transitional perturbation – characterized by acute labile glycogen-water pool depletion, elevated gluconeogenic proteolysis, and incomplete enzymatic remodeling – rather than stationary steady-state physiology. This manuscript formally quantifies this timescale mismatch using open-system mass balance equations, demonstrating that unadapted transients can generate a non-adipose mass efflux of 250–430 g/day, swamping the steady-state daily substrate oxidation signal. To resolve this, I propose a criterion-based, tiered run-in protocol. Participants undergo a controlled 3–6 week ward-based or hybrid outpatient stabilization phase, with strict biomarker gatekeeping (urinary nitrogen plateau, RQ-to-FQ convergence, β-hydroxybutyrate stationarity) required to verify physiological readiness before entering the chamber. I also address the ward-to-chamber confinement artifact and outline a multi-system adaptation matrix generalizing this framework to slow-adapting interventions beyond ketogenic diets.