CHAPTER 4
The Gastrointestinal Tract and
Nutrition
Introduction
• Many of the organic components of food
are in the form of large insoluble
molecules which have to be broken down
into simple compounds before they can
pass through the mucous membrane of
the alimentary cannel into the blood and
lymph. The breaking down process is
termed digestion, the passage of the
digested nutrients through the mucous
membrane absorption.
Digestion in monogastric mammals
• GI tract = gastrointestinal tract
• Digestion: mechanical forces
– chemical action
– hydrolysis of ingesta
• Absorption:
– small molecules from lumen →
– mucosal surface →
– blood, lymph
• monogastric (nonruminant) , ruminant.
• Carnivores:
– relatively short and uncomplicated intestine:
dog, mink.
• Omnivores:
– long small intestine, moderately caecum and
a sacculated large intestine.
• Coprophagy (feces eating) :
• rabbit
– soft feces, hard feces
– supply of vitamins and amino acid
GI tract of nonruminant
• Stomach:
• 1. oesophagus region:
– no gland, α-amylase activity continue, active
microbial population (mainly Lactobacillus,
Streptococci)
• 2. cardia region:
– produce gel- forming glycoprotein(mucus),
protecting the stomach,
• 3. fundus gland
region:
– peptic cells produce
proteolytic enzyme,
parietal cell secrete
HCl
• 4.pyloric region:
– like those in the cardia
region.
• Gastric juice
– water,
– pepsinogen,
– inorganic salts
– mucus
– HCl
– intrinsic factor.
• Four pepsins have been found in the pig
which have optimum pH with 2.0 and 3.5.
Pepsin preferentially attack those peptide
bonds of aromatic amino acids,
(phenylalanine, tryptophan, and tyrosine).
Rennin or chymosin are also occur in the
gastric juice.
Small intestine and digestion
1. duodenum:
– covered with finger like, or plate or tongue like
villi, brush border enzyme.
– The duodenum is the site for mixing digesta
and secretion and the jejunum being the site
of absorption.
• 2. jejunum
• 3. ileum
• Bile is secreted by the liver and passes to
the duodenum through the bile duct. It
contains the sodium and potassium salts
of bile acids, chiefly glycocholic and
taurocholic, phospholipids and bile
pigments.
• The bile salts play an important part in
digestion by activating lipase and
emulsifying fats.
• Pancreas is a gland which lies in the
duodenal loop and two secretory functions.
( Insulin and digestive enzymes)
Secretion stimulate route:
• Acid enter the duodenum→
• secretin is liberated from the epithelium of
the small intestine into blood →
• when it reach the pancreatic circulation,
stimulates the pancreatic cells to secrete
a water fluid containing bicarbonate ions
and little enzyme.
• Cholecystokinin(CCK) is liberated from
the mucosa when peptides and other
digestive products reach the duodenum.→
• CCK stimulates the secretion of
proenzyme into pancreas juice.
pancreas juice
• proenzyme such as trypsinogen
chymotrypsinogen
• Procarboxypeptidases A and B
• proelastase,
• α-amylase
• lipase
• lecithinases
• nucleases
• The inactive zymogen is converted to the
active enzyme by enterokinase, an
enzyme liberated from the duodenal
mucosa.
• Digestion in the small intestine include
– lumen digestion.
– membrane digestion.
• The hydrolysis of oligosaccharides to
monosaccharides and of small peptide to
amino acids is brought about by enzyme
associated with the intestinal villi. (called
brush border enzymes)
• Enzymes produced are sucrase, maltase,
lactase , oligo-1,6-glucosidases, and
aminopeptidase.
Large intestine and digestion
• Absorption of water
• secretion of inorganic element
• Extensive bacterial fermentation , Slow
rate of passage and abundant nutrient
sources encourage the prolific growth of
bacteria.
• Cecum, colon,rectum
•
•
•
•
•
Bacteria:
Indole, skatole
phenol, H2S
amines, NH3,
volatile fatty acids(VFA , include acetic
acid, propionic acid, butyric acid),
• H2, B vitamins.
GI tract of avian (fowl)
•
•
•
•
•
•
•
crop: fermentation (lactobacillus)
proventriculus: gastric juices
gizzard:
small intestine: enzyme
Cecum(ceca): bacterial fermentation
colonic – recturm
The digestive enzyme secretion are similar to
those of mammals. But lactase not been
detected.
Absorption of digested nutrients
• The duodenum has villi is primarily a
mixing and neutralizing site and the
jejunum is the major absorptive site.
• Role of the GI tract in transport of nutrients
• The passage route of nutrients from the
intestine lumen into the intestinal
epithelial cell and then into the blood or
lymph by active, passive transport.
•
Passive transport :
– passive diffusion(high concentration toa low
concentration diffusion)
– facilitative transport (carrier transport the
molecule down its concentration gradient).
•
Active transport:
– the carrier has two specific binding sites and
the nutrient is attached to one of these while
other site picks up a sodium(in case of
monosaccharides and amino acids) or a
hydrogen ion (in case of dipeptides).
– Sodium pump.
– Na/K transporting ATPase.
•
pinocytosis ( cell drinking),
phagocytosis
•
pinocytosis occurs in new born animal to
absorption of immunoglobulins, proteins ,
peptide from colostrum.
*transport route
1.
2.
3.
4.
penetration of the microvillus
migration through the cell interior
possible metabolism within the cell
extrusion from lateral and basal of the
cell
5. passage through the basement
membrane
6. penetration through the epithelium into
blood or lymph.
Blood and Nutrition
• Blood is the vehicle for transport of
nutrients and metabolites among organs,
tissues and cells of the body, Nutrients
transport by carrier or binding protein.
GI tract of ruminant
1. Saliva:
– 150L (cow), 10L(sheep), eating and
ruminating produced.
– Provide a source of N (urea and
mucoproteins). P, Na, Buffered
– maintaining a pH in the rumen
2. Stomach
(rumen, reticulum, omasum, and abomasum) .
• Rumen:
– high population of microorganism(1010 –
1012)fermentation.
• Reticulum:
– moving food into
rumen or into the
omasum
• Omasum:
– reducing particle size,
control passage of
ingesta.
• Abomasum:
– glandular stomach
• The reticulo-rumen provides a continuous
culture system for anaerobic bacteria,
protozoa and fungi.
• Food and water enter the rumen and the
food is fermented to yield volatile fatty
acids, microbial cells and gas methane
and carbon dioxide.
• The acids produced by fermentation are
capable of reducing the pH of rumen liquor
to 2.5-3.0, but under normal condition the
pH is maintained at 5.5-6.0.
• reticular groove ( esophageal groove)
– Begins at the lower end of the esophagus and,
when closed, forms a tube from the
esophagus into the omasum.
– Allow milk bypass the reticulorumen, and
escape bacterial fermentation.
– It is stimulated by the sucking, certain ions,
solid in liquid
• rumination:
– semi-liquid materials regurgitate up the
esophagus, swallowed the liquid, and
remastication of and swallowed the bolus.
– 8h/day
– inhibition of rumination result reduced in feed
intake
• Eructation:
• contraction of the upper sacs of the rumen
which force the gas forward and allow the
gas to escape.
Rumen microorganisms
• The bacteria number 109-1010 per mL of
rumen contents.Over 200 species have
been identified.
• Typical rumen bacteria showed in Table
8.3
• Protozoa are present in small numbers
(10 6 per mL), being lager, may equar the
bacteria in total mass.
Fungi
• strictly anaerobic.
• The fungi are capable of utilizing most
polysaccharides
• many soluble sugars, but not use the
pectin, polygalacturonic acid,
arabinose,fucose, mannose and galactose.
• As the microbial mass synthesized in the rumen
provides about 20 % of the nutrients absorbed
by the host animal, the composition of
microorganisms is important.
• The bacteria dry matter contains about 100 g/kg,
but only 80 % of this is in the form of amino
acids, the remaining 20% being present as
nuclic acid N.
• Some of the amino acids are contained in the
peptidoglycan of the cell wall membrane and are
not digested by the host animal.
Digestion of carbohydrates
• All the carbohydrates, but not lignin, are
attacked by the rumen microorganism.
• The breakdown of carbohydrates in the
rumen may be divided into two stages:
• First stage:
– Digestion of complex carbohydrates to simple
sugars (brought by extracellular microbial
enzymes). That as analogous to the digestion
of carbohydrate in non-ruminants.(Fig. 8.2)
• Second stage:
• The pathways involved are similar to those
in the metabolism of carbohydrates by the
animal itself.(Fig 8.3)
• Three volatile fatty
acids
– acetate 65%,
propionate 21%,
butyrate 14%.
• The total weight of
acids produced may
be as high as 4 kg per
day in cows.
• Much of the acid produced is absorbed
directly from
– rumen, reticulum, omasum,
• 20% may pass through the abomasum
and be absorbed in the
– small intestine.
• The rate of gas production in the rumen
may exceed 30 L/hour. It include
– 40% CO2, 30-40% methane, H2 5%, small
amount of O2 and nitrogen.
• Most of the gas produced is lost by
eructation, if gas accumulates it causes
bloat, in which the distension of the rumen
may be result in the collapse and death of
the animal.
Digestion of protein
• The digestion of protein in the rumen is
shown in Fig.8.4.
• Food proteins are hydrolysed to peptides
and amino acids by rumen
microorganisms, but some amino acids
are degraded further, to organic acids,
ammonia and carbon dioxide.
• The ammonia together with some small
peptides and free amino acids is utilized
by the rumen organism to synthesis
microbial protein.
• Some of the microbial protein is broken
down in the rumen and its nitrogen is
recycled.
Utilisation of NPN compounds by
the ruminant
• Dietary protein is not the only contributor
to the ammonia pool in the rumen. As
much as 30% of the nitrogen in the
ruminant foods may be in the form of
simple organic compounds such as
nitrates and amines or nitrates. Most of
these are readily degraded in the rumen,
and nitrogen entering the ammonia pool.
• Urea in the rumen is rapidly hydrolysed to
ammonia by bacterial urease. This NH3 to
be efficiently incorporated in microbial
source of energy for protein synthesis). It
protein. (must have a readily available is
important to avoid over consumption of
urea, since a rapid absorption of NH3,
hence increase blood NH3 concentration
to toxic level.
Digestion of lipids
• The TG present in the foods contain a high
proportion of the C18 polyunsaturated acids
be hydrolysed in the rumen by bacterial
lipase. Unsaturated fatty acids are
hydrogenated to monoenoic acid and to
stearic acid.
• The capacity of rumen microorganisms to
digest lipids is strictly limited. If fat level
increase above 100 g/kg, the fermentation
activity of the microbes are reduced.
• Synthesis of vitamins :
• vitamin B complex and K.
• vit B12 will need sufficient Co.
Dynamics of digestion in the
ruminant
• Food enter rumen→
1. soluble constituents (sugars) are quickly
degraded by the microbes.
2. insoluble constituents are colonized by
microbes and slowly broken down.
3. the component will consist of cell wall
encrusted with lignin or silica as to be
undegradable in the rumen.
• The movement of digesta from the rumen
(the particale size is to be 3-4 mm) can be
expressed as a rate of passage or
retention time.
• Typical rates of passage of particular
matter are 0.012-0.030 per hour, and
retention times, 30-80 hours.
Control and modification of
rumen fermentation
• Alter the pattern of digestion that should
improve the nutrition of ruminants.
• 1. To modify the microbial population in
order to suppress undesirable
process( methane production) or desirable
process( microbial protein). Microbial
population, genetic modified bacteria,
change protozoa population.
• 2. To protect nutrients from rumen fermentation
in order that they should be digested in the small
intestine.( control of rumen fermentation). Based
on heat treatment, chemical treatment . By-pass
carbohydrate or protein.
• Need to synchronise the supply of energy and
protein to the microorganism. For example, to
supplement a rapidly degraded source of
protein with a rapidly degradable source of
carbohydrate .
Alternative sites of microbial
digestion
• The microbial fermentation in the large
intestine is similar to that in the rumen.
VFA are produced and absorbed, methane
and other gas are present.In general, hind
gut fermentation is less effective than
rumen digestion, many of the product of
digestion are not absorbed.
• Practising coprophagy (the consumption of
faeces) in rabbits.
• Poultry have two caeca and a colon in
which to ferment residues, they gain little
of nothing from hind gut fermentation.
Blood evaluation in nutrition
• Anemia:
– Develop as a result of deficiencies of Fe, Cu,
Co, Vit B12, folic acid, or protein
– Confoirmed by
• Presence of low PCV,
• low hemoglobin content in blood,
• Microscopic exam of red cells
• blood component assay
73
•As the rate of migration of
the enterocytes up the villi
increases, the enterocytes are
shed from the top of the villi
at a greater rate; therefore,
the number of mature enterocytes decreases.
74