Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
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24
Te energy value of foods is measured in
kilocalo-
ries
(kcal) or “large calories” (C). One kilocalorie is the
amount of heat energy needed to raise the temperature of
1 kilogram of water 1°C (1.8°F). Tis unit is the “calorie” that
dieters count so conscientiously.
In Chapter 23, we talked about how foods are digested and
absorbed, but what happens to these foods once they enter the
blood? Why do we need bread, meat, and fresh vegetables? Why
does everything we eat seem to turn to fat? In this chapter we
will answer these questions as we examine both the nature of
nutrients and their metabolic roles.
Diet and Nutrition
Define nutrient, essential nutrient, and calorie.
List the six major nutrient categories. Note important
sources and main cellular uses.
A
nutrient
is a substance in food the body uses to promote
normal growth, maintenance, and repair. Te nutrients needed
for health divide neatly into six categories. Tree of these—
carbohydrates, lipids, and proteins—are
major nutrients
that
make up the bulk of what we eat. Te fourth and fi±h categories,
vitamins and minerals, though equally crucial for health, are
required in only minute amounts.
In a strict sense, water, which accounts for about 60% by vol-
ume of the food we eat, is also a major nutrient. We described its
importance in the body in Chapter 2, so here we consider only
the five nutrient categories listed above.
Most foods offer a combination of nutrients. A balanced diet
of foods from each of the different food groups normally guar-
antees adequate amounts of all the needed nutrients.
In 1992, the U.S. Department of Agriculture released a food
guide pyramid in which four food groups were stacked horizon-
tally in the shape of a pyramid. Since then, several alternative
versions have made an appearance claiming to be more healthy
nutritionwise and focusing on exercise as an aid to good health.
Te version put out in 1992 by Walter Willett, called the
Healthy Eating Pyramid
(Figure 24.1a)
, looks at six major
food groups, subdividing some of them further. It uses the tra-
ditional (horizontal) orientation of food groups; emphasizes
eating whole-grain foods and lots of fruits and vegetables; and
recommends substituting plant oils for animal fats and restrict-
ing red meat, sweets, and starchy foods.
Fresh out of the oven in 2011 is
MyPlate
a food guide with
a completely new symbol—a round dinner plate. Put out by
the USDA, it shows food categories in healthy proportions in
sections of a place setting rather than as segments of a pyra-
mid (Figure 24.1b). Te fact that each food group is shown as
part of a single meal suggests how consumers might plan their
meals relative to amounts and variety of foods selected from
each food group. Occupying half the plate are fruits and vegeta-
bles (more vegetable than fruit). Tese are balanced in the other
half by grains and proteins (more grain than protein). A glass
represents dairy, the fi±h food group. Links provide details on
healthy choices in each food group. You can personalize the My-
Plate diet by age, sex, and activity level by going to the MyPlate
website (www.choosemyplate.gov).
²o be perfectly honest, nutrition advice is constantly in flux
and o±en mired in the self-interest of food companies. None-
theless, basic dietary principles have not changed in years and
are not in dispute: Eat less overall; eat plenty of fruits, vegetables,
and whole grains; avoid junk food; and exercise regularly.
Te ability of cells, especially liver cells, to convert one type
of molecule to another is truly remarkable. Tese interconver-
sions allow the body to use a wide range of foods and to adjust
to varying food intakes. But there are limits to our ability to
conjure up new molecules from old.
At least 45 and possibly 50 molecules, called
essential nutri-
ents
, cannot be made fast enough to meet the body’s needs, so our
diet must provide them. As long as we ingest all the essential nutri-
ents, the body can synthesize the hundreds of additional molecules
(b)
USDA’s MyPlate
Red meat, butter:
use sparingly
Vegetables
in abundance
Whole-grain
foods at
most meals
Daily excercise and weight control
(a)
Healthy Eating Pyramid
Dairy or calcium
supplement: 1–2 servings
White rice, white bread,
potatoes, pasta, sweets:
use sparingly
Fish, poultry, eggs:
0–2 servings
Nuts, legumes:
1–3 servings
Fruits:
2–3 servings
Plant oils
at most
meals
Peanut
Oil
Vegetable
Oil
Olive
Oil
Figure 24.1
Two visual food guides.
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