Chapter: Nutrition and Diet Therapy: Digestion, Absorption, and Metabolism


Energy is constantly needed for the maintenance of body tissue and temperature and for growth (involuntary activity), as well as for voluntary activity.



Energy is constantly needed for the maintenance of body tissue and tempera-ture and for growth (involuntary activity), as well as for voluntary activity. Examples of voluntary activity include walking, running, swimming, garden-ing, and so on. The three groups of nutrients that provide energy to the body are carbohydrates, proteins, and fats. Carbohydrates are and should be the primary energy source.


Energy Measurement


The unit used to measure the energy value of foods is the kilocalorie, or kcal, commonly known as the large calorie, or calorie. In the met-ric system it is known as the kilojoule. One kilocalorie is equal to 4.184 kilojoules, but this may be rounded off to 4.2 kilojoules. A calorie is the amount of heat needed to raise the temperature of 1 kilogram of water 1 degree Celsius (C).


The number of calories in a food is its energy value, or caloric density. Energy values of foods vary a great deal because they are determined by the types and amounts of nutrients each food contains.

One gram of carbohydrate yields 4 calories; 1 gram of protein yields 4 calories; and 1 gram of fat yields 9 calories. One gram of alcohol yields 7 calories.


The energy values of foods are determined by a device known as a bombcalorimeter. The inner part of a calorimeter holds a measured amount offood, and the outer part holds water. The food is burned, and its caloric value is determined by the increase in the temperature of the surrounding water. The number of calories in average servings of common foods is listed in Table A-D of the appendix.

Basal Metabolic Rate


One’s basal metabolism is the energy necessary to carry on all involuntary vital processes while the body is at rest. These processes are respiration, circula-tion, regulation of body temperature, and cell activity and maintenance. The rate at which energy is needed only for body maintenance is called the basalmetabolism rate (BMR). The BMR may be referred to as the resting energy expenditure (REE).


Medical tests can determine one’s BMR (or REE). When such a test is given, the body is at rest and performing only the essential, involuntary func-tions. Voluntary activity is not measured in a BMR test. Factors that affect one’s BMR are lean body mass, body size, sex, age, heredity, physical condition, and climate.

Lean body mass is muscle as opposed to fat tissue. Because there is moremetabolic activity in muscle tissue than in fat or bone tissue, muscle tissue re-quires more calories than does fat or bone tissue. People with large body frames require more calories than do people with small frames because the former have more body mass to maintain and move than do those with small frames.


Men usually require more energy than women. They tend to be larger and to have more lean body mass than women do.


Children require more calories per pound of body weight than adults because they are growing. As people age, the lean body mass declines, and the basal metabolic rate declines accordingly. Heredity is also a determining factor. One’s BMR may resemble one’s parents’, just as one’s appearance may. One’s physical condition also affects the BMR. For example, women require more calories during pregnancy and lactation than at other times. The basal meta-bolic rate increases during fever and decreases during periods of starvation or severely reduced calorie intake. People living and working in extremely cold or warm climates require more calories to maintain normal body temperature than they would in a more temperate climate.


Thermic Effect of Food


The body requires energy to process food (digestion, absorption, transpor-tation, metabolism, and storage); this requirement represents 10% of daily energy (calorie) intake. Multiply BMR by 0.10 and add to the BMR (REE) before an activity factor is calculated.

Female: REE = 655 + (9.6 × weight in kg) + (1.8 × height in cm) – (4.7 × age)


Male: REE = 66 + (13.7 × weight in kg) + (5 × height in cm) – (6.8 × age)


= weight inkilograms (kg) (weight in pounds ÷ 2.2 =kg)


= height in centimenters (cm) (height in inches x 2.54 = cm)


= age in years

Estimating BMR.Dietitians commonly use the Harris-Benedict equa-tion to determine the BMR (REE) of persons over the age of 18. This equation uses height, weight, and age as factors and results in a more individualized estimate of the REE than some other methods (Figure 3-5).

Another method used to estimate one’s BMR, or REE, is the following:


·  Convert body weight from pounds to kilograms (kg) by dividing pounds by 2.2 (2.2 pounds equal 1 kilogram).


·  Multiply the kilograms by 24 (hours per day).


·  Multiply the answer obtained in step 2 above by 0.9 for a woman and by 1.0 for a man.


For example, assume that a woman weighs 110 pounds. Divide 110 by 2.2 for an answer of 50 kg. Multiply 50 kg by 24 hours in a day for an answer of 1,200 calories. Then multiply 1,200 calories by 0.9 for an answer of 1,080 calories. This is the estimated basal metabolic energy requirement for that particular woman.

Calculating Total Energy Requirements


An individual’s average daily energy requirement is the total number of calories needed in a 24-hour period. Energy requirements of people differ, depending on BMR (REE) and activities. More energy is burned playing soccer than playing the piano. Refer to Table 3-2 for calorie guidelines according to MyPyramid.


Table 3-3 shows suggested weights for adults according to height.

Energy Balance

A person who takes in fewer calories than she or he burns usually loses weight. If someone takes in more calories than she or he burns, the body stores them as adipose tissue (fat). Some adipose tissue is necessary to protect the body and support its organs. Adipose tissue also helps regulate body temperature, just as insulation helps regulate the temperature of a building. An excess of adipose tissue, however, leads to obesity, which can endanger health because it puts extra burdens on body organs and systems. For the healthy person, the goal is energy balance. This means that the number of calories consumed matches the number of calories required for one’s BMR (REE) and activity.

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