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Energy Requirements

The amount of energy required to support metabolic activities for twenty-four hours varies from person to person. The factors that influence energy needs include the individual's basal metabolic rate, degree of muscular activity, body temperature, and rate of growth.

The basal metabolic rate (ba'sal met"ah-bol'ik rat) or BMR measures the rate at which the body expends energy under basal conditions—when a person is awake and at rest, after an overnight fast, and in a comfortable, controlled environment. Tests of thyroid function can be used to estimate a person's BMR.

The amount of oxygen the body consumes is directly proportional to the amount of energy cellular respiration releases. The BMR, therefore, reveals the total amount of energy expended in a given time period to support the activities of such organs as the brain, heart, lungs, liver, and kidneys.

The average adult BMR indicates a requirement for approximately one calorie of energy per hour for each kilogram of body weight. However, this requirement varies with such factors as sex, body size, body temperature, and level of endocrine gland activity. For example, since heat loss is directly proportional to the body surface area, and a smaller person has a greater surface area, he or she will have a higher BMR. Males tend to have higher metabolic rates than females. As body temperature increases, BMR increases, and as the blood level of thyroxine or epinephrine increases, so does the BMR. The BMR can also increase when the level of physical activity increases during the day.

Maintaining the basal metabolic rate usually requires the body's greatest expenditure of energy. The energy required to support voluntary muscular activity comes next, though this amount varies greatly with the type of activity (table 18.4). For example, the energy to maintain posture while sitting at a desk might require 100 calories per hour above the basal need, whereas running or swimming might require 500-600 calories per hour.

Maintenance of body temperature may require additional energy expenditure, particularly in cold weather.

In this case, extra energy may be expended by involuntary muscular contractions, such as shivering, or through voluntary muscular actions, such as walking. Growing children and pregnant women, because their bodies are actively producing new tissues, also require more calories.

Energy Balance

A state of energy balance exists when caloric intake in the form of foods equals caloric output from the basal metabolic rate and muscular activities. Under these conditions, body weight remains constant, except perhaps for slight variations due to changes in water content.

If, on the other hand, caloric intake exceeds the output, a positive energy balance occurs, and tissues store excess nutrients. This increases body weight, since an excess of 3,500 calories can be stored as a pound of fat. Conversely, if caloric output exceeds input, the energy balance is negative, and stored materials are mobilized from the tissues for oxidation, causing weight loss.

H What is basal metabolic rate? Q What factors influence the BMR?

What is energy balance?

Desirable Weight

The most obvious and common nutritional disorders reflect calorie imbalances, which may result from societal and geographic factors. Obesity is a common problem in nations where food is plentiful and diverse. The tendency to become obese may be a holdover from thousands of years ago, when the ability to store energy in the form of fat was a survival advantage when food supplies were scarce or erratic. Today in many African nations, natural famines combined with political unrest cause mass starvation. Starvation is considered later in the chapter.

It is difficult to determine a desirable body weight. In the past, weight standards were based on average weights and heights within a certain population, and the degrees of underweight and overweight were expressed as percentage deviations from these averages. These standards reflected the gradual gain in weight that usually occurs with age. Later, medical researchers recognized that such an increase in weight after the age of twenty-five to thirty years is not necessary and may not be conducive to health. Consequently, standards of desirable weights were prepared. More recent height-weight guidelines are based upon the characteristics of people who live the longest. These weights are somewhat more lenient than those in the desirable weight charts.

Overweight is defined as exceeding desirable weight by 10% to 20%. A person who is more than 20% above the desired weight is considered to be obese, although obesity is more correctly defined as excess adipose tissue. Therefore, overweight and obesity are not the same.

Figure 18.8

(a) An obese person is overweight and has excess adipose tissue.

(b) An athlete may be overweight due to muscle overgrowth but is not considered obese. In fact, many athletes have very low percentages of body fat.

Figure 18.8

(a) An obese person is overweight and has excess adipose tissue.

(b) An athlete may be overweight due to muscle overgrowth but is not considered obese. In fact, many athletes have very low percentages of body fat.

For example, as figure 18.8 shows, an athlete or a person whose work involves heavy muscular activity may be overweight, but not obese. Clinical Application 18.1 discusses obesity.

When a person needs to gain weight, diet can be altered to include more calories and to emphasize particular macronutrients. For example, a person recovering from a debilitating illness might consume more carbohydrates, whereas a bodybuilder might eat extra protein to hasten muscle development. An infant also needs to gain weight rapidly, which is best accomplished by drinking human milk. The high fat content of human milk is important for the rapid growth of the infant's brain.

Shier-Butler-Lewis: Human Anatomy and Physiology, Ninth Edition

V. Absorption and Excretion

18. Nutrition and Metabolism

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