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Digestion and Absorption
Introduction
Digestion is a very important topic
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The gastrointestinal system is regarded as the portal through which these
food molecules enter the body
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In the small intestine, absorption occurs at the epithelial junction of the small intestine cells
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Moreover, epithelial
cells have microvilli which together form the brush border
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Carbohydrate Digestion
The main dietary carbohydrates are polysaccharides, disaccharides, and monosaccharides
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In glycogen, the
majority of glucose molecules are in long chains (glucose molecules in 1,4 α linkage),
however, there is a minority of chain branching (produced by 1,6 α linkages)
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Carbohydrate digestion is carried out by salivary and pancreatic enzymes (α-amylases) and a
variety of oligosaccharides of the small intestine walls
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Glucose
Maltose,

Maltase


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Intestine
mucosa

Lactose

Glucose and Galactose

Sucrose,
Sucrase


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maltose,

Glucose

maltotriose

Starch is attacked by the amylase of the salivary gland Ptyalin
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7, and stomach acid inhibits the enzyme’s actions when food enters the
stomach
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Both the salivary and pancreatic α-amylases hydrolyze 1,4 α linkages,
however, leave 1,6 α linkages, terminal 1,4 α linkages and the 1,4 α linkages next to
branching points
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The oligosaccharides which are responsible for further digestion of starch derivatives are
found in the brush border on the surface of the mucosal cells, principally in the ileum
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Most of the glucose molecules
which are formed enter the mucosal cells, a minority of glucose molecules reenter the
intestinal lumen and are absorbed farther along
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Deficiency of one or more of these disaccharidases leads to diarrhea, bloating

and flatulence after ingesting sugar
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The bloating and flatulence happen because
of the production of gas (CO2 and H2) from disaccharide residues in the lower small intestine
and colon
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Summary of Carbohydrate Digestion
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Main dietary carbohydrates = polysaccharides (starch), disaccharides (lactose and
sucrose) and monosaccharides (fructose and glucose)
Digestion happens through salivary and pancreatic enzymes (α-amylases) and
oligosaccharides
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Glucose and galactose are absorbed by a process which involves active
transport
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Some of the transport of sugars is uniquely affected by the amount of sodium present in the
intestinal lumen
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Intracellular sodium is low, so it moves into the cell along
its concentration gradient
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Sodium is transported into the lateral intercellular spaces and the glucose diffuses
into the interstitum and then to the capillaries
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This
maintains the concentration gradient across the luminal border of the cell in order for more
sodium and hence more glucose to be able to enter
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Fructose makes use of a different carrier, thus its absorption is independent of sodium or
the transport of glucose and galactose
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Pepsins are secreted in their inactive precursor form (proenzyme),
which is activated in the intestinal tract
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Human gastric mucosa consists of 3
chromatographically distinct pepsinogens, and these produce 3 pepsins which have slightly
varying properties
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This peptic digestion
produces polypeptides of very diverse sizes
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6 to 3
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5
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The pancreatic carboxypeptidases and intestinal aminopeptidases and dipeptidases are
responsible for further protein digestion into smaller peptides and free amino acids
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Endopeptidases cleave polypeptides at
the interior peptide bonds
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Carboxy and aminopeptidases are subtypes of exopeptidases
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These exopeptidases are located
in the brush border of the mucosal cells
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g carboypeptidases (cleave at
carboxyl end) and aminopeptidases cleave at amine end – these enzymes found at
brush border of mucosal cells
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L-Amino acid absorption occurs more rapidly than the
D-amino acids
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There are three different transport systems: One transports neutral amino acids; one
transports basic amino acids; one transports proline, hydroxyproline and some other
compounds
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A big difference, however, between sugar influx and amino acid
influx is that sodium increases the maximum velocity of sugar influx without affecting the
apparent affinity of its carrier, whereas sodium has no effect upon the maximum velocity of
amino acid influx but seems to increase carrier affinity
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In addition, some absorbed di- and tripeptides are able to enter the portal blood
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50% of digested protein originates from ingested food, 25% from digestive juice
proteins and another 25% from desquamated mucosal cells
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Protein contained within the
stool has come from bacteria and cellular debris
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In the case of infants, some undigested proteins are also absorbed
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Foreign proteins entering the blood circulation
stimulate antibody formation
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The products of its actions are free fatty acids and 2-monoglycerides
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The vast majority of dietary cholesterol is in
the form of cholesterol esters, and these esters are hydrolysed by pancreatic esterase
within the intestinal lumen
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Bile salts on their own are unable to effectively be emulsifying
agents, however, when in the presence of the phospholipid and monoglycerides, particles
which are 200-5000nm in diameter are formed
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They consist of fatty acids, monoglycerides
and cholesterol
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The lipids diffuse out of the micelles and a saturated aqueous solution of the
lipids is maintained in contact with the brush border of the mucosal cells
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The rate of
uptake of bile salts by the jejuna mucosa is low, thus the bile salts diffuse back into the
intestinal lumen, where they are available for the formation of new micelles
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Steatorrhea is partly due to lipase deficiency and also partly due to depressed micelles
formation which comes about due to the absence of the bicarbonate normally secreted
from the pancreas, the relatively acid milieu in the duodenum inhibits the incorporation of
fatty acids in the micelles
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Fatty acids consisting of less than 10-12 carbon atoms pass from mucosal
cells directly into the portal blood, where they are transported as free (unesterified) fatty
acids
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Both the triglycerides and cholesterol esters are then
coated with a layer of lipoprotein, cholesterol and phospholipid to form chylomicrons
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Most of the triglyceride in mucosal cells is formed through the acylation of the absorbed 2monoglycerides, primarily in the smooth endoplasmic reticulum
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Glycerophosphate is also converted into glycerophospholipids which participate in
chylomicron formation
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Carbohydrate moieties are added to the
proteins in the Golgi apparatus, and the completed chylomicrons are extruded by exocytosis
from the basal or lateral aspects of the cell
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On moderate fat intake, over 95% of ingested fat
is absorbed
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The fat absorption
processes are not fully mature at birth, infants usually fail to absorb 10-15% of ingested fat,
hence they are more vulnerable to the adverse effects of disease processes reducing fat
absorption

Fat Absorption Summary
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Sunstances from micelles passively diffuse into mucosal cells – small fatty acids go
into portal blood as free fatty acids – large fatty acids reesterified to triglycerides –
converted to chylomicrons and enter lymphatics
Triglycerides formed through acylation of 2-monoglycerides in SER and
glycerophosphate
Chylomicrons exocytosed from basal or lateral aspects of cell
Most fat absorption in upper small intestine, some in ileum – 5% stool is fat
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98% of this fluid is reabsorbed, and only 200 ml of
fluid is lost daily in stools
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Na +
diffuses into and out of the small intestine depending on the salinity of the intestinal
contents, it is also actively transported out of the lumen in the small intestine and colon
through pumps on the basilateral walls of the cells
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In the small intestine, active transport of Na+ is vital in bringing about the absorption of
substances such as glucose and amino acids
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This is the reason why Na+ and water loss in diarrhoea is
treated by oral administration of solutions consisting of NaCl and glucose
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The osmolality of the duodenal contents could
be either hyper of hypotonic depending on the ingested meal, but by the timet the meal
enters the jejunum, the osmolality is almost equivalent to that of the plasma
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In the colon, Na + is pumped
out and water moves passively with it, again along the osmotic gradient
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Absorption of Vitamins and Minerals
Vitamins
Water-soluble vitamin absorption is very quick, but in contrast the absorption of fat soluble
vitamins A, D, E and K is deficient if fat absorption is depressed due to lack of pancreatic
enzymes or if bile is excluded from the intestine through the obstruction of the bile duct
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This vitamin binds to intrinsic factors, which are proteins secreted in the
stomach, and the complex is absorbed across the ileal mucosa
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Calcium
30-80% of ingested calcium is absorbed
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This is facilitated by 1, 25dihydroxycholecalciferol, which is the metabolite of vitamin D produced in the kidney
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The
production rate of 1, 25-dihydroxycholecalciferol is enhanced when the plasma Ca2+ is
decreased and reduced when the plasma Ca2+ is elevated
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Ca2+ absorption is also facilitated by lactose and
protein, but is inhibited by phosphates and oxalates as these form insoluble salts with Ca2+
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Iron
The loss of iron from the body is usually very little
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6 mg/d, and the
number is double in women due to blood loss in menstruation
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Hence, the amount of iron absorbed normally ranges from
about 3 to 6% of the amount ingested
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In the stomach, only a trace of iron is absorbed, however, the gastric
secretions dissolve the iron and provide a milieu favourable to its reduction to the Fe 2+
form
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Ascorbic acid and other
reducing substances in the diet facilitate conversion of ferric to ferrous iron
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Iron absorption is an active process which mostly takes place in the upper part of the small
intestine
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The mucosal cells pass part of the

iron directly into the bloodstream, however, most of it is bound to the protein apoferritin,
which combines with iron to form ferritin
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Iron forms a micelle of ferric hydroxyphosphate, and in ferritin, the subunits
surround this micelle
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The absorption of iron into the bloodstream is enhanced when body iron stores are
depleted or when erythropoiesis is increased, and decreased in the opposite conditions
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Iron in the mucosal cells stays bound in
ferritin and is lost with the cells when they are shed into the intestinal lumen and passed in
the stool
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This is known by a term ‘mucosal block’
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If more iron
is absorbed than excreted, it results in iron overload
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Large deposits of ferritin and hemosiderin
are associated with hemochromatosis, which is a syndrome characterised by pigmentation
of the skin, pancreatic damage with diabetes, liver cirrhosis and gonadal hypertrophy

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