948
UNIT 4
Maintenance of the Body
24
Tere are a number of other enzyme defects, classified ac-
cording to the impaired process as carbohydrate, lipid, or min-
eral metabolic disorders. Te carbohydrate deficit
galactosemia
results from an abnormality in or lack of the liver enzymes
needed to transform galactose to glucose. Galactose accumu-
lates in the blood and leads to mental deficits.
In the carbohydrate disorder
glycogen storage disease
,
glycogen synthesis is normal, but one of the enzymes
needed to convert it back to glucose is missing. As excessive
amounts of glycogen are stored, its storage organs (liver and
skeletal muscles) become glutted with glycogen and enlarge
tremendously.
With the exception of
type 1 diabetes mellitus
, children free
of genetic disorders rarely exhibit metabolic problems. How-
ever, by middle age and particularly old age,
type 2 diabetes
mellitus
becomes a major problem, particularly in people who
are obese.
Metabolic rate declines throughout the life span. In old age,
muscle and bone wasting and declining efficiency of the endo-
crine system take their toll. Because many elderly are also less
active, the metabolic rate is sometimes so low that it becomes
nearly impossible to obtain adequate nutrition without gaining
weight. Te elderly also use more medications than any other
group, at a time of life when the liver has become less efficient in
its detoxifying duties.
Many drugs and popular remedies influence nutrition. For
example:
Some diuretics prescribed for congestive heart failure or hy-
pertension (to flush fluids out of the body) can cause severe
hypokalemia by promoting excessive loss of potassium.
Some antibiotics—for example, sulfa drugs, tetracycline, and
penicillin—interfere with food digestion and absorption.
Tey may also cause diarrhea, further decreasing absorption.
Although physicians discourage its use because it interferes
with absorption of fat-soluble vitamins, mineral oil is still a
popular laxative with the elderly.
About half the elderly in the U.S. consume alcohol. When it
is substituted for food, nutrient stores can be depleted. Ex-
cessive alcohol intake leads to absorption problems, certain
vitamin and mineral deficiencies, deranged metabolism, and
damage to the liver and pancreas.
Although malnutrition and a waning metabolic rate present
problems to some elderly, certain nutrients—notably glucose—
appear to contribute to the aging process in all of us. Nonen-
zymatic reactions (the so-called
browning reactions
) between
glucose and proteins, long known to discolor and toughen
foods, may have the same effects on body proteins. When en-
zymes attach sugars to proteins, they do so at specific sites and
the glycoproteins produced play well-defined roles in the body.
By contrast, nonenzymatic binding of glucose to proteins (a
process that increases with age) is haphazard and eventually
causes cross-links between proteins. Tis type of binding prob-
ably contributes to lens clouding, and the general tissue stiffen-
ing and loss of elasticity so common in the aged.
Sweating begins and the skin becomes flushed and warm. Phy-
sicians have long recognized these signs as signals that body
temperature is falling (aah, she has passed the crisis). As we
explained in Chapter 21, fever speeds healing by increasing the
metabolic rate, and it also appears to inhibit bacterial growth.
Check Your Understanding
32.
What is the body’s core?
33.
Cindy is flushed and her teeth are chattering even though
her bedroom temperature is 72°F. Why do you think this is
happening?
34.
How does convection differ from conduction in causing heat
loss?
For answers, see Appendix H.
Developmental Aspects
of Nutrition and Metabolism
Describe the effects of inadequate protein intake on the
fetal nervous system.
Describe the cause and consequences of the low metabolic
rate typical of the elderly.
List ways that medications commonly used by aged people
may influence their nutrition and health.
Good nutrition is essential in utero, as well as throughout life.
If the mother is ill nourished, the development of her infant is
affected. Most serious is the lack of adequate calories, proteins,
and vitamins needed for fetal tissue growth, especially brain
growth. Additionally, inadequate nutrients during the first three
years a±er birth will lead to mental deficits or learning disorders
because brain growth continues during this time. Proteins are
needed for muscle and bone growth, and calcium is required
for strong bones. Although anabolic processes are less critical
a±er growth is completed, sufficient nutrients are still essential
to maintain normal tissue replacement and metabolism.
Homeostatic Imbalance 24.9
Tere are many inborn errors of metabolism (or genetic disor-
ders), but perhaps the two most common are
cystic fibrosis
(see
Chapter 22) and
phenylketonuria
(
PKU
) (fen
0
il-ke
0
to-nu
9
re-ah).
In PKU, tissue cells are unable to use the amino acid phenyl-
alanine (fen
0
il-al
9
ah-nēn), which is present in all protein foods.
Te defect involves a deficiency of the enzyme that converts
phenylalanine to tyrosine. Phenylalanine cannot be metabo-
lized, so it and its deaminated products accumulate in the blood
and act as neurotoxins that cause brain damage and retarda-
tion within a few months. Tese consequences are uncom-
mon today because most states require a urine or blood test
to identify affected newborns, and these children are put on
a low-phenylalanine diet. Expectant women with PKU are
sometimes encouraged to follow a controlled phenylalanine
diet during their pregnancy.
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