Maintenance of the Body
(Table 24.1). Legumes (beans and peas), nuts, and cereals are
protein-rich, but their proteins are nutritionally incomplete be-
cause they are low in one or more of the essential amino acids.
±e exception to this generalization is soybeans, plant-derived
Strict vegetarians must carefully plan their diets to obtain
all the essential amino acids and prevent protein malnutrition.
When ingested together, cereal grains and legumes provide all
the essential amino acids (Figure 24.2b). Some combination of
these foods is found in the diets of all cultures (for instance,
the rice and beans seen on nearly every plate in a Mexican res-
taurant). For nonvegetarians, grains and legumes are useful as
partial substitutes for more expensive animal proteins.
Uses in the Body
Proteins are important structural materials of the body, includ-
ing, for example, keratin in skin, collagen and elastin in con-
nective tissues, and muscle proteins. In addition, functional
proteins such as enzymes and some hormones regulate an in-
credible variety of body functions. Whether amino acids are
used to synthesize new proteins or burned for energy depends
on a number of factors:
The all-or-none rule.
All amino acids needed to make a par-
ticular protein must be present in a cell at the same time and
in suﬃcient amounts. If one is missing, the protein cannot
be made. Because essential amino acids cannot be stored,
those not used immediately to build proteins are oxidized for
energy or converted to carbohydrates or fats.
Adequacy of caloric intake.
For optimal protein synthesis,
the diet must supply suﬃcient carbohydrate or fat calories
for ATP production. When it doesn’t, dietary and tissue pro-
teins are used for energy.
In healthy adults the rate of protein
synthesis equals the rate of protein breakdown and loss, a
homeostatic state called
. ±e body is in
nitrogen balance when the amount of nitrogen ingested in
proteins equals the amount excreted in urine and feces.
±e body is in
positive nitrogen balance
when the amount
of protein incorporated into tissue is greater than the amount
being broken down and used for energy—the normal situ-
ation in growing children and pregnant women. A positive
balance also occurs when tissues are being repaired following
illness or injury.
negative nitrogen balance
, protein breakdown for energy
exceeds the amount of protein being incorporated into tis-
sues. ±is occurs during physical and emotional stress (for
example, infection, injury, or burns), when the quality or
quantity of dietary protein is poor, or during starvation.
Certain hormones, called
, accelerate protein synthesis and growth. ±e eﬀects
of these hormones vary continually throughout life. For ex-
pituitary growth hormone
stimulates tissue growth
during childhood and conserves protein in adults, and the
trigger the growth spurt of adolescence. Other
hormones, such as the adrenal
Unlike triglycerides, cholesterol is not used for energy. It is
important as a stabilizing component of plasma membranes
and is the precursor from which bile salts, steroid hormones,
and other essential molecules are formed.
Fats represent over 40% of the calories in the typical American
diet. ±ere are no precise recommendations on amount or type
of dietary fats, but the American Heart Association suggests:
Fats should represent 30% or less of total caloric intake.
Saturated fats should be limited to 10% or less of total fat
Daily cholesterol intake should be no more than 300 mg (the
amount in one egg yolk).
±e goal of these recommendations is to keep total blood
cholesterol below 200 mg/dl. Because a diet high in saturated
fats and cholesterol may contribute to cardiovascular disease,
these are wise guidelines. Table 24.1 summarizes sources of the
various lipid classes and consequences of deﬁciency or excessive
In an attempt to reduce fat intake without losing fat’s appetiz-
ing aspects, many people have turned to fat substitutes. Perhaps
the oldest fat substitute is air (beaten into a product to make it
ﬂuﬀy). Other fat substitutes are modiﬁed starches and gums,
and milk whey protein. Except for those based on dietary ﬁ-
bers (gums), these products are metabolized and provide calo-
ries. One of the newest substitutes, Olestra, a fat-based product
made from cottonseeds, is not metabolized because it is not di-
gested or absorbed.
Most fat substitutes have two drawbacks: (1) ±ey don’t stand
up to the intense heat needed to fry foods, and (2) although
manufacturers claim otherwise, they don’t taste nearly as good
as the “real thing.” ±e ones that are not absorbed tend to cause
ﬂatus (gas) or diarrhea, and may interfere with absorption of
fat-soluble drugs, vitamins, and beta-carotene, a precursor of
Distinguish between nutritionally complete and incomplete
Indicate uses of proteins in the body.
Deﬁne nitrogen balance and indicate possible causes of
positive and negative nitrogen balance.
Animal products contain the highest-quality proteins, in other
words, those with the greatest amount and best ratios of
sential amino acids
. Proteins in eggs, milk, ﬁsh,
and most meats are
that meet all the body’s
amino acid requirements for tissue maintenance and growth