908
UNIT 4
Maintenance of the Body
24
American adults typically consume about 46% of dietary
food energy in the form of carbohydrates. Because starchy foods
(rice, pasta, breads) cost less than meat and other high-protein
foods, carbohydrates make up an even greater percentage of
the diet in low-income groups. Highly processed carbohydrate
foods such as candy and soF drinks provide concentrated energy
sources only—so-called “empty calories.” Eating refined, sugary
foods instead of the more complex carbohydrates may cause
nutritional deficiencies as well as obesity.
Table 24.1
lists other
possible consequences of excessive intake of carbohydrates.
Lipids
Indicate uses of lipids in the body.
Distinguish between saturated, unsaturated, and trans
fatty acid sources.
Dietary Sources
Te most abundant dietary lipids are triglycerides, also called
neutral fats (Chapter 2). We eat saturated fats in animal prod-
ucts such as meat and dairy foods, in a few tropical plant prod-
ucts such as coconut, and in hydrogenated oils (trans fats) such
as margarine and solid shortenings used in baking. Unsaturated
fats are present in seeds, nuts, olive oil, and most vegetable oils.
±ats are digested to monoglycerides and fatty acids, and then
reconverted to triglycerides for transport in the lymph.
Major sources of cholesterol are egg yolk, meats and organ
meats, shellfish, and milk products. However, it is estimated that
the liver produces about 85% of blood cholesterol regardless of
dietary intake.
Te liver is also adept at converting one fatty acid to another,
but it cannot synthesize
linoleic acid
(lin
0
o-le
9
ik), a fatty acid
component of
lecithin
(les
9
ĭ-thin). ±or this reason, linoleic acid,
an omega-6 fatty acid, is an
essential fatty acid
that must be in-
gested. Linolenic acid, an omega-3 fatty acid, may also be es-
sential. ±ortunately, most vegetable oils contain both linoleic
and linolenic acids.
Uses in the Body
±ats have fallen into disfavor, particularly among those for
whom the “battle of the bulge” is constant. But fats make foods
tender, flaky, or creamy, and make us feel full and satiated (satis-
fied). ±urthermore, dietary fats
are
essential for several reasons:
±ats help the body absorb fat-soluble vitamins.
²riglycerides are the major energy fuel of hepatocytes and
skeletal muscle.
Phospholipids are an integral component of myelin sheaths
and cellular membranes.
±atty deposits in adipose tissue provide (1) a protective cushion
around body organs, (2) an insulating layer beneath the skin,
and (3) an easy-to-store concentrated source of energy fuel.
Prostaglandins
(pros
0
tah-glan
9
dinz), regulatory molecules
formed from linoleic acid via arachidonic acid (ah
0
rah-kĭ-
don
9
ik), play a role in smooth muscle contraction, control of
blood pressure, and inflammation.
required for life and good health. While “essential” is a standard
way to describe the chemicals that must be obtained from outside
sources, this term is misleading because both essential and nones-
sential nutrients are equally vital for normal functioning.
Carbohydrates
Distinguish between simple and complex carbohydrate
sources.
Indicate the major uses of carbohydrates in the body.
Dietary Sources
Except for milk sugar (lactose) and negligible amounts of glyco-
gen in meats, all the carbohydrates we ingest are derived from
plants. Sugars (monosaccharides and disaccharides) come from
fruits, sugar cane, sugar beets, honey, and milk. Te polysaccha-
ride starch is found in grains and vegetables.
²wo varieties of polysaccharides provide fiber. Cellulose,
plentiful in most vegetables, is not digested by humans but pro-
vides roughage, or
insoluble fiber
, which increases the bulk of
the stool and facilitates defecation.
Soluble fiber
, such as pectin
found in apples and citrus fruits, reduces blood cholesterol lev-
els, a desirable goal for most of us.
Uses in the Body
Te monosaccharide
glucose
is
the
carbohydrate molecule ulti-
mately used as fuel by body cells to produce A²P. Carbohydrate
digestion also yields fructose and galactose, but the liver con-
verts these monosaccharides to glucose before they enter the
general circulation.
Many body cells also use fats as energy sources, but neurons
and red blood cells rely almost entirely on glucose for their en-
ergy needs. Because even a temporary shortage of blood glucose
can severely depress brain function and lead to neuron death,
the body carefully monitors and regulates blood glucose levels.
Any glucose in excess of what is needed for A²P synthesis is
converted to glycogen or fat and stored for later use.
Other uses of monosaccharides are meager. Small amounts
of pentose sugars are used to synthesize nucleic acids, and a
variety of sugars are attached to externally facing plasma mem-
brane proteins and lipids.
Dietary Requirements
Te low-carbohydrate diet of the Inuit (Eskimos) and the high-
carbohydrate diet of peoples in the ±ar East indicate that humans
can be healthy even with wide variations in carbohydrate intake.
Te minimum requirement for carbohydrates is not known, but
100 grams per day is presumed to be the smallest amount needed
to maintain adequate blood glucose levels. Te recommended di-
etary allowance (130 g/day) is based on the amount needed to fuel
the brain, not the total amount needed to supply all daily activities.
Te recommended intake to maintain health is 45–65% of total
calorie intake, with the emphasis on
complex
carbohydrates (whole
grains and vegetables), rather than simple carbohydrates (mono-
saccharides and disaccharides). When less than 50 grams per day
are consumed, tissue proteins and fats are used for energy fuel.
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