707
CHAPTER EIGHTEEN
Nutrition and Metabolism
surement called the basal metabolic rate, the degree of mus-
cular activity, body temperature, and rate of growth.
The
basal metabolic rate
(ba
sal met
ah-bol
ik ra
¯t), 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 envi-
ronment. 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 released by cellular
respiration. The BMR indicates the total amount of energy
expended in a given time to support the activities of such
organs as the brain, heart, lungs, liver, and kidneys.
The BMR for an average adult indicates a requirement
for approximately 1 calorie of energy per hour for each kilo-
gram of body weight. However, this requirement varies with
sex, body size, body temperature, and level of endocrine
gland activity. For example, because heat loss is directly pro-
portional to the body surface area, and a smaller person has
a greater surface area relative to body mass, he or she will
have a higher BMR. Males typically have higher metabolic
rates than females. As body temperature, blood level of thy-
roxine or blood level of epinephrine increase, so does the
BMR. The BMR can also increase when the level of physical
activity increases during the day.
Maintaining the BMR usually requires the body’s great-
est expenditure of energy. The energy required to sup-
port voluntary muscular activity comes next, though this
amount varies greatly with the type of activity
(table 18.5)
.
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 addi-
tional energy expenditure, particularly in cold weather. In
this case, extra energy is expended in involuntary muscular
contractions, such as shivering, or through voluntary muscu-
lar actions, such as walking. Growing children and pregnant
women, because their bodies are actively producing new tis-
sues, also require more calories.
temperature of the surrounding water, and the change in
temperature is measured. The volume of the water is known,
so the amount of heat released from the food can be calcu-
lated in calories.
Caloric values determined in a bomb calorimeter are
somewhat higher than the amount of energy that metabolic
oxidation releases, because nutrients generally are not com-
pletely absorbed from the digestive tract. Also, the body
does not completely oxidize amino acids, but excretes parts
of them in urea or uses them to synthesize other nitrogenous
substances. When such losses are considered, cellular oxida-
tion yields on the average about 4.1 calories from 1 gram
of carbohydrate, 4.1 calories from 1 gram of protein, and
9.5 calories from 1 gram of fat. More than twice as much
energy is derived from equal amounts by weight of fats as
from either proteins or carbohydrates. This is one reason
why avoiding fatty foods helps weight loss, if intake of
other nutrients does not substantially increase. Fats encour-
age weight gain because they add fl avor to food, which can
cause overeating. However, fatty foods satisfy hunger longer
than carbohydrate-rich foods.
PRACTICE
20
What term designates the potential energy in a food?
21
How is the energy value of a food determined?
22
What is the energy value of a gram of carbohydrate? A gram of
protein? A gram of fat?
Energy Requirements
The amount of energy required to support metabolic activi-
ties for twenty-four hours varies from person to person. The
factors that infl
uence individual energy needs include a mea-
Thermometer
Food sample
Insulation
Platinum dish
Water
Stirrer
Oxygen
Electric wires
for ignition
of sample
FIGURE 18.8
A bomb calorimeter measures the caloric content of a
food sample.
TABLE
18.5
|
Calories Used During
Various Activities
Activity
Calories (per Hour)
Walking up stairs
1,100
Running (jogging)
570
Swimming
500
Vigorous exercise
450
Slow walking
200
Dressing and undressing
118
Sitting at rest
100
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