2-Digestion and Absorption of Carbohydrates.2023.pdf

biochemistry

1. Digestion and Absorption of
Carbohydrate
Advanced Nutritional Biochemistry
(NF 765)
Fall 2023
Dr. Sana Janakat

2. Nondigestible Carbohydrates (Chapter 8)
Alpha galactosidic (raffinose and
stachyose)
Beta glucosidic (Cellulose and Beta
glucans)
Digestible Carbohydrates
Most are polymers, oligomers and dimers
of hexoses
Most digestion occur via luminal and
intestinal hydrolysis ( table 1)
Major Categories of Carbohydrates

3. Table 5.1
Digestion of Dietary Carbohydrates
Intestinal
Hydrolysis
Yield
Luminal
Hydrolysis
Yield
Food Source
GlucoseMaltose,
matotriose and
α-dextrins
Starches
(Amylose and
amylopectin
Glucose and
galactose
NoneLactose
Glucose and
fructose
NoneSucrose

4. α-amylase is produced via Salivary and
pancreatic exocrine glands.
In infants salivary α-amylase is active in
the mouth and in the small intestines
Generallyinactive in the stomach and
resumes its activity in the small intestines
Luminal Digestion of starches by
α-amylase

5. Factors affecting α-amylase
secretion
1-Cell specific nuclear factors (pancreatic
transcription factor 1)
2-Hepatocyte type nuclear factors
3-Presence of ingested carbohydrate in the
lumen
4-CCK

6. Secretion of α-amylase is not a limiting
factor because secretion exceed
requirements.
Starches/Hydrophobic protein complex
digestion (α-amylase is H☺)
Cooking starch release from protein
digestion
Starches/nondigestible fibers Digestion
Factors Affecting Starches Digestion

7. Types of starches
•Amylose
•Amylopectin
•Glycogen

8. Reducing and non-reducing ends
Non-reducing end
Reducing end

9. Digestion of starches
α-amylase (interior links) (Fig 1)
Reducing end
It has low affinity to polysaccharides shorter than 5 glucose residues long

10. Digestion of starches
β-amylase(Exterior links)
Debranching enzyme )α16)(Fig 3)
* Debranching enz or isomaltase. ** Maltase
*
**

11. Oligosaccharide Digestion
Synthesis of intestinal oligosaccharidases

12. Table 5-2Intestinal Surface Membrane
Oligosaccharideses

13. •Sucrase-isomaltase
1 polypeptide 2 functions
Trehalase (brush border enzyme
(trehalose in insects and mushrooms
only [α,α(1 1) glucose units)
Intestinal Surface Membrane Oligosaccharideses

14. Lactase (the only β-galactosidase)
Hypolactasia
Prevalence in healthy populations

15. Table 5-3 Prevalence of Hypolactasia in Healthy populations

16. Detecting Hypolactasia

17. SGLT 1 and GLUT 2 for glucose and
galactose
GLUT 5 and GLUT 2 for Fructose (Fig
5)
mRNA synthesis of the three
transporters increase priorto feeding.
SGLT absorbs huge quantities of water
(260 molecules/ 1molcules of glucose
and 2 Na ions = 3L/100g glucose)
Carbohydrates absorption

18. Fig 5-5 Hexose carrier proteins on the
enterocyte membrane

19. Rate of chewing
Meal composition
Physical state of starch
Rate limiting steps
Lactase production (1/3 of sucrase)
Assimilation in lactose fed subjects is only 50% of
Glucose or Galactose fed subjects.
α-amylaseproduction(10 folds more than
required)
Factors influencing CH
2O
assimilation

20. Better digestion leads to efficient
absorption
If the upper part of the small intestines is
removed the efficiency of the ileum
increases.
Efficiency
Duodenum Jejunum Ileum
Factors influencing CH
2O assimilation

21. Glucose-Galactose malabsorption
Point mutation in SGLT :
Leu
147
Arg or
Cys
353
Ser
Fructose malabsorption
The only treatment to all types is elimination
Genetic factors affecting assimilation

22. Genetic defects
Defective oligosaccharidases
•Hypolactasia due to:
•abnormal mRNA synthesis or
•Normal mRNA but low translation
Decreased sucrase-αdextranase synthesis
(10% in Eskimos and 0.2% in North
Americans).
 Point mutation Gln
1098
Pro
(The product cant be glycosylated nor
transported to apical membrane leading to
degradation)

23. Fig 5-6 Defective
oligosaccharidase
s leading to
carbohydrate
malabsorption
syndrome