What is a negative nitrogen balance? A positive nitrogen balance?
How can inadequate nutrition cause edema?
Carbohydrates, fats, and proteins supply energy, which is
required for all metabolic processes and therefore important
to cell survival. If the diet is deF cient in energy-supplying
nutrients, structural molecules may gradually be consumed,
leading to death. On the other hand, excess intake of energy-
supplying nutrients may lead to obesity, which also threat-
ens health.
Energy Values of Foods
The amount of potential energy a food contains is expressed
¯z), units of heat. Although a calorie is
deF ned as the amount of heat required to raise the tempera-
ture of a gram of water by 1 degree Celsius (°C), the calo-
rie used to measure food energy is 1,000 times greater. This
large calorie
(Cal.) equals the amount of heat required to
raise the temperature of a kilogram (1,000 grams) of water
by 1°C (from 14.5°C to 15.5°C) and is also equal to 4.184
joules. A joule is the international unit of heat and energy.
A large calorie is sometimes called a
but it is cus-
tomary in nutritional studies to refer to it as a calorie.
Figure 18.8
shows a bomb calorimeter, used to measure
the caloric contents of foods. It consists of a metal cham-
ber submerged in a known volume of water. A food sample
is dried, weighed, and placed in a nonreactive dish inside
the chamber. The chamber is F
lled with oxygen and sub-
merged in the water. Then, the food is ignited and allowed
to completely oxidize. Heat released from the food raises the
ans)—a condition in which the amount of nitrogen taken
in equals the amount excreted.
A person who is starving has a
negative nitrogen bal-
because the amount of nitrogen excreted as a result of
amino acid oxidation exceeds the amount the diet replaces.
Conversely, a growing child, a pregnant woman, or an ath-
lete in training is likely to have a
positive nitrogen balance
because more protein is being built into new tissue and less
is being used for energy or excreted.
Protein Requirements
In addition to supplying essential amino acids, proteins pro-
vide nitrogen and other elements for the synthesis of non-
essential amino acids and certain nonprotein nitrogenous
substances. The amount of dietary protein individuals require
varies according to body size, metabolic rate, and nitrogen bal-
ance condition.
±or an average adult, nutritionists recommend a daily
protein intake of about 0.8 gram per kilogram (0.4 gram per
pound) of body weight or 10% of a person’s diet. Another
way to estimate desirable protein intake is to divide weight
in pounds by 2. Most people should consume 60–150 grams
of protein a day. ±or a pregnant woman, who needs to main-
tain a positive nitrogen balance, the recommendation adds
30 grams of protein per day. Similarly, a nursing mother
requires an additional 20 grams of protein per day to main-
tain a high level of milk production.
Protein deficiency causes tissue wasting and also
decreases the level of plasma proteins, which decreases the
colloid osmotic pressure of the plasma. As a result, fl
collect in the tissues, producing a condition called
Table 18.4
summarizes the sources, requirements,
and uses for carbohydrate, lipid, and protein nutrients.
To Chapter 15, Exchanges in the Capillaries,
pages 578–579.
Carbohydrate, Lipid, and Protein Nutrients
Sources and
RDA* for Adults
per Gram
Conditions Associated with
Primarily from starch and
sugars in foods of plant origin
and from glycogen in meats
125–175 g
Oxidized for energy; used in production of ribose,
deoxyribose, and lactose; stored in liver and muscles as
glycogen; converted to fats and stored in adipose tissue
Obesity, dental
caries, nutritional
Meats, eggs, milk, lard,
plant oils
80–100 g
Oxidized for energy; production of triglycerides,
phospholipids, lipoproteins, and cholesterol, stored in
adipose tissue; glycerol portions of fat molecules may be
used to synthesize glucose
Obesity, increased
serum cholesterol,
increased risk of
heart disease
Weight loss,
skin lesions
Meats, cheese, nuts, milk,
eggs, cereals, legumes
0.8 g/kg body weight
Production of protein molecules used to build cell structure
and to function as enzymes or hormones; used in the transport
of oxygen, regulation of water balance, control of pH, formation
of antibodies; amino acids may be broken down and oxidized
for energy or converted to carbohydrates or fats for storage
Extreme weight
loss, wasting,
anemia, growth
RDA = recommended dietary allowance.
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