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8/6/2019 Absorption of Carbohydrates & Amino Acids
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Presented by :
Osama Sharafeddin
MBBS U of K
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Introduction :
There is no or little absorption in the stomach
; except for short & medium chain F.A andEthanol .
Small intestine is the main absorptive organ ;
90% .
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The contents change gradually from fluid to
semisolid state due to absorption of water
which starts in small intestine & continue incolon.
Amino acid &Carbohydrates are water
soluble; they are transported in the hepaticportal system.
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Products of digestion are : monosaccharides
;chiefly hexoses (Glucose , fructose, Galactose )
and pentoses (ribose) . Some oligo and disaccharides are hydrolysed
to monosaccharides in the mucosal surface ofsmall intestinal epithelium , by pancreatic
amylase &disaccharidases .
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Two main mechanisms of absorption :
1-facilitative transport ; with the gradient
2- active transport ; against the gradient .
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Glucose and other hexoses could be
transported by facilitative transporters
GLUTs ; mainly GLUT5 in the intestine ,GLUT2also participate in release of hexoses
into the capillaries. Fructose uses GLUT5 ,
but more slowly than glucose and galactose .
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Via the SGLT1 transporter ,Glucose is actively
transported against gradient ,coupled with
Sodium ; the former against the gradient andthe latter with the gradient .
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the energy required for this process is
obtained from ATP hydrolysis linked to a
pump that expels Sodium in exchange forPotassium .
This transporter is inhibited by Oubain , acardiac glycoside; and Phlorhizin in the renal
tubules.
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As an adaptive mechanism for increased
Carbohydrates in the diet , there is an
increase in the number of transporters in thebrush border .
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1- Lactose intolerance
may be attributable to lactase deficiency,
which is a rate limiting enzyme for Lactoseabsorption .
It should not be confused with milk
intolerance ; a sensitivity condition to milkprotein , usually -lactoglobulin .
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a- Inherited Lactase deficiency : rare ;
symptoms appear after birth ; management
by feeding lactose free diet , yoghurt feeding, Calcium supplementation and energy.
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b- Primary low lactase deficiency: relatively
common particularly among non whites;
gradual decrease of lactase action due todecreased expression .(failure of translation) .
c- Secondary low lactase activity : aconsequence of intestinal disease s: Tropical
sprue , kwashiorkor ,colitis, G.E, after surgeryof peptic ulcer .
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2-Sucrase deficiency :
both sucrase and isomaltase are deficient ;they coexist as a complex enzyme ;
symptoms appear from early childhood .
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3- Disacchariduria :
due to disaccharidase deficiency; more than300 mg of disaccharides appear in urine ; also
occur in patients with intestinal damage .
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4- monosaccharide malabsorption :
a- Mutation in SGLT1: slow glucose andgalactose absorption ; normal fructoseabsorption .
b- Fructose Sorbitol malabsorption .
Symptoms : diarrhea , flatulence , abdominalcramps .
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Products of protein digestion are absorbed as
individual amino acids .
Some dipeptides are hydrolyzed in theintestinal brush border .
A.A absorption is an energy dependentprocess; including carriers depending on a
mechanism similar to SGLT1 (coupled toNa+).
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Some carriers are specialized in Neutral a.a ;
others in Phe & Met ; and a special carrier for
Imino acids Pro & Hyp.
Na+ independent carriers include :
Neutral and lipophilic a.a carriers e.g Phe ,Leu
Cationic a.a e.g Lys .
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Large intestinal bacteria can form toxic
amines from amino acids by decarboxylation;
known as Ptomaines , some examples ofptomaines are:
Cadaverine LysAgmatine Arg
Tyramine TyrPutrescine Orn
Histamine His
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There is some evidence that some
protein fragments larger than amino
acids are absorbed in some conditions ;the situation which can lead to an
antigenic reaction .
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An example for the above theory is non
tropical sprue ; a disease characterized
by circulating antibodies to wheatgluten or its fractions , the harmful
molecule is a six or seven amino acid
particle .
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Thanks!