700
UNIT FIVE
Carbohydrate Sources
Carbohydrates are ingested in a variety of forms. Complex
carbohydrates include the
polysaccharides,
such as starch
from plant foods and glycogen from meats. Foods contain-
ing starch and glycogen usually have many other nutrients,
including valuable vitamins and minerals. The simple carbo-
hydrates include
disaccharides
from milk sugar, cane sugar,
beet sugar, and molasses and
monosaccharides
from honey
and fruits. Digestion breaks complex carbohydrates down to
monosaccharides, which are small enough to be absorbed
into the bloodstream.
glycogen, a storage carbohydrate. Eating stimulates adipocytes
to secrete
leptin,
which acts on target cells in the hypothala-
mus. The effect suppresses appetite by inhibiting release of
the hypothalamic neurotransmitter
neuropeptide Y
that stim-
ulates eating. This produces a negative feedback response to
ingested calories. Leptin also stimulates metabolic rate
(f g.
18.1)
. Low leptin levels indicate depleted fat stores, a con-
dition in which metabolism slows to conserve energy and
appetite increases. Inherited leptin de± ciency is rare, but the
resultant loss of this appetite “brake” results in obesity.
Conversely,
ghrelin,
another recently discovered hor-
mone that the stomach secretes, enhances appetite by stimu-
lating the release of neuropeptide Y. Therefore, a compound
that blocks ghrelin production or activity might help people
lose weight. Fasting and a low-calorie diet stimulate ghre-
lin production. The success of gastric bypass surgery may be
due in part to decreased ghrelin secretion resulting from loss
of stomach tissue.
Adiponectin is another hormone that affects body weight. It is a
protein hormone synthesized in adipose cells and secreted into the
bloodstream that mediates response to insulin and regulates fatty
acid catabolism. Adiponectin also has an anti-inF
ammatory e±
ect.
The hormone boosts basal metabolic rate (calories burned at rest)
without a±
ecting appetite. Levels of adiponectin are higher in people
who do not have diabetes and have healthy body weights, so boost-
ing its levels may one day help people to lose weight. Most informa-
tion about adiponectin, however, is still from work on nonhuman
animals.
PRACTICE
1
Identify and distinguish among macronutrients and
micronutrients.
2
Describe how hormones control appetite.
18.2
CARBOHYDRATES
Carbohydrates
are organic compounds and include the sug-
ars and starches. The energy held in their chemical bonds is
used to power cellular processes.
TABLE
18.2
|
Substances That Control
Appetite
Substance
Site of Secretion
Function
Insulin
Pancreas
Stimulates adipocytes to admit
glucose and store fat; glycogen
synthesis
Leptin
Adipocytes
Suppresses appetite and increases
metabolic rate after eating
Neuropeptide Y
Hypothalamus
Enhances appetite
Ghrelin
Stomach
Enhances appetite
Adipocytes synthesize
fat and secrete
leptin
into bloodstream
Fat cells
Bloodstream
Bloodstream
excessive
insufficient
Appetite
Food intake in excess
of caloric needs
Leptin
inhibits neuropeptide
Y release from the
hypothalamus
Leptin
increases
metabolic rate.
Appetite suppressed
Appetite enhanced
Cells in stomach
secrete
ghrelin
Ghrelin
stimulates release
of neuropeptide Y from
the hypothalamus
Fasting
+
+
+
+
+
Release into
bloodstream
Stimulation
Inhibition
FIGURE 18.1
Appetite control is complex. Illustrated here are the
ects of leptin and ghrelin on appetite.
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