Transcript CHO digestion and absorption

CHO METABOLISM
BY Dr. Naglaa Ibrahim Azab
Assistant professor of medical
biochemistry
CHO IN DIET
Mostly
Polysaccharides
Small amounts
disaccharides
Monosaccharides
Polysaccharides
Starch
Glycogen
Cellulose
Disaccharides
Sucrose
Lactose
Maltose
Monosaccharides
Glucose
Fructose
Starch
In the
mouth
Salivary amylase
Dextrins
In the
intestine
sucrose
Lactose
Isomaltose
Sucrase
Lactase
Isomaltase
Fructose
Galactose
Glucose
Maltose
Maltase
Glucose
What about the stomach?
Is there digestion of CHO in the
stomach???
NO. ……….. Why?
No digestive enzymes for CHO in the
stomach
& the gastric HCL can not digest CHO
What about the salivary amylase?
Why does not it digests CHO in the
stomach???
It is destroyed by
the gastric HCL
CHO in diet: Starch, glycogen, cellulose, sucrose,
maltose, lactose, glucose, fructose & fructose
Does digestion of cellulose occur???
NO. ……….. Why?
Because salivary amylase (α- amylase acts
only on α-1,4 glucosidic linkage found in
starch and glycogen but not in cellulose in
which the linkage is β -1,4 glucosidic
linkage
So, what about glycogen???
salivary amylase acts on α-1,4 glucosidic
linkage found in glycogen giving…………..
End products of CHO digestion
Glucose
Pentose
Galactose
Fructose
Mannose
Occurs in duodenum and upper
jejunum
Glucose uptake by the intestinal
cells not need insulin
What if it needs insulin???
Mechanisms of absorption of CHO
Simple diffusion
Facilitated transport
Active transport
Simple diffusion
Intestinal
mucosal cell
Intestinal lumen
Fructose and pentoses
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No need for energy
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Facilitated diffusion
Intestinal
mucosal cell
Intestinal lumen
Glucose, Fructose and
galactose
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Glucose
transporter -5
which is Na
independant
No need for energy
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Active transport
Intestinal
mucosal cell
Intestinal lumen
Glucose,
galactose
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Na Glucose
transporter -1
Glucose,
galactose
Na
Na
Na
Na /K pump
(Na/K
ATPase)
K
K
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Fate of absorbed sugars
Intestinal
mucosal cell
Monosaccharides
Portal
blood
Liver
Glucose
Glucose
Fructose
Galactose
Fructose
Galactose
Systemic
circulation
Glucose
Fate of absorbed glucose
1- Oxidation
Major pathways
Glycolysis
Minor pathways
HMP shunt Uronic acid
pathway
Krebs cycle
ATP(Energy)
NADPH+H Glucuronic
Ribose 5- P
acid
Fate of absorbed glucose
2- Storage
Glycogenesis
Glycogen
Lipogenesis
Triglycerides
Fate of absorbed glucose
3- Conversion to other substances
Fructose
In semen
Galactose
Aminoacids
Lactose of milk( in
the lactating
mammary gland
Aminosugars
Disorders of CHO digestion
Cause and
pathogenesis
Lactose intolerance
Sucrase deficiency
Congenital or Acquired
Only congenital
(Occurs at infancy and may
recover at old age(
Deficient intestinal lactase 
lactose not digested 
accumulation of lactose in the
intestine  fermented by the
intestinal bacteria  acids and
gases
Deficient intestinal sucrase
 sucrose not digested 
accumulation of sucrose in
the intestine  fermented
by the intestinal bacteria 
acids and gases
Manifestations
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Treatment
-
Abdominal distension
- Abdominal cramps
- Diarrhea
- Acid stool
- Lactose free milk
No milk or milk products
The same
-Sucrose free diet
CASE (1)
A young man entered the physician office
complaining of intestinal gases and diarrhea. He
had signs of dehydration . The patient temperature
was normal. He said that this occurred after a
birthday party in which he had participated in
icecream eating contest . The patient reported
prior episodes of similar nature following
ingestion of a significant amount of dairy products
.
The clinical picture is probably due to deficiency in :
A - Salivary α- amylase.
B - Isomaltase.
C - Pancreatic α- amylase.
D - Sucrase.
E – Lactase.
Comment
lactose, a prominent component of milk and
most other dairy products.
Undigested lactose -------------------> acids + gases
ANSWER : E – Lactase.
Lactose intolerance
.
Definition
.
The inability to digest lactose
(the main sugar in milk)
Due to
deficiency of the intestinal enzyme lactase
giving rise to gastrointestinal symptoms.
Lactase deficiency is not the same as lactose
intolerance.
•Persons with milder deficiencies of lactase often
have no symptoms after the ingestion of milk.
• For unclear reasons, even persons with
moderate deficiencies of lactase may not have
symptoms.
• A diagnosis of lactase deficiency is made when
the amount of lactase in the intestine is reduced,
but a diagnosis of lactose intolerance is made
only when the reduced amount of lactase causes
symptoms.
Causes
1-Congenital lactase deficiency
because of a congenital absence (absent from birth) of lactase
due to a mutation in the gene that is responsible for producing
lactase. This is a very rare cause of lactase deficiency, and the
symptoms of this type of lactase deficiency begin shortly after
birth.
2-Primary lactose intolerance
The most common cause of lactase deficiency is a decrease in
the amount of lactase that occurs after childhood and persists
into adulthood, referred to as adult-type hypolactasia. This
decrease in lactase is genetically programmed, and the
prevalence of this type of lactase deficiency in different ethnic
groups is highly variable.
3-Secondary lactose intolerance
This type of deficiency is due to diseases that destroy the lining
of the small intestine along with the lactase. An example of
such a disease is celiac sprue.
Biochemical background
.
Normally
Disaccharides cannot be absorbed through the
wall of the small intestine into the bloodstream
Lactose
Small intestinal
Lactase
Glucose + Galactose
Absorbed
In lactose intolerance
Lactose
Colon
Small intestinal
Lactase
Glucose + Galactose
In colon
Lactose
Colonic bacteria
secreting lactase
Glucose + Galactose
Used and some splitted by these
bacteria(fermentation)
CO2 gas+ Hydrogen gas + acids as lactic acid
A small proportion Some is absorbed changed into methane gas
Most of the
is expelled
from the colon and by another type of colonic
hydrogen is
bacterium present in some
into the body
used up in
people. These people will
the colon
excrete only methane or
increased flatulence
expelled by the
by other
(passing gas)
lungs in the breath both hydrogen and methane
bacteria
gas in their breath and
flatus.
In colon
• The copious amounts of gas (a mixture of
hydrogen, carbon dioxide, and methane), may
cause a range of abdominal symptoms,
including stomach cramps, nausea, bloating,
acid reflux and flatulence.
• Not all of the lactose that reaches the colon is
split and used by colonic bacteria. The unsplit
lactose in the colon and its fermentation
products draws water into the colon (by
osmosis). This leads to loose, diarrheal stools.
The severity of the symptoms of lactose intolerance
varies greatly from person to person as the severity
depends on:
1- The amounts of lactose in diet; the more lactose in
the diet, the more likely and severe the symptoms.
2- The severity of lactase deficiency, that is, they may
have mild, moderate, or severe reduction in the
amounts of lactase in their intestines. Thus, small
amounts of lactose will cause major symptoms in
severely lactase deficient people but only mild or no
symptoms in mildly lactase deficient people.
3- The different responses of people to the same
amount of lactose reaching the colon. Whereas some
may have mild or no symptoms, others may have
moderate symptoms. The reason for this is not clear
but may relate to differences in their intestinal
bacteria.
Diagnosis
1-Elimination diet
-- A diet that eliminates milk and milk
products, continued long enough to clearly
evaluate whether or not symptoms are
better
--Elimination of all milk products should
eliminate symptoms completely if lactose
intolerance alone is the cause of the
symptoms
2-Milk challenge
A person fasts overnight and then drinks a glass of
milk in the morning. Nothing further is eaten or drunk
for 3-5 hours. If a person is lactose intolerant, the
milk should produce symptoms within several hours of
ingestion. If there are no symptoms or symptoms are
substantially milder than the usual symptoms, it is
unlikely that lactose intolerance is the cause of the
symptoms.
•Milk used must be fat-free to eliminate the possibility
that fat in the milk is the cause of symptoms.
• It is not possible to eliminate the possibility that
symptoms are due to milk allergy, however, this
usually is not confusing since allergy to milk is rare and
primarily occurs in infants and young children. (If milk
allergy is a consideration, pure lactose can be used
instead of milk.)
3-Breath test
The hydrogen breath test is the most convenient and reliable test
for lactase deficiency and lactose intolerance. For the breath test,
pure lactose, usually 25 grams , is ingested with water after an
overnight fast.
Samples of breath are collected every 10 or 15 minutes for 3-5
hours after ingestion of the lactose, and the samples are
analyzed for hydrogen and/or methane.
•If hydrogen and/or methane are found in the breath, it means
that the person is lactase deficient.
•The amount of hydrogen or methane excreted in the breath is
roughly proportional to the degree of lactase deficiency, that is,
the larger the amount of hydrogen and/or methane produced,
the greater the deficiency.
•The amount of hydrogen and/or methane in the breath,
however, is not proportional to the severity of the symptoms. In
other words, a person who produces little hydrogen and/or
methane may have more severe symptoms than a person who
produces a large amount hydrogen and/or methane.
4-Blood glucose test
Lactose is ingested (usually 0.75 to 1.5 gm of lactose per kg of body
weight) after an overnight fast, and serial blood samples are drawn
and analyzed for glucose.
• If the level of blood glucose rises more than 25 mg/100ml, it
means that the lactose has been split in the intestine and the
resulting glucose has been absorbed into the blood. This implies
that lactase levels are normal
•Not often used due to false positive tests, that is, an abnormal test
in people who have normal lactase levels and no lactose
intolerance. .
5- stool acidity test
•Is a test for lactase deficiency in infants and young children. For
the stool acidity test, the infant or child is given a small amount of
lactose orally. Several consecutive stool samples then are tested for
acidity. With a deficiency of lactase, unabsorbed lactose enters the
colon and is split into glucose and galactose. Some of the glucose
and galactose is broken down by the bacteria into acids, for
example, lactic acid. Lactic acid turns the stool acidic. Therefore, a
lactase deficient infant or child will develop an acidic stool following
the test dose of lactose.
•The stool acidity test is not done frequently due to superiority of
breath testing has led to modifications in the equipment for
collecting breath samples that makes it easier to do breath testing
in young children and even infants
6-Intestinal biopsy:
• For measurement of lactase levels in the lining.
• The analysis of lactase levels in the biopsy requires
specialized procedures that are not often available, and,
as a result, lactase levels are not often measured except
for research purposes.
Treatment
1- Dietary changes
•Reducing the amount of lactose in the diet . Lactose free milk formulas
are used for infants with lactose intolerance
•Though yoghurt contains large amounts of lactose, it often is welltolerated by lactose intolerant people. This may be so because the
bacteria used to make yoghurt contain lactase, and the lactase is able to
split some of the lactose during storage of the yoghurt as well as after the
yogurt is eaten (in the stomach and intestine). Yoghurt also has been
shown to empty more slowly from the stomach than an equivalent
amount of milk. This allows more time for intestinal lactase to split the
lactose in yoghurt, and, at least theoretically, would result in less lactose
reaching the colon.
•Supermarkets may carry milk that has had the lactose already split by the
addition of lactase. Substitutes for milk also are available, including soy
and rice milk. Acidophilus-containing milk is not beneficial since it
contains as much lactose as regular milk, and acidophilus bacteria do not
split lactose.
• For individuals who are intolerant to even small amounts of lactose,
the dietary restrictions become more severe. Any purchased product
containing milk must be avoided. It is especially important to eliminate
prepared foods containing milk purchased from the supermarket and
dishes from restaurants that have sauces.
2- Lactase enzyme
Caplets or tablets of lactase are available to take with milk-containing
foods.
3- Adaptation
Some people find that by slowly increasing the amount of milk or milkcontaining products in their diets they are able to tolerate larger amounts
of lactose without developing symptoms. This adaptation to increasing
amounts of milk is not due to increases in lactase in the intestine.
Adaptation probably results from alterations in the bacteria in the colon.
Increasing amounts of lactose entering the colon change the colonic
environment, for example, by increasing the acidity of the colon. These
changes may alter the way in which the colonic bacteria handle lactose.
For example, the bacteria may produce less gas. There also may be a
reduction in the secretion of water and, therefore, less diarrhea.
4- Calcium and vitamin D supplements
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