As discussed in Chapter 71, energy intake is balanced with energy output in healthy adults who maintain a stable body weight. About 45 per cent of daily energy intake is derived from carbohydrates, 40 per cent from fats, and 15 per cent from proteins in the average American diet. Energy output can also be partitioned into several measurable components, including energy used for (1) performing essential metabolic functions of the body (the "basal" metabolic rate); (2) performing various physical activities; (3) digesting, absorbing, and processing food; and (4) maintaining body temperature.
An average man who weighs 70 kilograms and lies in bed all day uses about 1650 Calories of energy. The process of eating and digesting food increases the amount of energy used each day by an additional 200 or more Calories, so that the same man lying in bed and eating a reasonable diet requires a dietary intake of about 1850 Calories per day. If he sits in a chair all day without exercising, his total energy requirement reaches 2000 to 2250 Calories. Therefore, the approximate daily energy requirement for a very sedentary man performing only essential functions is 2000 Calories.
The amount of energy used to perform daily physical activities is normally about 25 per cent of the total energy expenditure, but it can vary markedly in different individuals, depending on the type and amount of physical activity. For example, walking up stairs requires about 17 times as much energy as lying in bed asleep. In general, over a 24-hour period, a person performing heavy labor can achieve a maximal rate of energy uti lization as great as 6000 to 7000 Calories, or as much as 3.5 times the energy used under conditions of no physical activity.
Basal Metabolic Rate (BMR)— 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 per cent of the daily energy expenditure in most sedentary individuals (Figure 72-3).
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 of time under the following conditions:
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.
6. No physical activity is permitted during the test. 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 per cent 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 72-4.
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. However, there are other factors that can influence the BMR, as discussed next.
Thyroid Hormone Increases Metabolic Rate. When the thyroid gland secretes maximal amounts of thyroxine, the metabolic rate sometimes rises 50 to 100 per cent above normal. Conversely, total loss of thyroid secretion decreases the metabolic rate to 40 to 60 per cent of normal. As discussed in Chapter 76, thyroxine increases the rates of the chemical reactions of many cells in the body and therefore increases metabolic rate. Adaptation of the thyroid gland—with increased secretion in cold climates and decreased secretion in hot climates— contributes to the differences in BMRs among people living in different geographical zones; for example, people living in arctic regions have BMRs 10 to 20 per cent higher than those of persons living in tropical regions.
Male Sex Hormone Increases Metabolic Rate. The male sex hormone testosterone can increase the metabolic rate about 10 to 15 per cent. 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 15 to 20 per cent as a result of direct stimulation of cellular metabolism.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.