Digestion
in
Poultry:
Proteins
and
Fats
The complex components of poultry diets must be sequentially degraded by

complex system of enzymes

other secretions delivered into the gut lumen
The digestion of fats and proteins is somewhat similar in that digestive activity
occurs predominantly in the duodenum and small intestine where most of the pH
sensitive enzymes are produced. The generic enzyme secretions necessary for
protein and fat digestion are detailed, together with description of overall
digestibility expected for an array of the most common feed ingredients.
Enzyme presence is rarely a limiting factor to digestion, although in the very
young chick there is an indication of marginal levels of lipase enzymes and
important co-factors necessary for digestion and absorption of fats and fatty
acids. Reduction in protein and amino acid digestibility is more commonly
associated with the presence of anti nutritional factors and/or the heat processing
applied to inactivate such diet contaminants. The role of such compounds as
tannins and antitrypsins, commonly found in some ingredients, is discussed in
relation to the potential for reducing their impact on digestion.
Introduction
Digestion is the process of breaking down large, complex molecules, as provided
by the birds’ feed, into smaller components that can be absorbed into the portal
blood system.
The process involves changes in both physical and chemical structures of most
dietary components.
Poultry feeds consist of a complex array of particles differing not only in chemical
composition, but also in size, hardness, solubility and ionic characteristics.
Under ideal conditions, this array of particles and chemicals with different
characteristics degrade slowly in a step-wise manner as feed passes from the
mouth to the large intestine. Particle breakdown is a constant process, although
the gizzard provides the major site of this activity.
Enzymes are largely responsible for molecular degradation, although their pH
greatly influences their efficacy.
It seems unlikely that sub-optimal digestion of any nutrient is caused by lack of
available enzyme. Most frequently, digestion is adversely affected by factors
which render unavailable the site of normal enzyme cleavage.
There is concern about the quantity of enzymes produced by very young birds,
and this becomes of significance as the regular market age of meat birds has
declined, so increasing the proportional time of juvenile digestive capacity.
When digestion is reduced, there will be reduced bird growth and/or increased
feed intake.
Indigestion may also cause problems with manure/litter management, because
non-digested residues in the large intestine often adsorb more water or produce
feces that are more viscous.
Table 1 shows an outline of normal digestive processes for proteins and fats,
together with detailed accounts of those factors that can inhibit digestion because
of chemical or physical change to the digesta.
1.1. Processes for Protein Digestion
Gastric Region:
There is no digestion of protein in the mouth or crop.
However, lubrication and softening of the feed by the mucus secreted by the crop
wall and saliva from the mouth.
The proventriculus represents the "glandular stomach" where digestive juices are
secreted.
The juices contain HCI and the enzyme pre-cursor (zymogen) pepsinogen which
is converted to the active enzyme pepsin as the pH of the feed decreases in the
proventriculus and gizzard.
The acid condition of the proventriculus and gizzard serve to break down the
protein to expose most of the pepsin-sensitive peptide bonds.
Pepsin enzyme catalyses the hydrolysis of proteins to peptides and, to a limited
degree, free amino acids. There is a wide range of specificities of peptide bonds
hydrolysed by pepsin, but those containing aromatic amino acids and/or leucine
and/or valine appear most susceptible.
G.I
Tract
Enzyme
(or
Substrate
End Product
pH
Region
secretion)
Mouth
Saliva
Lubricates and softens food
Crop
Mucus
Lubricates and softens food
4.5
Stomach
HCI
Lowers stomach pH
2.5
(Gizzard and
Pepsin
Protein
Polypeptides
Trypsin,
Proteins,
Peptones,
Chymotrypsin
Peptones
and amino acids
6.0
and Elastases
and Peptides
Peptides and amino
to
Carboxy-
Peptides
acids
6.8
petidases
Collagen
Peptides
Peptidases
Peptides
Dipeptides
Polynucleotidase
Nucleic acids
amino acids
to
Mono-nucleotides
6.6
proventiculus)
Duodenum
Peptides
Collagenase
Jejunum
and
5.8
Table 1. Protein Digestion in Poultry
In the gizzard, the ingesta is further mixed with the fluid secreted by the
proventriculus.
There is also mechanical grinding of the feed facilitated by ingredients, such as
insoluble grit, that may be added to the diet.
Grit provides additional surfaces for grinding as well as acting to stimulate
motility in the gizzard. The digestibility of coarse feed particles such as whole
grains, grains with a minimal amount of processing or pelleted feed is improved
by addition of grit in the diet.
However, protein digestibility of ground feed is little affected by the absence of a
fully developed gizzard.
Intestinal Region:
The pancreas produces fluids containing zymogens that are converted to their
active enzyme forms at the sites of digestion.
Trypsinogen is activated to trypsin in the duodenum by enterokinase, an enzyme
secreted from the intestinal mucosa.
Once started, this process is autocatalytic and the trypsin so formed initiates
activation of the other zymogens.
Thus, trypsin is central in the development of full proteolysis activity. The
consequence of trypsin inhibitors found in certain legumes and cereals is poor
digestibility of dietary proteins.
Trypsin, chymotrypsin and elastase catalyse the break down of proteins,
peptones and peptides into smaller peptides and amino acids in the duodenum.
Trypsins catalyse breakdown of bonds which involve lysine or arginine,
Whereas, bonds involving aromatic amino acid residues are susceptible to
chymotrypsin catalysis.
Elastases are relatively non-specific with regard to the type of peptide bonds.
The pancreas also secretes exopeptidases containing carboxypeptidases A and B
which catalyse the hydrolysis of the terminal bonds in polypeptide chains. They
hydrolyse the carboxyl- terminal ends of the peptides, thereby removing the
amino acid residues in sequence.
Carboxypeptidase A is inhibited in sequence hydrolysis by the proximity of
charged side-chains and proline, while carboxypeptidase B is also inhibited by
proline but requires cationic side-chains for maxima1 activity. The pancreatic
secretion also contains certain collagenases that catalyse the hydrolysis of
collagen to small peptides. The jejunal wall secretes the proteolytic enzymes
peptidase (erepsin) and polynucleotidedase which catalyse the hydrolysis of
small peptides into amino acids and dipeptides, and also convert nucleic acids to
mononucleotides, respectively, which can then be absorbed.
There seems to be an increase in enzyme activity as the bird gets older. Nitsan et
al. (1991) and Nir et al. (1993) suggest trypsin activity in the intestinal contents to
increase around lo-fold up to 30 days of age. Trypsin activities of 2-4 units/g
intestinal con-tents were recorded at Id of age, while values of around 30 units/g
were seen at 15-20d. Comparable values for chymotrypsin reported by these
same authors were 2-5 units/g at Id, and 6-14 units at 15-20d. Krogdahl and Sell
(1989) also reported a 70% increase in trypsin activity of birds from 1-21 d of age.
Peptide uptake is most rapid in the jejunum whereas amino acid uptake is most
rapid in the ileum (Crampton, 1972). Peptides are absorbed into the mucosal cells
by pinocytosis. Peptidases located in the cytoplasm of the mucosal cell complete
the peptide hydrolysis into free amino acids. Amino acid transport into the
mucosal cells is by active transport involving Na+, similar to that of glucose, with
different carrier systems for different categories of amino acids. The amino acids
enter the blood stream from the mucosal cell by facilitated diffusion as free amino
acids, with only trace amounts of peptides found in plasma.
The natural isomers (L-amino acids) are generally absorbed more rapidly than
the D-forms. A common but L-preferring site exists for the transport of both
isomers of methionine. Other neutral L-amino acids have a high affinity for this
site, whereas the D-isomers, except for D-methionine, have a very low affinity
(Lerner and Taylor, 1967; Griminger and Scanes, 1976).
1.2. Normal Ranges of Protein Digestibility
Feedstuffs commonly used in poultry diets vary widely in content and
digestibility of protein. There is also significant variance in values reported for
protein digestibility of the same feedstuff. Digestibility of protein in different
samples of hydrolysed feather meal was, for example, estimated to be from 32 to
76% (Han and Parsons, 1991). Different factors, such as strain of plant, and source
and processing conditions in case of animal feedstuffs, are likely the causes of
variation. Table 2 shows mean crude protein contents and ranges of digestibilities
for protein and selected amino acids in some common feedstuffs.
Dietary supplementation with synthetic amino acids or their analogues is a
common practice in com-mercial poultry production, Results of many studies
show that the digestibilities of such additives are much higher than for
comparable amino acids fed as intact proteins. Sibbald and Wolynetz (1985) using
cockerels, compared synthetic lysine (L-lysine HCl) to lysine provided by
conventional ingredients and obtained bioavailability of 93% for the synthetic
form as opposed to 88% for the natural form of lysine. Even higher digestibility
(up to 97%) has been shown for synthetic lysine when fed to broiler chickens.
Feedstuff
C.
Digestibility (%)
Protein
(%)
C.
Lys
Met
Cys
81
91
85
Protein
Vegetable sources (cereals)
Yellow
maize
8
82 - 86
Wheat
12
78 - 82
81
87
87
Barley
10
70 -82
78
79
81
Sorghum
10
67 - 72
78
89
83
Vegetable sources (oil seed meals)
Peanut
49
88 - 91
83
88
78
46
83 - 87
91
92
82
43
61 - 76
67
73
73
88
82-92
86
91
76
Fish meal1
66
86 - 90
88
92
73
Meat
60
75 - 80
79
85
58
87
36 - 77
66
76
59
meals
Soybean
meals
Cottonseed
meal
Animal sources
Blood
meal1
meal1
Feather
meal
1defatted
Table 2. Normal Crude Protein Contents and Digestibilities of Common Poultry
Feedstuffs
1.3. Causes of Reduced Protein Digestibility
Naturally Occurring Anti-Nutritional Compounds
Certain feedstuffs that are commonly used in poultry diets contain naturally
occurring compounds that significantly reduce their nutritional value. In most
cases this is due to reduced digestibility (Liener, 1980). The feeding value of grain
sorghum (milo) is markedly influenced by its tannin content that causes a
binding and precipitation of dietary proteins and digestive enzymes (Butler et al.,
1984). High levels of tannins (up to 5% in some sorghum strains) have been
shown to reduce dry matter and protein digestibilities in chicks (Nelson et al.,
1975; Singh, 1984; Halley et al., 1986), and consequently depress growth (Chubb,
1982). Armstrong et al. (1974) and Longstaff and McNab (1991) showed that
dietary tannins also reduce amino acid digestibility in chicks. More recent studies
by Douglas et al. (1993) showed that there is less adverse effects of tannins in
older turkeys over eight weeks of age. This is probably due to the more mature
digestive processes in older birds which are able to accommodate the antinutritional effects of tannins.
Feeding raw soybeans is well known to cause growth depression, poor feed
efficiency and pancreatic enlargement in young chickens, and also small egg size
in laying hens (Chubb, 1982). These effects are due to the anti-trypsins in
soybeans that reduce digestibility of proteins (Zhang and Parsons, 1993).
Antitrypsins inhibit the activities of the proteolytic enzyme trypsin, which results
in lower activities of other proteolytic enzymes that require trypsin for activation.
The growth depression in chicks is due to slow release of all essential amino acids
by proteolytic enzymes in the presence of the trypsin inhibitors (Chubb, 1982).
This is in contrast to the effects in rats where there is specific retardation of
release of mainly methionine. Heat treatment is known to be effective in
deactivating the anti nutritional compounds (Waldroup et al., 1985; Parsons et al.,
1992; Anderson-Hefermann et al., 1993). The trypsin inhibitor activity of
conventional soybean meal is substantially decreased during commercial heat
treatment and solvent extraction processing (Waldroup, 1982; Anderson-
Hefermann et al., 1993). Adding enzymes to soybean meal has also been shown to
improve its digestion and utilization Marsman et al. (1997). Cottonseed meal
contains gossypol which, when heated during processing, forms indigestible
complexes with lysine, (NRC, 1994). Processing methods such as prepress solvent
extraction significantly lowers the free gossypol content, thereby improving
lysine digestibility (NRC, 1994). The nutritive value of alfalfa meal is sometimes
influenced by its saponin content. The mechanism of action of saponins is not
entirely known, but they are thought to inhibit the functions of digestive
enzymes, and so lower the absorption of amino acids, sugars and other nutrients
(Krogdahl, 1986). Recent studies by Gerendai and Gippert (1994) showed a
decrease in crude protein digestibility by chickens that were fed alfalfa meal diets
containing 1.0% saponin as opposed to those fed 0.08% saponin. Normal saponin
content of alfalfa meal is about 1.0%.
Proteins found in animal hide, scales, hair, feather, and bone are not easily
digested and contain high concentrations of keratin and collagen. The major
component of feather meal, for example, is keratin, which necessitates that it be
prehydrolysed to make it digestible by poultry. Moran et al. (1966) reported that
heating feathermeal results in cleavage of disulfide bonds of cystine, allowing
better digestion by protease in growing chicks. Due to differences in processing
conditions, commercial feather meals vary widely in their protein digestibilities
(Papadopoulos et al., 1985, 1986; Han and Parsons, 1991). Burgos et al. (1974)
reported that the amino acids in feather meal are almost completely digestible
(95%), while other workers (Parsons et al., 1982; Sibbald, 1986) reported much
lower digestibilities of about 32%.
b) Effects of Processing
Excess heat applied during processing may either destroy or render unavailable
certain heat- sensitive amino acids thereby greatly reducing the nutritive value of
such feedstuffs. This can happen due to either very high processing temperature,
prolonged processing time or both (Clandinin et al., 1947; Moran et al., 1963;
Parsons et al., 1991). Many of the adverse effects of overheating result from
reduced protein quality and decreased amino acid digestibility. Parsons et al.
(1992) reported that the true digestibilities of several amino acids in soybean meal
decreased linearly with increased heating time at 1210C. The largest decreases in
digestibility were observed for lysine and histidine. This is in agreement with the
results of other workers who reported decreases in amino acid digestibilities as a
result of heat treatment under similar conditions with canola meal (AndersonHefermann et al., 1993) and sunflower meal (Zhang and Parsons, 1994). The
effects of prolonged heating on digestibility observed in soybean meal was
greatest for lysine where 90 minutes of heating reduced lysine digestibility by
about 60%. This situation becomes most critical for some vegetable ingredients,
such as sunflower meal, that are normally very deficient in total lysine.
The large decrease in lysine digestibility can largely be attributed to the formation
of Maillard reaction products created during heat treatment (Hurrell, 1990).
Many of the Maillard products result from the nonenzymatic browning reactions
between lysine and oligosaccharides such as fructose, stachyose, sucrose and
raffinose. Soybean meal contains substantial quantities of these soluble sugars,
making it very susceptible to Maillard reactions during heat treatment (Coon et
al., 1990; Araba et al., 1994). Chemical treatments, such as ethanol extraction of
soybean meal, have been reported to remove the oligosaccharides, thereby
decreasing the potential for lysine destruction occurring during heat treatment
(Hanock et al., 1990).
Measures of amino acid availability are further confounded by the production of
so-called early Maillard reaction products known as Amadori rearrangement
compounds. Many of the amino acids in Amadori compounds are released
during
acid
hydrolysis
and
therefore
detectable
by
ion-exchange
chromatography. But they are, however, unavailable to birds for protein
synthesis (Carpenter, 1973). These compounds have been isolated in heated
soybean meal (Parsons et al., 1992) and particularly affect availability of lysine,
cystine and histidine. Cystine digestibility may also be affected by heat treatment
through its conversion to lanthionine, a crosslinked sulphur amino acid that is
apparently indigestible (Robbins et al., 1980; Parsons et al., 1992). Although steam
pelleting is widely used in production of poultry feeds, there are some
inconsistencies in performance of birds fed mash versus pelleted diets (Sibbald,
1977; Araba and Dale, 1990). This effect has been attributed to occurrence of
Maillard reaction between free sugars and free lysine under the hot, moist
conditions of pelleting (Dale, 1992). The discrepancies in the response of feeding
pelleted diets may be due to differences in pelleting conditions and the soluble
sugar content of the feedstuffs. A commonly used protein source like soybean
meal that contains over 6% sucrose and other oligosaccharides may, therefore, be
more susceptible to reduced lysine digestibility during pelleting.
2. Fat
G.I
Tract
Enzyme (or
Substrate
End Product
pH
Region
secretion)
Mouth
Saliva
Lubricates and softens food
Crop
Mucus
Lubricates and softens food
4.5
Stomach
HCI
Lowers stomach pH
2.5
(Gizzard
and
Lipase
proventiculus)
Fats
Fatty
acids,
mono-
glycerides and glycerol
Duodenum
Bile
Fats
Emulsification
and Jejunum
Lipase
Fats
Fatty
Cholesterol
Fatty acid -
glycerides and glycerol
to
esterase
cholesterol
Fatty acid, cholesterol
6.6
acids,
mono-
5.8
esters
Table 3. Fat Digestion in Poultry
2.1. Normal Digestive Processes (Fat)
There is no evidence for hydrolysis of fats in the upper alimentary tract in
poultry. Although by analogy with the human, a limited acid-catalyzed
hydrolysis in the proventriculus and gizzard cannot be precluded. In carnivore
avian species, however, there is secretion of lipase from the walls of the stomach
which suggests the beginning of fat digestion (Ensminger, 1980). The digestion
and absorption of fats in the chicken occurs mainly in the small intestine (Table
3). Lipase secretion in the chick is active from a very early age and increases very
rapidly during the first three weeks (Laws and Moore, 1963). Duodenal activity of
lipase, for example, was shown to increase almost 100-fold between 4 and 21 days
post-hatch in the chicken (Noy and Sklan, 1994).
Fat digestion is enhanced by emulsification with bile salts secreted into the lumen
of the small intestine from the gall bladder. Lipases act as an oil- water interface
and this explains why emulsification is required for fat digestion. Pancreatic
lipase brakes down the emulsified fats into fatty acids, monoglycerides and
glycerol. Cholesterol esterase, also secreted from the pancreas, hydrolyses
cholesterol-fatty acid esters into cholesterol and free fatty acids. Short-chain fatty
acids and free glycerol are then absorbed directly into the mucosa of the small
intestine and transported to the portal circulation. Other free fatty acids,
monoglycerides and cholesterol molecules are emulsified by bile salts, forming
micelles which are essential for normal fat absorption. Water-insoluble
compounds such as polar unsaturated fatty acids and monoglycerides that
cannot form micelles alone, readily form stable mixed micelles with conjugated
bile salts. Saturated fatty acids such as palmitic and stearic acids on the other
hand, which are non-polar and that have high melting points are only slightly
soluble in emulsions with bile salts. They are, however, markedly solubilized in
the presence of a mixed micelle. In this form, the fatty acids and other lipid-like
materials are solubilized in the aqueous phase of the lumen and are transported
to
the
mucosal
cell
membrane.
Consequently,
the
levels
of
dietary
saturated:unsaturated fatty acids and bile salt secretion are important factors in
fat absorption.
Nitsan et al. (1991) reported levels of lipase in contents of the small intestine to
increase from 4 units/g at Id of age, up to 8 units/g at 20d. Nir et al. (1993)
reported comparable values of 0.8 and 2 units/g for Id and 15d old birds,
respectively. Krogdahl and Sell (1989) showed a 100% increase in intestinal lipase
activity in birds fed a high fat diet, while there was little change over time for
birds on a low fat diet. Green and Kellog (1987) indicated a relatively high level of
bile salts (13.9 mM/L) in intestinal contents of 2d old birds, and while this value
doubled over a 44d period, there was a noticeable decline (8.2 mM/L) at 9d of
age.
Micelles attach to the surface of the epithelial cells, where upon break-up, the
components are absorbed into the jejunum by passive diffusion. Once inside the
mucosal cells, the monoglycerides and fatty acids are re-esterified, and, together
with free and esterified cholesterol, lipoproteins and phospholipids, are
assembled into chylomicrons. In poultry, unlike mammals, lipoproteins rich in
resynthesised triglycerides are absorbed directly into the portal blood system and
transported to the liver. These types of lipoproteins have been referred to as
portomicrons as opposed to chylomicrons in mammals (Bensadoun and Rothfeld,
1972).
2.2. Normal Ranges of Fat Digestibility
Digestibility
Metabolizable
(kcal/kg)
energy
Fat
0-21d
>21d
0-21d
>21d
Tallow
80
86
7400
8000
Poultry Fat
88
97
8200
9000
Fish oil
92
97
8600
9000
Vegetable oil
95
99
8800
9200
Coconut oil
70
84
6500
7800
Palm oil
77
86
7200
8000
Vegetable
84
87
7800
8100
87
96
8100
8900
Type/Age
soapstock
Restaurant
grease
Table 4. Digestibility and Metabolizable Energy of Some Common Feed Grade
Fats
as
Affected by Age of Bird
Dietary fats vary appreciably in composition and in their contribution of energy
and fatty acids to poultry. The digestibility of fats by poultry has also been shown
to vary widely, depending upon many factors including type of fat, level of
saturation, age of bird, level of fat inclusion in the diet and presence of other
dietary components (Wiseman, 1984). The energy value of fats depends mainly
upon the digestion and absorption of fatty acids from the intestinal tract. Because
fatty acids are not excreted in the urine, their metabolizable energy values should
be directly related to their absorbability. Table 4 shows the effect of age of bird on
digestion and metabolizability of fats and oils that are commonly used in poultry
diets.
2.3. Causes of Reduced Fat Digestibility
a. Impurities and Anti-Nutritional Factors
Impurities are most often referred to collectively as M.I.U. (moisture, impurities
and unsaponifiables). Most of these compounds will have little nutritional value,
and so obviously digestibility values must be adjusted corresponding to total fat
content. During oxidation at both high and low temperatures, a vast range of
unusual polymers can be produced, and Wiseman (1986) has extensively
described their structure and formation and the adverse effect of feeding such
polymers in oxidized fats. Wiseman (1986) cites evidence for the dramatic effect
of oxidation of a fat caused by overheating on available energy where loss in
digestibility can reach up to 30%. A number of naturally occurring fatty acids can
also adversely affect overall fat utilization, although their mode of action is most
likely via general well-being of the animal rather than through any specific mode
of action related to digestion or absorption, etc. Two such components are erucic
acid present in rapeseed oils and some other Brassica sp., and the cyclopropenoid
fatty acids in cottonseed.
b. Fatty Acid Composition
Fat composition can influence overall fat digestion because different components
can be digested and/or absorbed with varying efficiency. It is generally
recognized that following digestion micelle formation is an important
prerequisite to absorption into the portal system. Micelles are conjugations of bile
salts, fatty acids, lipase, some monoglycerides and perhaps glycerol. The
conjugation of bile salts with fatty acids is an essential prerequisite for
transportation to and absorption through the microvilli of the small intestine. As
previously described, polar unsaturated fatty acids and monoglycerides form this
important association. However, already formed mixed micelles of unsaturates
and bile salts themselves have the ability to solubilize non-polar compounds.
Efficiency of fat utilization is, therefore, dependent upon there being an adequate
supply of bile salts and an adequate balance of unsaturates:saturates.
Consideration of the balance between unsaturated fatty acids, provided by most
oils, and saturates, found in animal fats can be used to advantage. For example,
Muztar et al. (198 1) indicate a significant synergistic effect from mixtures of
tallow and vegetable soapstocks in terms of energy contribution. In this study,
the digestibility of the fat mixture was improved by 4%, and assuming no effect
on soapstock utilization within the 50:50 mixture, then tallow utilization was in
fact increased by 8%. The same type of effect is seen with pure fatty acids (Atteh
and Leeson, 1985). This type of synergism can confound data concerning
estimates of fat digestibility. In such assays, when fat is substituted into a basal
diet, then differences in digestibility between the basal and test-fat diets are
assumed to relate solely to the test fat. When the test fat improves the digestibility
of fat or other nutrients in the basal diet, an erroneously high value is ascribed to
the test fat. Lewis (1989) concludes that such maximum synergism occurs with
fats added at about 3% of the diet and that ME values determined from
digestibility studies, rather than conventional ME studies, are "not applicable in
the real world." Similarly Ketels and DeGroote (1989) determined the optimum
ratio of unsaturates:saturates in terms of overall fat digestibility and ME.
Contrary to the conclusion of Lewis (1989), Ketels and DeGroote (1989) show a
similar ratio for optimum fat digestibility and fat ME at 3:1 (unsaturated:
saturated).
While unsaturated fatty acid content of a diet, therefore, has a marked effect on
overall fat digestion, there is often concern over a fat’s content of free-fatty acids
(EEA). Although a large proportion of fatty acids is released in the lumen after
hydrolysis, the presence of monoglycerides plays an important role in
solubilizing non-polar long-chain saturates. Sklan (1979) also showed that overall
absorption of fatty acids was highest in 3-week-old chicks fed triglycerides and
lowest when pure fatty acids were fed. This may be due to less efficient micelle
formation, or less bile production. Sklan (1979) suggests that when products high
in FFA content are used, such problems may be corrected by supplying a source
of monoglyceride. This type of research data suggests feed manufacturers should
be wary of fats containing high levels of FFA. However, Alao and Balnave (1985)
showed no difference in the utilization of tallow samples containing 2 versus 16%
FFA’s. These authors concluded that the level of linoleic acid and/or unsaturates
in tallow probably has a larger effect on its digestion than does the level of FFA’s.
Another important factor associated with micelle formation is availability of bile
salts. The age-related effect on digestion of fats is partly accounted for by
inefficient bile-salt recycling in very young birds. Atteh and Leeson (1985) clearly
showed the advantage to be gained by adding cholic acid (bile salt) to the diet of
broiler chickens. With approximately 9% palmitate in the diet or 9% of an
oleic/palmitate mixture there was a dramatic increase in bird performance and
fat utilization in response to added cholic acid. Unfortunately, the use of 0.2%
cholic acid in the diet as used in this study is not economically viable.
While it is possible to define ratios and levels of fatty acids needed to optimize
digestion, it is some-times difficult to envisage how such information could be
accommodated in a formulation matrix. Miller et al. (1983) outlined two methods
for taking into account variable fat utilization related to inclusion level,
suggesting overall savings of some 1% in total feed costs. Leeson and Summers
(1997) also described the relative values that can be applied to fats based on bird
age, content of free fatty acids, and inclusion level of the fat.
c. Intestinal Factors, Rate of Passage and Interaction With Other Ingredients
The status of the intestinal lumen will obviously have an effect on the digestion
and/or absorption of any nutrient. Freeman (1969) indicates that digesta pH can
influence fat digestion in that acidic conditions reduce micellar solubilization. In
rats, fat digestibility is reduced when the diet contains lactic acid. This concept
warrants further study considering the use of organic acid mold inhibitors and
the use of various feed additives to modify gut pH. Freeman et al. (1968) indicate
that the capacity of the pig’s small intestine to absorb micellar lipid is far in
excess of normal flow into the intestine, suggesting absorption, per se, to not be a
rate-limiting process. However, Katongole and March (1980) indicate that
mechanisms of absorption may be affected by an age-related availability of fatty
acid binding protein (FABP). Dror et al. (1976) likewise concludes that pancreatic
lipase output is little affected by diet fat concentration. A fatty acid-albumin
complex has been shown to be absorbed less efficiently than are micellar fatty
acids (Sklan, 1975) and the formation of a complex of these fatty acids with
undigested protein may be partly responsible for the poorer fat digestion seen
when animals are fed improperly processed soybean meal.
Birds infected with coccidiosis often exhibit inferior fat digestibility. Steatorrhoea
occurring with intestinal coccidiosis may, therefore, result from the loss of
reconstituted fat globules following the rupture of parasitized epithelial cells. Fat,
per se, and linoleic acid in particular may also affect the microbial population in
the intestine. Groneuer and Hartfield (1975) indicate reduction in coliform
bacterial population in layers fed corn oil. Fat digestion also seems to be
adversely affected by high levels of indigestible fiber (Cherry and Jones, 1982).
Increased levels of cellulose apparently result in reduced fat digestion, possibly
through complexing of fiber with bile salts.
Diet fat, per se, can affect rate of passage of digesta, and this can influence overall
diet digestibility. Sell and co-workers at Iowa State have used this argument to
account for the so-called "extra metabolic" effect of fat. Mateos et al. (1982)
suggest that fats and oils likely inhibit stomach emptying and intestinal digesta
movement. However, this effect is influenced by diet constituents, the rate of
passage being more affected when the diet contains sucrose versus starch
(Mateos and Sell, I98 1). Decreased rate of passage suggests that digesta spends
more time in contact with digestive enzymes, carriers or co-factors and
absorptive sites, etc. Addition of fats to the diet may, therefore, lead to increased
digestion of non-fat components of the diet.
d. Bird Age
The fact that young birds cannot digest fats as well as older birds (Table 4), has
been documented for many years, and yet this fact has rarely been incorporated
in formulation matrices. Sell et al. (1986) demonstrate the ability of young turkeys
to metabolize various fat sources where, from 2 to 8 weeks of age, the energy
contribution of tallow increased by 25%. Katongole and March (1980) likewise
show a 20-30% improvement in digestion of tallow for 6 versus 3 week-old
broilers and Leghorns. The effect of age on ability to digest fats is most
pronounced for the saturates (Whitehead and Fisher, 1975). This age-related
phenomenon does not seem to be of significance with other nutrients (Fisher and
McNab, 1987). The reason why adult birds are better able to’ digest fats, and
particularly saturated fats, is not clear. Young birds recycle bile salts less
efficiently, and this may be a factor as described previously. Also there is an
indication that fatty acid binding protein is not produced in adequate quantities
by young birds. Both Sell et al. (1986) and Katongole and March (1980) cite
evidence for up to 5x increase in FABP with chicks from hatch through 8 weeks of
age.
e. Soap Formation
Once fats have been digested, free-fatty acids have the opportunity to react with
other nutrients within the digesta. One such possible association is with minerals
to form soaps that may or may not be soluble. If insoluble soaps are formed, there
is the possibility that both the fatty acid and the mineral will be unavailable to the
bird. Atteh and Leeson (1984) indicate substantial soap formation in the digesta
of broiled? Chicks which is most pronounced with saturated fatty acids and with
high levels of diet minerals. Differences in fat digestibility were mirrored in
changes in fecal soap formation. In other studies, Atteh and Leeson (1983)
indicated such increased fecal soap production to be associated with reduced
bone ash and bone calcium content of broilers. Soap production seems to be less
of a problem with older birds. This is of importance to laying hens that are fed
high levels of calcium. In addition to calcium, other minerals, such as
magnesium, can form soaps with saturated fatty acids. In older birds and some
other animals, there is an indication that while soaps form in the upper digestive
tract, they are subsequently solubilized in the lower tract due to changes in pH.
Under these conditions both the fatty acid and mineral are available to the bird.
Control over digesta pH may, therefore, be an important parameter for control
over soap formation.
Summary
Protein and fat present in poultry diets are broken down by an array of enzymes
throughout the digestive tract. It seems unlikely that poor digestibility will be
caused by lack of enzyme production, with the exception of that for saturated
fatty acids by the very young bird. Reduced availability of amino or fatty acids
are most likely a consequence of naturally occurring anti-nutrients and/or
adverse processing conditions of ingredients or diets. The digestion and
absorption of fats is known to be influenced by factors, such as bird age, inclusion
level in the diet and fatty acid composition, and yet even with this knowledge
available, nutritionists invariably use single energy values in describing a specific
type of fat.
Protein and amino acid availability are of greatest concern in animal and
vegetable protein ingredients. Protein content and availability from cereals and
their by-products seem to be more consistent and little affected by processing
conditions. For most of these high protein ingredients, the main concerns are
presence of antinutrients and conditions of heat processing. In most situations, it
is variability in processing that causes most variance in amino acid digestibility,
and it is ironic that such processing is used specifically to combat this situation.
With relatively quick digesta rate of passage, there is little opportunity for the
bird to accommodate any hinderante to the normal processes of enzymatic
digestion and subsequent absorption.
Dr. Steve Leeson
Educated in the United Kingdom at the University of Nottingham, Steve Leeson
began his career with the University of Guelph in 1974. He is now a Professor of
Poultry Nutrition. Over the last few years he has conducted numerous studies on
various aspects of layer, broiler and breeder nutrition. Dr. Leeson’s most recent
research has involved nutrient enrichment of poultry products, and the
interaction between the environment and nutritional needs of layers and broilers.
Dr. Leeson has published many research papers and has authored four books in
poultry nutrition and management.
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