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Summary Animal Nutrition-ANU20306
- Instelling
- Wageningen University (WUR)
A summary of the chapters 3,4,5 of the reader for ANU
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Animal NutritionChapter 1:
Chapter 3 Digestion and absorption
Animals can be classified into three groups based on their digestive tracts: 1) simple nonruminants 2)
ruminants 3) nonruminant herbivores. Nonruminants have a single stomach and do not rely on
microbial digestion. They digest by secreting digestive enzymes into the GI tract. Ruminant animals
rely on microbial digestion in the large part of their compartmentalised stomach. Nonruminant
herbivores have digestive tract modifications to facilitate microbial fermentation. They can be
divided into foregut, colon and caecal fermenters. Ruminants consuming relatively low quality diet
with high amounts of indigestible material have a highly developed GI tract with large compartments
for microbial fermentation. Carnivores have a very simple GI tract, because meat is nutrient dense.
The liver secretes bile into the duodenum. Either directly or indirectly via the gall bladder. The gall
bladder stores bile and releases it when food is present in the duodenum. Bile salts are necessary for
lipid and fat-soluble vitamin absorption. Bile salts also promote pancreatic lipase activity and
resynthesis of triglycerides in the intestinal mucosal cells. The pancreas have an exocrine and
endocrine function. The exocrine part releases digestive juice into the duodenum that contains
bicarbonate and digestive enzymes. The endocrine part produces insulin and glucagon, which play an
important part in carbohydrate (sugars) metabolism.
The objective of digestion is to convert complex nutrients into compounds that can be absorbed by
the animal. The order of digestion is different for the different digestive tracts. In carnivores like cats,
hindgut fermentation does not play a role, whereas cattle have both foregut and hindgut
fermentation. Horses are only hindgut fermenters. Chickens store their ingested food in the crop,
then it moves to the proventriculus where the digestive processes are similar to those occurring in
monogastric stomachs. Afterwards the food moves to the gizzard where it is reduced in size before
moving on to the small intestine.
Micro-organisms contribute to the digestion process by enzymatic degradation of dietary
components, detoxification of toxic factors and by the synthesis of vitamins (B and K). A negative
effect of micro-organisms is that they use a part of the dietary energy and nutrient for themselves,
also they can produce metabolites that can be harmful to the intestinal wall and physiology of the
animal. The microbial composition depends on the place in the GI tract, differences within animal
species (age, genetic background). Stress affects microbial composition through gut motility and
secretion of enzymes that have less availability of nutrients or absorption of nutrients as a
consequence. Disturbances in the GI microbial population are usually temporary. The amount of a
nutrient that is ingested is very important in indicating the amount of nutrient that can be delivered
to the animal, but also it’s digestibility which is also depends on the animal’s characteristics (species,
age) or the environmental circumstances (water temperature in fish).
Absorption is the process where nutrients cross the cellular lining of the GI tract. Most absorption
happens in the jejunum (small intestine). Ruminants have similar digestive and absorptive processes
happening in the small intestine, but also the absorption of volatile fatty acids and ammonia in the
rumen. Absorption can take place by 1) passive diffusion, 2) active or carrier-mediated diffusion , 3)
pinocytosis. Passive transport can happen through the cell membrane, protein channels or facilitated
by carrier-mediated proteins. Some sugars and most amino acids move across a concentration
,gradient by the use of energy. A carrier with two binding sites moves Na+ or H+ on one side and the
nutrient on the other side. The ion moves down it’s concentration gradient across the intestinal
membrane, depositing the nutrient and ion inside the cell. Immunoglobulins in colostrum are
absorbed by pinocytosis, where a cell engulfs large molecules in solution.
Proteins must be hydrolysed before the dipeptides, tripeptides and amino acids can be absorbed.
Pancreatic proteases convert proteins into oligopeptides, brush border peptidases convert these
oligopeptides into amino acids, di- and tripeptides. Only in early postnatal life polypeptides and
proteins can be transported as whole across the epithelium. Later the smaller particles are moved
across the membrane by sodium-dependent active amino acid transporters. Some amino acids
compete with each other for transport.
Lipids are emulsified by bile salts, which allows for greater exposure for pancreatic lipases. Pancreatic
lipases convert triglycerides into monoglycerides and free fatty acids. These monoglycerides and free
fatty acids form micelles , the micelles join with cholesterol and fat-soluble vitamins to form more
complex micelles. They form micro-emulsions that render the lipid ready for absorption as hydrolysis
by pancreatic lipase proceeds. The main site for absorption is the proximal part of the jejunum.
Glycerol and short-chain fatty acids are absorbed by passive transport. Monoglycerides and long-
chain fatty acids enter the brush border by diffusion. Micelles diffuse among the microvilli, as
digestion products are absorbed from the micelles, more digestion products slowly partition out of
the micelles. Before leaving the mucosal cell, the mixed lipid droplets become coated with a thin
layer of protein. The droplets are now called chylomicrons. They consist mainly of triglycerides with
some phospholipids, cholesterol and protein. Mammals absorb most of their long-chain fatty acids
into the lymphatic system. Absorbed lipoproteins are carried in the blood as chylomicrons and
lipoproteins. Free fatty acids are transported as a complex with albumin. Chylomicrons and other
lipids are removed rapidly from the blood by the liver, fat depots and other tissues.
Non structural carbohydrates such as starch, sucrose, lactose and maltose are easily digested by
enzymatic hydrolysis. Sucrose is digested by sucrase into glucose and fructose. Pre-ruminant calves
show difficulty with sucrose. Lactose (milk sugar) is digested by lactase into glucose and galactose.
Young animals have a large capacity for lactase, which declines with age. Poultry do not produce
lactase. Maltose is the main product in the breakdown of starch to glucose. Young animals have
difficulty with the breakdown of starch, because they have little amylase to break down starch. They
prefer disaccharides. Most sugars require active transport across the gastrointestinal wall. Glucose
and galactose are absorbed through an active transport system using specific carrier proteins and a
sodium gradient. Fructose is absorbed by a separate carrier system that is not sodium dependent.
Ruminants do not absorb large quantities of monosaccharides, because the microorganisms degrade
them in the rumen to convert them into volatile fatty acids. Glucose can be dissolved in blood.
Non-starch polysaccharides (structural carbohydrates) cannot be hydrolysed by enzymes produced
by mammals or birds themselves. Animals are dependent on microbial fermentation to use the
energy in structural carbohydrates. Example of NSPs are cellulose, hemicellulose and pectin.
Cellulose can be hydrolysed by cellulase. Hemicelluloses are encrusted with lignin, which prevents
degradation by microbes and are thus less hydrolysable. When breakdown of structural
carbohydrates occur they are mainly converted into volatile fatty acids (acetic, propionic and butyric
acids). The composition of feedstuffs considerably influence the digestibility of NSPs. Many structural
carbohydrates encapsulate other feed components (protein, minerals) rendering them also
unavailable for digestion. The volatile fatty acids are mostly absorbed in the rumen. The rate of
volatile fatty acids absorption increases with chain length and decreases with pH. Butyrate is oxidized
by the intestinal mucosa and serves as the preferred energy substrate of colonocytes in the hindgut
, fermenters. In ruminants butyrate is extensively metabolised within the rumen and abomasum’s
epithelial cells to form acetoacetate and beta-hydroxy butyrate. Any butyrate entering circulation is
either oxidized or contributes to fatty acids synthesis. A negligible amount of propionate is oxidized.
All VFA except for acetate is removed from the blood by the liver.
Fat-soluble vitamins (A, D, E, K) require bile salts and fat to form micelles for absorption, they are
then passively absorbed. They are then transported to the liver by the lymph system as chylomicrons.
Water-soluble vitamins are absorbed in the small intestine into the portal blood via different
transport systems. Most are absorbed via active transport, but passive diffusion also takes place.
The availability of minerals for absorption is dependent on age, species, presence of interfering
substances as phytate or certain fibres, mineral-mineral interaction. The absorption of minerals and
trace elements mainly takes place in the small intestine, although in ruminants Mg absorption in the
rumen also be important. Minerals are transported as compounds and within/ bound to compounds.
Factors affecting digestibility
Animals may differ in the amount of digestive juices secreted or their microbial population.
Differences between species also exist of course. Ruminants are better at digesting a diet high in fibre
than non-ruminants.
If the feeding level is increased, the feed moves more rapidly through the GI tract, decreasing the
exposure to the action of digestive enzymes. This has the most negative effect on more slowly
digested components. The digestibility of a feedstuff is closely related to its chemical composition.
Feedstuffs that show little variation in composition between one sample and another, will show little
variation in digestibility.
The digestibility of one ingredient or feedstuff is influenced not only by its own composition but also
by the composition of other dietary ingredients consumed with it.
The diet should be prepared in a way to increase digestibility for that particular diet.
Enzyme preparations may be added to the diet to increase digestibility.
Nutritionally active factors (NAFs) depress the availability of nutrients for the animal. After ingestion,
they can produce effects such as depressing feed intake, retarded growth, poor feed conversion.
Classification of NAFs can be as follows:
1. Factors depressing digestion or metabolic utilisation of proteins:
a. Protease inhibitors
b. Lectins’
c. Polyphenolic compounds (tannins)
d. Phytate
2. Factors depressing the digestion of carbohydrates
a. A-amylase inhibitors’
b. Polyphenolic compounds (tannins)
3. Factors affecting mineral availability for absorption
a. Phytate
b. Oxalic acid
4. Factors that alternate the immune system
a. Antigenic proteins
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