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Resting Metabolic Rate (RMR). The term metabolism describes the sum total of the thousands of dynamic chemical reactions that occur in the body to sustain life RMR is the sum total of energy needed to keep the body going at rest. It can be likened to an idling engine. The synthesis of new tissue, for example, during growth and pregnancy have significant metabolic cost. Body size is also related—a bigger person has a generally higher metabolic rate. To extend the analogy of the idling engine, this would be a truck that is required to carry a heavier load. To do this might require a bigger engine which would then burn more fuel while idling. Resting metabolic rate (RMR) can be estimated from equations based on height and weight.

There are two basic factors which determine metabolic rate (MR) at any given moment: the number and size of respiring cells, and the metabolic intensity of those cells as determined by the work being done and how efficiently energy transformation occurs to meet the body’s demands. Resting metabolic rate (RMR) is the energy required to sustain life while an individual is at rest, but still awake. Basal metabolic rate (BMR) is often used synonymously with RMR and the energy required to sustain life at the lowest level, which is during sleep, is called the sleeping metabolic rate (SMR). The main determinants of metabolic rate are fat-free mass (FFM), fat mass (FM), age and sex. A ‘determinant’ is a factor which explains differences in metabolic rate between individuals.

RMR accounts for 60-70 per cent of daily energy expenditure. Excess consumption of energy or overfeeding causes an increase in RMR, while underfeeding, such as very-low-calorie diets (VLCDs) and fasting result in a decreased RMR. Most of these changes are due to changes in fat-free mass (FFM) and the thermic effect of food (which is dependent on the calorie load). There may also be some ‘adaptive’ changes which result in a metabolic rate (MR) which is greater (for overfeeding), or less (for underfeeding) than predicted.

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