Table 6.

Key considerations of measuring resting energy expenditure (REE) and total daily energy expenditure (TDEE) in field-based human performance research

REE
Athletes	Ability to adhere to pre-test protocols (e.g., fasting, avoiding vigorous exercise) Schedule to minimize interference with down time, training sessions or competitions Sleep, stress, and altered circadian rhythms (if jet lagged from traveling) may impact measurements
Researcher	Experience (and/or training) in proper use/maintenance of equipment Availability aligns with athlete schedules
Equipment	Ensuring equipment can withstand transport and various environmental conditions Known and acceptable accuracy and precision of measurements
Logistics	Ability to conduct measurements in a controlled environment Transporting equipment Ensuring access to compatible and reliable power sources Allocating sufficient time for set-up, calibrations, and testing
TDEE with DLW
Athletes	Ability of athletes to provide urine samples (e.g., limited by time, training, and potentially, dehydration)
Researcher	Ability to follow DLW dosing and urine collection/storage protocols Need clear communication on who will lead sample collection, storage, shipping, and analyses Need for highly-trained researchers for analyses of isotope concentrations
Equipment	Need for highly technical, costly, and specific analytical techniques (e.g., isotope ratio mass spectrometry) only available at a few institutions worldwide
Logistics	Sample collection procedures (especially on days with multiple urine samples), including when and where samples will be collected and stored Shipping requirements (e.g. procuring shipping materials, country-specific regulations on biological samples)

DLW, doubly labeled water