Energy expenditure can be estimated by measuring macronutrient or oxygen consumption, or heat production or carbon dioxide production. Most measurement approaches in use today involve the measurement of oxygen consumption and/or production of carbon dioxide via indirect calorimetry.
Construct Definition (and Source):
A measurement of the amount of energy used to carry out a physiological or physcial function. (NCI)
Mifflin M, St. Jeor S, Hill L, Scott B, Daugherty S, Koh Y. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51:241-247.