722
UNIT FIVE
A compulsive disorder that may result from mineral deF
ciency is
pica,
in which people consume huge amounts of nondietary substances
such as ice chips, soil, sand, laundry starch, clay and plaster, and even
hair, toilet paper, matchheads, inner tubes, mothballs, and charcoal.
The condition is named for the magpie bird,
Pica pica,
which eats a
range of odd things.
Pica a±
ects people of all cultures and was noted as early as 40
B
.
C
.
The connection to dietary deF
ciency stems from the observation that
slaves su±
ering from pica in colonial America recovered when their
diets improved, particularly when given iron supplements. Another
clue comes from a variation on pica called geophagy—”eating dirt”—
that a±
ects many types of animals, including humans. Researchers
discovered that when parrots eat a certain claylike soil in their native
Peru, soil particles bind alkaloid toxins in their seed food and carry
the toxins out of the body. Perhaps pica in humans is protective in
some way, too.
18.8
HEALTHY EATING
An
adequate diet
provides sufficient energy (calories),
essential fatty acids, essential amino acids, vitamins, and
minerals to support optimal growth and to maintain and
repair body tissues. Individual nutrient requirements vary
greatly with age, sex, growth rate, level of physical activ-
ity and stress, as well as with genetic and environmental
factors, so it is not possible to design a diet adequate for
everyone. However, nutrients are so widely found in foods
that satisfactory amounts and combinations can usually be
obtained despite individual food preferences, assuming that
foods are available.
It is difF
cult to keep track of the different nutrients in
a diet and be certain that an adequate amount of each is
consumed daily. Nutritionists have devised several ways to
help consumers make healthy food choices, recognizing that
people can meet dietary requirements in many and diverse
ways. Most familiar is the RDA guideline that has appeared
on several tables in the chapter.
RDA
stands for United
States Recommended Daily Allowance. An RDA is the upper
limit of another measurement, the Recommended Dietary
Allowance, which lists optimal calorie intake for each sex
at various ages, and the amounts of vitamins and minerals
needed to avoid deF ciency or excess conditions. The RDA
values on food packages are set high, ensuring that most
people who follow them receive sufF cient amounts of each
nutrient. Government panels meet every F
ve years to evalu-
ate the RDAs in light of new data.
Placing foods into groups is a simpler way to follow
a healthy diet. Diagrams called
food pyramids
organize
foods according to suggested proportions of the diet, often
in serving sizes. One food pyramid, developed by the U.S.
Department of Agriculture, dominated for years, but new
ones offer more speciF
c suggestions geared to age, health,
ethnicity, food preferences such as vegetarianism, or
A daily intake of 2 mg of copper is sufF cient to supply
cells. A typical adult diet has about 2–5 mg of this mineral,
so adults seldom develop copper deF
ciencies. ±oods rich in
copper include liver, oysters, crabmeat, nuts, whole-grain
cereals, and legumes.
Iodine
(I) is found in minute quantities in all tissues but
is highly concentrated in the thyroid gland. Its only known
function is as an essential component of thyroid hormones.
(±igure 13.20 shows the molecular structures of two of these
hormones, thyroxine and triiodothyronine.)
A daily intake of 1 microgram (0.001 mg) of iodine per
kilogram of body weight is adequate for most adults. The
iodine content of foods varies with the iodine content of soils
in different geographic regions, so many people use
iodized
table salt to season foods to prevent deF
ciencies.
Cobalt
(Co) is widely distributed in the body because it
is an essential part of cyanocobalamin (vitamin B
12
). It is also
necessary for the synthesis of several important enzymes.
The amount of cobalt required in the daily diet is
unknown. This mineral is found in a great variety of foods,
and the quantity in the average diet is apparently sufF -
cient. Good sources of cobalt include liver, lean meats,
and milk.
Zinc
(Zn) is most concentrated in the liver, kidneys, and
brain. It is part of many enzymes involved in digestion, res-
piration, and bone and liver metabolism. It is also necessary
for normal wound healing and for maintaining the integrity
of the skin.
The daily requirement for zinc is about 15 mg, and
most diets provide 10–15 mg. Only some may be absorbed,
so zinc deF
ciencies may occur. The richest sources of zinc
are meats; cereals, legumes, nuts, and vegetables provide
lesser amounts.
Fluorine
(±), as part of the compound fluoroapatite,
replaces hydroxyapatite in teeth, strengthening the enamel
and preventing dental caries.
Selenium
(Se) is stored in the
liver and kidneys. It is a constituent of certain enzymes and
participates in heart function. This mineral is found in lean
meats, whole-grain cereals, and onions.
Chromium
(Cr) is
widely distributed throughout the body and regulates glu-
cose use. It is found in liver, lean meats, yeast, and pork
kidneys.
Table 18.11
summarizes the characteristics of trace
elements.
The term “dietary supplement” traditionally refers to
minerals, vitamins, carbohydrates, proteins, and fats—the
micronutrients and macronutrients. Clinical Application
18.2 discusses the more commercial meaning of “dietary
supplement.”
PRACTICE
72
How is copper used?
73
What is the function of iodine?
74
Why might zinc deF
ciencies be common?
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