The resting metabolic rate (RMR) is defined as the amount of energy that is required to maintain normal metabolic activity in the absence of physical activity. The basal metabolic rate (BMR) is similar to the RMR, but differs in that the former is a measure only of the energy needs of life supportive metabolism, whereas the latter includes energy required to digest food. Basal metabolic rate is therefore often measured following a sleep while fasting, while the RMR can be measured during the day following food consumption, but at rest. However, the terms are often used interchangeably. Measuring BMR and RMR is complex and requires specialist equipment. Therefore mathematical estimates of metabolic rate have been devised. A number of factors have been assessed in order to allow predictions of the BMR of an individual, including body size and body surface area, sex and age. However, while many of these factors have and are used in traditional equations of prediction, lean body mass remains the single best predictor of the BMR.
For example, multiple regression analysis has been used to investigate the strength of the predictability of a number of factors for accurate determination of BMR1. The data used was taken from the seminal work on BMR by Harris and Benedict in 1919, and included BMR and anthropometric data from 223 subjects. In addition, 16 healthy males were included in the study because they were trained athletes. Regression analysis is useful in such cases because it allows the testing of variables against one another in order to determine those that have the most significant effects. Following such analysis it was concluded that the lean body mass of the individual was the single greatest predictor of BMR, and accounted for 70 % of the variability. It could be further speculated that skeletal muscle mass is actually the single greatest predictor of BMR because skeletal muscle is highly metabolically active, and subject to constant turnover, which is an inefficient process that requires large amounts of energy.
The observation that younger male subjects have a higher metabolic rate than older or female subjects therefore may simply be a reflection that such younger males have a higher amount of skeletal muscle that increases their lean mass. These results have important implications for fat loss because it is now being recognised that low calorie diets cause a significant loss of lean body mass in the form of skeletal muscle, and that this loss of skeletal muscle may be permanent. Those that follow low energy diets may therefore do permanent damage to their RMR and this may increase the likelihood of future weight gain. In contrast, those with a high RMR are protected from future fat gain because their body is less efficient due to the relatively larger mass of skeletal muscle that must be maintained at great energy cost. Such individuals also have larger capacity to store glycogen, because of a larger pool of skeletal muscle, and this may decrease the availability of glucose for conversion to fat for storage in adipose tissue.
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