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Sedentary women (60 kg)

Figure 4 The three classical components of total energy expenditure. (Inactive person).

Sedentary women (60 kg)

Figure 4 The three classical components of total energy expenditure. (Inactive person).

5-10%, the difference being explained by the effect of arousal. BMR is known to be depressed during starvation.

The major part of the whole-body RMR stems from organs with high metabolic activity such as the liver, kidneys, brain, and heart, although these account for a small proportion of the total body weight (5%). Per unit body weight, the kidneys and heart have a metabolic rate more than twice as high as the liver and the brain. In contrast, the metabolic rate of muscle per unit body weight is nearly 35 times lower than that of the heart and kidneys. Since the proportion of muscle to non-muscle changes with age from birth to adulthood, the RMR per unit body weight is not constant with age. The tissue with the lowest metabolic activity per unit body weight is adipose tissue, which accounts for only 4% of the whole-body RMR in nonobese subjects. Calculations show that this value can increase up to 10% or more in obese subjects with a large excess in body fat. Skin and intestines (which have a relatively large protein mass and protein turnover), as well as bones and lungs, also contribute significantly to RMR.

Numerous studies have demonstrated that major factor explaining the variation in RMR between individuals is FFM. FFM is a heterogeneous component that can be partitioned into muscle mass and nonmuscle mass. Unfortunately, there is no simple and accurate way to assess these two subcomponents. Owing to the larger variation between individuals in fat mass, as compared to FFM, and because in grossly obese women fat mass can represent a nonnegligible component of total RMR, the prediction models for RMR that include both FFM and fat mass explain significantly more variance in RMR than FFM alone. In addition, age, sex, and family membership are additional factors that should be taken into account.

The effects of gender on resting metabolic rate are explained by differences in body composition. Caution should be used when comparing resting metabolic rate expressed per kilogram FFM in men and women, because the composition of FFM is influenced by gender. The muscle mass of men is greater than that is greater of women and this tends to give a lower value of RMR per kilogram FFM in men when compared to that of women. This is explained by a greater component of tissue with a low metabolic rate (resting muscle) in men than in women.

In clinical work, where body composition is difficult to assess, body weight, gender, and age can be used to estimate BMR and RMR (Table 2), bearing in mind that many important determinant of RMR, in addition to body size, have been tracked (Table 3).

Table 2 Simple formulae for the prediction of resting metabolic rate in men and women of different ages (equations for predicting basal metabolic rate from body weight alone)

Age range (years)

kcal per day

MJ per day

Males

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