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Lecture
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Carbohydrates
and Protein Digestion and Absorption
Vay Liang
W. Go, MD
Professor of
Medicine
A
discussion of the digestion and absorption
of macronutrients (protein, fat, carbohydrate).
Carbohydrates:
Simple
Carbohydrates
(Sugars)
|
1.Monosaccharides |
1. glucose
2. fructose
3. galactose
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2.
Disaccharides
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1. sucrose =
glucose + fructose
2. lactose
= glucose + galactose
3. maltose
= glucose + glucose
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Complex
Carbohydrates
(Starches)
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Amylose |
a-1, 4 links of glucose, straight chain
Average
chain length of 600 glucose residues |
Amylopectin
( branched chain) |
a-1, 4 linked
straight chains with a-1, 6 linked branch pts.
Contains
about 6000 glucose residues
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Dietary
fiber
(plant materials that resist digestion)
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Soluble
(pectin, guar gum,
oat bran) |
1. Absorbs water
2. Delays
absorption of sugar
3. Binds
bile salts
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Insoluble
(cellulose, lignin) |
1. Decreases
transit time
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Enzymes present in
gut flora partially break down dietary fiber to short-chain fatty acids
(acetate, propionate, butyrate), hydrogen, carbon dioxide and methane.
Short-chain fatty
acids derived from metabolism of fiber stimulate colonic salt and water
absorbtion and regulate colonocyte proliferation and differentiation.
Butyrate is a principle energy source for the large intestine.
Average
CHO Consumption
Adult male:
300g/d
Adult female: 200 g/d
Carbohydrates
provide about 45-50% of the average American's daily caloric intake.
In food, carbohydrates
are present in association with proteins.
Carbohydrate
Digestion (small intestine)
| å-amylase |
1.
Hydrolyzes å-1, 4 bonds |
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2.
Salivary: degraded in acidic environment (pH 3 - 3.5) in
neonates with immature pancreatic function, salivary amylase is significant
in starch digestion |
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3.
Pancreatic: responsible for intraluminal starch hydrolysis |
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(oligosaccharide
products of a-amylase digestion are hydrolyzzed by enzymes on the
brush border membrane surface. Only free glucose is transported
into the enterocyte.):
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enzymes
in the brush border responsible for
hydrolosis of oligosaccharides to glucose |
1.
Glucoamylase: removes single glucose residues sequentially from the
non-reducing end of the chain |
| 2.
Sucrase: efficiently hydrolyzes short å-1,4-linked oligosaccharides
such as maltose and maltotriose |
| 3.
Isomaltase: cleaves the non-reducing, terminal å-1, 6 bond |
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4. Lactase:
digests lactose to glucose and galactose
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Levels of
enzymes are modified by dietary alterations
-A
CHO free diet inactivates sucrase
-A high sucrose
diet increases level of sucrase m-RNA
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