Resting metabolic rate

Resting metabolic rate (RMR) refers to whole-body mammal (or other vertebrate) metabolism during a time period of strict and steady resting conditions that are defined by a combination of assumptions of physiological homeostasis and biological equilibrium. RMR differs from basal metabolic rate (BMR) because BMR measurements must meet total physiological equilibrium whereas RMR conditions of measurement can be altered and defined by the contextual limitations. Therefore, BMR is measured in the elusive "perfect" steady state, whereas RMR measurement is more accessible and thus, represents most, if not all measurements or estimates of daily energy expenditure.

Indirect calorimetry is the study or clinical use of the relationship between respirometry and bioenergetics, where measurements of the rates of oxygen consumption (VO₂) and the generation of waste products such as carbon dioxide, metabolic water, and less often urea are used to quantify rates of resting energy expenditure. These parameters approximate direct calorimetry measurements of body heat generation to about 98%, and they are the ones most commonly used to represent RMR, expressed as the ratio between i) energy and ii) the time frame of the measurement. For example, following analysis of oxygen consumption of a human subject, if 5.5 kilocalories of energy were estimated during a 5-minute measurement from a rested individual, then the resting metabolic rate equals = 1.1 kcal/min rate. Unlike some related measurements (e.g. METs), RMR itself is not referenced to body mass and has no bearing on the rate of cellular energy metabolism itself.

A comprehensive treatment of confounding factors on BMR measurements is demonstrated as early as 1922 in Massachusetts by Engineering Professor Frank B Sanborn, wherein descriptions of the effects of food, posture, sleep, muscular activity, and emotion provide criteria for separating BMR from RMR.