Components of Energy Expenditure
Components of 24-Hour Energy Expenditure
The 24-hour daily energy expenditure has several components as listed below:
1. Basal metabolic rate (~60%)
2. Energy used for physical activity (~25%)
Basal Metabolic Rate—The Minimum Energy Expenditure for the Body to Exist
Even when a person is at complete rest, considerable energy is required to perform all the chemical reactions of the body. This minimum level of energy required to exist is called the basal metabolic rate (BMR) and accounts for about 50 to 70% of the daily energy expenditure in most sedentary individuals (Figure 127-1).
Because the level of physical activity is highly variable among different individuals, measurement of the BMR provides a useful means of comparing one person’s metabolic rate with that of another. The usual method for determining BMR is to measure the rate of oxygen utilization over a given period. The conditions for measuring BMR are listed below in Box 127-1
Box 127-1 Conditions To Be Maintained During BMR Measurement
1. The person must not have eaten food for at least 12 hours.
2. The BMR is determined after a night of restful sleep.
3. No strenuous activity is performed for at least 1 hour before the test.
4. All psychic and physical factors that cause excitement must be eliminated.
5. The temperature of the air must be comfortable and between 68° and 80°F.
The BMR normally averages about 65 to 70 Calories per hour in an average 70-kilogram man. Although much of the BMR is accounted for by essential activities of the central nervous system, heart, kidneys, and other organs, the variations in BMR among different individuals are related mainly to differences in the amount of skeletal muscle and body size.
Skeletal muscle, even under resting conditions, accounts for 20 to 30% of the BMR. For this reason, BMR is usually corrected for differences in body size by expressing it as Calories per hour per square meter of body surface area, calculated from height and weight. The average values for males and females of different ages are shown in Figure 127-2.
Much of the decline in BMR with increasing age is probably related to loss of muscle mass and replacement of muscle with adipose tissue, which has a lower rate of metabolism. Likewise, slightly lower BMRs in women, compared with men, are due partly to their lower percentage of muscle mass and higher percentage of adipose tissue.
Other Factors Can Influence the BMR
Thyroid Hormone Increases Metabolic Rate
When the thyroid gland secretes maximal amounts of thyroxine, the metabolic rate sometimes rises 50 to 100% above normal. Conversely, total loss of thyroid secretion decreases the metabolic rate to 40 to 60% of normal. Thyroxine increases the rates of the chemical reactions of many cells in the body and therefore increases metabolic rate.
Male Sex Hormone Increases Metabolic Rate
The male sex hormone testosterone can increase the metabolic rate about 10 to 15%. The female sex hormones may increase the BMR a small amount, but usually not enough to be significant. Much of this effect of the male sex hormone is related to its anabolic effect to increase skeletal muscle mass.
Growth Hormone Increases Metabolic Rate
Growth hormone can increase the metabolic rate by stimulating cellular metabolism and by increasing skeletal muscle mass. In adults with growth hormone deficiency, replacement therapy with recombinant growth hormone increases basal metabolic rate by about 20%.
Fever Increases Metabolic Rate
Fever, regardless of its cause, increases the chemical reactions of the body by an average of about 120% for every 10°C rise in temperature. This is discussed in more detail in Chapter 128.
Malnutrition Decreases Metabolic Rate
Prolonged malnutrition can decrease the metabolic rate 20 to 30%, presumably due to the paucity of food substances in the cells. In the final stages of many disease conditions, the inanition that accompanies the disease causes a marked decrease in metabolic rate, to the extent that the body temperature may fall several degrees shortly before death.
Energy Used for Physical Activities
The factor that most dramatically increases metabolic rate is strenuous exercise. Short bursts of maximal muscle contraction in a single muscle can liberate as much as 100 times its normal resting amount of heat for a few seconds. For the entire body, maximal muscle exercise can increase the overall heat production of the body for a few seconds to about 50 times normal, or to about 20 times normal for more sustained exercise in a well-trained individual.
Table 127-1 shows the energy expenditure during different types of physical activity for a 70-kilogram man. Because of the great variation in the amount of physical activity among individuals, this component of energy expenditure is the most important reason for the differences in caloric intake required to maintain energy balance. However, in industrialized countries where food supplies are plentiful, such as the United States, caloric intake often periodically exceeds energy expenditure, and the excess energy is stored mainly as fat.
Table 127-1
Energy Expenditure During Different Types of Activity for a 70-Kilogram Man∗
| Form of Activity | Calories per Hour |
| Sleeping | 65 |
| Awake lying still | 77 |
| Sitting at rest | 100 |
| Standing relaxed | 105 |
| Dressing and undressing | 118 |
| Typewriting rapidly | 140 |
| Walking slowly (2.6 miles per hour) | 200 |
| Carpentry, metalworking, industrial painting | 240 |
| Sawing wood | 480 |
| Swimming | 500 |
| Running (5.3 miles per hour) | 570 |
| Walking up stairs rapidly | 1100 |
∗Extracted from data compiled by Professor M.S. Rose.
Even in sedentary individuals who perform little or no daily exercise or physical work, significant energy is spent on spontaneous physical activity required to maintain muscle tone and body posture and on other nonexercise activities such as “fidgeting.” Together, these nonexercise activities account for about 7% of a person’s daily energy usage.
Energy Used for Processing Food—Thermogenic Effect of Food
After a meal is ingested, the metabolic rate increases as a result of the different chemical reactions associated with digestion, absorption, and storage of food in the body. This is called the thermogenic effect of food because these processes require energy and generate heat.
After a meal that contains a large quantity of carbohydrates or fats, the metabolic rate usually increases about 4%. However, after a high-protein meal, the metabolic rate usually begins rising within an hour, reaching a maximum of about 30% above normal, and this lasts for 3 to 12 hours. This effect of protein on the metabolic rate is called the specific dynamic action of protein. The thermogenic effect of food accounts for about 8% of the total daily energy expenditure in many persons.
Energy Used for Nonshivering Thermogenesis—Role of Sympathetic Stimulation
Although physical work and the thermogenic effect of food cause liberation of heat, these mechanisms are not aimed primarily at regulation of body temperature. Shivering provides a regulated means of producing heat by increasing muscle activity in response to cold stress, as discussed in Chapter 128. Another mechanism, nonshivering thermogenesis, can also produce heat in response to cold stress. This type of thermogenesis is stimulated by sympathetic nervous system activation, which releases norepinephrine and epinephrine, which in turn increase metabolic activity and heat generation.
In certain types of fat tissue, called brown fat, sympathetic nervous stimulation causes liberation of large amounts of heat. This type of fat contains large numbers of mitochondria and many small globules of fat instead of one large fat globule. In these cells, the process of oxidative phosphorylation in the mitochondria is mainly “uncoupled.” That is, when the cells are stimulated by the sympathetic nerves, the mitochondria produce a large amount of heat but almost no ATP, so almost all the released oxidative energy immediately becomes heat.
A neonate has a considerable number of brown fat cells, and maximal sympathetic stimulation can increase the child’s metabolism more than 100%. The magnitude of this type of thermogenesis in an adult human, who has virtually no brown fat, is probably less than 15%, although this might increase significantly after cold adaptation.
Nonshivering thermogenesis may also serve as a buffer against obesity. Recent studies indicate that sympathetic nervous system activity is increased in obese persons who have a persistent excess caloric intake. The mechanism responsible for sympathetic activation in obese persons is uncertain, but it may be mediated partly through the effects of increased leptin, which activates pro-opiomelanocortin neurons in the hypothalamus. Sympathetic stimulation, by increasing thermogenesis, helps to limit excess weight gain.
Metabolic Rate and Its Measurement
Heat Is the End-Product of Almost All the Energy Released in the Body
In discussing many of the metabolic reactions in the preceding chapters, we noted that not all the energy in foods is transferred to ATP; instead, a large portion of this energy becomes heat. On average, 35% of the energy in foods becomes heat during ATP formation. Then, still more energy becomes heat as it is transferred from ATP to the functional systems of the cells, so even under optimal conditions, no more than 27% of all the energy from food is finally used by the functional systems. Even when 27% of the energy reaches the functional systems of the cells, most of this eventually becomes heat.
Essentially all the energy expended by the body is eventually converted into heat. The only significant exception occurs when the muscles are used to perform some form of work outside the body. For instance, when the muscles elevate an object to a height or propel the body up steps, a type of potential energy is created by raising a mass against gravity. But when external expenditure of energy is not taking place, all the energy released by the metabolic processes eventually becomes body heat.
The Calorie
Most often, the Calorie is the unit used to quantitatively discuss the metabolic rate of the body. It will be recalled that 1 calorie—spelled with a small “c” and often called a gram calorie—is the quantity of heat required to raise the temperature of 1 gram of water 1°C. The calorie is much too small a unit when referring to energy in the body. Consequently, the Calorie—sometimes spelled with a capital “C” and often called a kilocalorie, which is equivalent to 1000 calories—is the unit ordinarily used in discussing energy metabolism. More recently, kilojoules (KJ) or megajoules (MJ) are used to express energy expenditure.
Measurement of the Whole-Body Metabolic Rate
Direct Calorimetry Measures Heat Liberated from the Body
Because a person ordinarily is not performing any external work, the whole-body metabolic rate can be determined by simply measuring the total quantity of heat liberated from the body in a given time.
In determining the metabolic rate by direct calorimetry, one measures the quantity of heat liberated from the body in a large, specially constructed calorimeter. The subject is placed in an air chamber that is so well insulated that no heat can leak through the walls of the chamber. Heat formed by the subject’s body warms the air of the chamber. However, the air temperature within the chamber is maintained at a constant level by forcing the air through pipes in a cool water bath. The rate of heat gain by the water bath, which can be measured with an accurate thermometer, is equal to the rate at which heat is liberated by the subject’s body.
Direct calorimetry is physically difficult to perform and is used only for research purposes.
Indirect Calorimetry—The “Energy Equivalent” of Oxygen
Because more than 95% of the energy expended in the body is derived from reactions of oxygen with the different foods, the whole-body metabolic rate can also be calculated with a high degree of accuracy from the rate of oxygen utilization. When 1 liter of oxygen is metabolized with glucose, 5.01 Calories of energy are released; when metabolized with starches, 5.06 Calories are released; with fat, 4.70 Calories; and with protein, 4.60 Calories.
Using these figures, it is striking how nearly equivalent are the quantities of energy liberated per liter of oxygen, regardless of the type of food being metabolized. For the average diet, the quantity of energy liberated per liter of oxygen used in the body averages about 4.825 Calories. This is called the energy equivalent of oxygen; using this energy equivalent, one can calculate with a high degree of precision the rate of heat liberation in the body from the quantity of oxygen used in a given period of time.
If a person metabolizes only carbohydrates during the period of the metabolic rate determination, the calculated quantity of energy liberated, based on the value for the average energy equivalent of oxygen (4.825 Calories/L), would be about 4% too little. Conversely, if the person obtains most energy from fat, the calculated value would be about 4% too great. Table 127-2 summarizes the determinants of basal metabolic rate.
Table 127-2
Determinants of Basal Metabolic Rate
| Determinants | |
| 1. Hormones | Thyroid hormones increase BMR Testosterone increases BMR Growth hormone increases BMR Catecholamines increase BMR |
| 2. Muscle mass | BMR increases with increase in muscle mass |
| 3. Body size | BMR increases proportional to body size |
| 4. Age | BMR decreases with age due to loss of muscle mass |