Total Energy Expenditure and its Components

It is customary to consider energy expenditure as being made up of three components: the energy spent for basal metabolism (or basal metabolic rate), the energy spent on physical activity, and the increase in resting energy expenditure in response to a variety of stimuli (in particular food, cold, stress, and drugs). These three components are depicted in Figure 4.

Basal Metabolic Rate (BMR) or Resting Metabolic Rate (RMR)

This is the largest component of energy expenditure accounting for between half to three-quarters of daily energy expenditure. It is measured under standardized conditions, i.e., in an awake subject lying in the supine position, in a state of physical and mental rest in a comfortable warm environment, and in the morning in the postabsorptive state, usually 10-12 h after the last meal. There is an arbitrary distinction between RMR and BMR in the literature. RMR may be considered equivalent to BMR if the measurements are made in postabsorptive conditions. It seems difficult to partition RMR into various subcomponents since the metabolic rates of individual organs and tissues are hard to assess in humans under noninvasive experimental conditions. BMR can vary up to ±10% between individuals of the same age, gender, body weight, and fat-free mass (FFM), suggesting that genetic factors are also important. Day-to-day intraindividual variability in BMR is low in men (coefficient of variation of 1-3%) but is greater in women because the menstrual cycle affects BMR. In both women and men, sleeping metabolic rate is lower than BMR by

2000

1000

2000

1000

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

0-3

60.9 W - 54

0.255 W - 0.226

3-10

22.7 W + 495

0.0949 W + 2.07

10-18

17.5 W + 651

0.0732 W + 2.72

18-30

15.3 W + 679

0.0640 W + 2.84

30-60

11.6 W + 879

0.0485 W + 3.67

>60

13.5 W + 487

0.0565 W + 2.04

Females

0-3

61.0 W - 51

0.255W - 0.214

3-10

22.5 W + 499

0.0941 W + 2.09

10-18

12.2 W + 746

0.0510W + 3.12

18-30

14.7 W + 496

0.0615W + 2.08

30-60

8.7 W + 829

0.0364 W + 3.47

>60

10.5 W + 596

0.0439 W + 2.49

W, body weight expressed in kilograms; MJ, megajoules. (Data from WHO (1986) Energy and Protein Requirements. Report of a Joint FAO/WHO/UNU Expert Consultation. Technical Report Series 724. Geneva: World Health Organization.)

W, body weight expressed in kilograms; MJ, megajoules. (Data from WHO (1986) Energy and Protein Requirements. Report of a Joint FAO/WHO/UNU Expert Consultation. Technical Report Series 724. Geneva: World Health Organization.)

Thermic Effect of Food or Postprandial Thermogenesis

The energy expenditure increases significantly after a meal. The thermic effect of food is mainly due to the energy cost of nutrient absorption and storage. The total thermic effect of food over 24 h represents -10% of the total energy expenditure in sedentary subjects. The thermic effect of nutrients mainly depends on the energy costs of processing and/or storing the nutrient. Expressed in per cent of the energy content of the nutrient, values of 8%, 2%, 20-30%, and 22% have been reported for glucose, fat, protein, and ethanol, respectively.

Glucose-induced thermogenesis mainly results from the cost of glycogen synthesis and substrate cycling. Glucose storage as glycogen requires 2 mol ATP/mol. In comparison with the 38 mol ATP produced on complete oxidation of glucose, the energy cost of glucose storage as glycogen corresponds to

Table 3 Determinants of resting (basal) metabolic rate

• Body composition (lean vs. obese)

• Physiological status (growth, pregnancy, and lactation)

• Hormonal status (e.g., Follicular ve luteal phase)

- Temperature (body internal and environment)

- Pharmacological agents (e.g., nicotine and caffeine)

5% (or 2/38) of the energy content of glucose stored. Cycling of glucose to glucose-6-phosphate and back to glucose, to fructose-1,6-diphosphate and back to glucose-6-phosphate, or to lactate and back to glucose, is occurring at variable rates and is an energy-requiring process that may increase the thermic effect of carbohydrates.

The thermic effect of dietary fat is very small; an increase of 2% of its energy content has been described during infusion of an emulsion of triglyceride. This slight increase in energy expenditure is explained by the ATP consumption in the process of free fatty acid reesterification to triglyceride. As a consequence, the dietary energy of fat is used very efficiently.

The thermic effect of proteins is the highest of all nutrients (20-30% of the energy content of proteins). Ingested proteins are degraded in the gut into amino acids. After absorption, amino acids are deaminated, their amino group transferred to urea, and their carbon skeleton converted to glucose. These biochemical processes require the consumption of energy amounting to ^25% of the energy content of amino acids. The second pathway of amino acid metabolism is protein synthesis. The energy expended for the synthesis of the peptide bonds also represents ^25% of the energy content of amino acids. Therefore, irrespective of their metabolic pathway, the thermogenesis induced after absorption of amino acids represents ^25% of their energy content.

Energy Expenditure Due to Physical Activity

The energy spent on physical activity depends on the type and intensity of the physical activity and on the time spent in different activities. Physical activity is often considered to be synonymous with 'muscular work', which has a strict definition in physics (force x distance) when external work is performed in the environment. During muscular work (muscle contraction), the muscle produces 3-4 times more heat than mechanical energy, so that useful work costs more than muscle work. There is a wide variation in the energy cost of any activity both within and between individuals. The latter variation is due to differences in body size and in the speed and dexterity with which an activity is performed. In order to adjust for differences in body size, the energy cost of physical activities are expressed as multiples of BMR. These generally range from 1 to 5 for most activities, but can reach values between 10 and 14 during intense exercise. In terms of daily energy expenditure, physical activity accounts for 15-40% of total energy expenditure but it can represent up to 70% of daily energy expenditure in an individual involved in heavy manual work or

Table 4 Exogenous and endogenous factors influencing the three components of energy expenditure

Components

Endogenous

Exogenous

• Basal

Fat-free mass

metabolic rate

Thyroid

hormones

Protein turnover

• Thermogenesis

Nutritional status

• Macronutrient

Sympathetic

intake (+alcohol)

nervous system

• Cold exposure

activity

• Stress

Insulin

• Thermogenic

resistance

stimuli (coffee,

(obesity)

tobacco)

• Thermogenic

drugs

• Physical

'Fidgeting'

• Duration intensity,

activity

Muscular mass

and frequency of

Work efficiency

physical activity

Fitness level

(V O2max)

competition athletics. For most people in industrialized societies, however, the contribution of physical activity to daily energy expenditure is relatively small. The numerous factors influencing the 3 components of energy expenditure are outlined in Table 4.

The effect of body weight in average women (~60 kg) on energy expenditure is illustrated in Figure 5. The relationship is slightly curvilinear because of differences in body composition in terms of leanness and fatness. Resting metabolic rate is shown as a baseline value.

Just as described above for a specific activity, it has been customary to express total energy expenditure

3500

7 3000 f

2 2000

1000

7 3000 f

2 2000

1000

Physical activity

>RMR

50 60 70 80 90 100 110 120 130 Body weight (kg)

Figure 5 Effect of body weight on total energy expenditure at two levels of physical activity in young women. A physical activity level (PAL) of 1.2 represents minimal physical activity compatible with health, whereas a value of 1.6 represents a 'medium' level of physical activity.

Physical activity

>RMR

50 60 70 80 90 100 110 120 130 Body weight (kg)

Figure 5 Effect of body weight on total energy expenditure at two levels of physical activity in young women. A physical activity level (PAL) of 1.2 represents minimal physical activity compatible with health, whereas a value of 1.6 represents a 'medium' level of physical activity.

(TEE) relative to RMR (TEE/RMR or TEE/BMR) to offset the large variation in RMR among subjects of difference body weight & body composition. This quotient is called physical activity level (PAL) and reflects multiples of RMR. A PAL of 1.5 indicates that TEE is 50% greater than RMR over 24 h.

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