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Physiology
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Lecture Series
Lecture 1
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Lecture 3
Lecture 4
Lecture 5
Lecture 6
Lecture 7
Lecture 8

 

Lecture 3 page 1 2 3 4 5

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

2. Disaccharides

1. sucrose = glucose + fructose
2. lactose = glucose + galactose
3. maltose = glucose + glucose

   
Complex Carbohydrates
(Starches)
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

   

Dietary fiber
(plant materials that resist digestion)

Soluble
(pectin, guar gum,
oat bran)

1. Absorbs water
2. Delays absorption of sugar
3. Binds bile salts

Insoluble
(cellulose, lignin)

1. Decreases transit time

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)

-amylase 1. Hydrolyzes -1, 4 bonds
  2. Salivary: degraded in acidic environment (pH 3 - 3.5) in neonates with immature pancreatic function, salivary amylase is significant in starch digestion
  3. Pancreatic: responsible for intraluminal starch hydrolysis

(oligosaccharide products of a-amylase digestion are hydrolyzzed by enzymes on the brush border membrane surface. Only free glucose is transported into the enterocyte.):


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 -1,4-linked oligosaccharides such as maltose and maltotriose
3. Isomaltase: cleaves the non-reducing, terminal -1, 6 bond

4. Lactase: digests lactose to glucose and galactose

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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Lecture 1
:Introduction to Nutrition in Western Civilization
Lecture 2:
Dietary Macronutrients, Body Fat, and Blood Lipids
Lecture 3:
Digestion and Absorption of Macronutrients
Lecture 4:
Basic Principles of Nutrient Metabolism
Lecture 5:
Obesity
Lecture 6:
Fuel Utilization During Exercise
  Lecture 7:Biochemistry of Oxidant Stress in Health and Disease Antioxidants
Lecture 8:Nutrition for the 21st Century

 

 

 

 

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