Basal metabolic rate BMR

Basal metabolic rate is the energy expenditure by the body when at rest, but not asleep, under controlled conditions of thermal neutrality, and about 12 hours after the last meal. It is the energy requirement for the maintenance of metabolic integrity, nerve and muscle tone, circulation and respiration (see Figure 1.2 for the contribution

Table 5.2 Definitions in energy metabolism


Basal metabolic rate

Energy expenditure in the post-absorptive state; measured under standardized conditions of thermal neutrality (environmental temperature 26-30 °C) , awake but completely at rest


Resting metabolic rate

Energy expenditure at rest, not measured under strictly standardized conditions


Physical activity ratio

Energy cost of physical activity, on a minute by minute basis, expressed as ratio of BMR


Integrated energy index

Energy cost of an activity over a period of time, including time spent pausing or resting, expressed as the average (integrated) value over the time, as a ratio of BMR


Physical activity level

Sum of PAR or IEI x time spent in each activity over 24 hours, expressed as ratio of BMR


Diet-induced thermogenesis

Increased energy expenditure after a meal


Total energy expenditure


of different organs to BMR). It is important that the subject is awake, as some people show an increased metabolic rate (and hence increased heat output), while others have a reduced metabolic rate and a slight fall in body temperature, when asleep. Where the measurement of metabolic rate has been made under less strictly controlled conditions, the result is more correctly called the resting metabolic rate.

Figure 5.2 shows the variation of BMR with body weight, age and gender:

  • Body weight affects BMR because there is a greater amount of metabolically active tissue in a larger body.
  • The decrease in BMR with increasing age is due to changes in body composition. With increasing age, even when body weight remains constant, there is loss of muscle tissue and replacement by adipose tissue, which is metabolically very much less active, as 80% of the weight of adipose tissue consists of reserves of triacylglycerol.
  • Similarly, the gender difference (women have a significantly lower BMR than do men of the same body weight) is accounted for by differences in body composition. As shown in Figure 5.3, the proportion of body weight that is adipose tissue reserves in lean women is considerably higher than in men. (See also section 6.1.2 for a discussion of methods of estimating the proportions of fat and lean tissue in the body.)
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