The resting metabolic rate (RMR) is the energy expended by the body at rest with the individual sitting or lying. In other words it is the energy required for normal metabolic and homeostatic regulation. The basal metabolic rate is similar to the resting metabolic rate but is usually taken first thing in the morning after a fast. In addition, the total energy expenditure also comprises of the thermic effect of food (TEF), the thermic effect of activity (TEA), adaptive thermogenesis (AT) and the energy expended during exercise (here). These components in combination with the resting metabolic rate gives the total 24-hour expenditure, a measure of all the energy expended by the body. It is often assumed that the overweight have a lower resting metabolic rate and 24-hour energy expenditure than lean individuals. In fact some studies have reported lower resting and basal metabolic rates in obese individuals compared to lean counterparts.
However, it is worth looking in more detail at the association between resting metabolic rate and body weight because it can help with understanding of effective weight loss strategies. For example, in one study1 the resting metabolic rate and 24-hour energy expenditure of subjects was measured in a respiratory chamber. In addition, the body composition of the subjects was also measured. The mean body weights for the individuals were 61.2, 78.8 and 97.9 kg in lean, moderately overweight or obese subjects, respectively. The results of the study showed that in absolute terms as body weight increase, resting metabolic rate and total 24-hour energy expenditure also increased. The total energy expenditures were 2400, 2294 and 2017 kcal in obese, moderately obese and lean subjects, respectively. The resting metabolic rates were 1815, 1590 and 1462 kcal per day in obese, moderately obese and lean subjects respectively.
The larger total energy expenditure in the obese subjects was due mainly to an increased energy usage during the day (as measured by VO2). Most of the increase energy utilisation during the day in the obese subject was accounted for by a higher resting metabolic rate, while only 8 % was accounted for by the extra cost of moving a larger body mass. Therefore these results support other data showing that the obese tend to have a greater total energy expenditure than lean counterparts. When we look at the data for the amount of lean mass in the subjects the reason for the discrepancy in the resting metabolic rates of the subjects becomes apparent. The lean mass of the subjects was 29 kg, between 20 and 26 kg and 16 kg in the obese, moderately obese and lean subjects, respectively. Therefore the higher resting metabolic rates of the obese are explainable by their higher degrees of lean tissue that would include a greater amount of skeletal muscle, a highly metabolically active tissue.
Dr Robert Barrington’s Nutritional Recommendation: The obese have greater amounts of lean mass, presumably as a result of higher nitrogen intakes in combination with a heavier body weights. Forced calorie restrictive diets cause negative nitrogen balances as well and metabolic changes in the body that catabolise muscle and reduce the amount of skeletal muscle. Much of the weight loss on such calorie counting diets is often in the form of lean mass. This reduces resting metabolic rate and increases the risk of future weight gain in the form of body fat. Because the reductions in resting metabolic rate seen in calorie counting forced calorie restrictive diet are seen to be semi-permanent, such diets are not recommended. Instead the resting metabolic rate should be maintained during fat loss by consuming a high protein, high quality diet in combination with resistance training.
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