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. References Alao, S.J., and D. Balnave, 1985. Nutritional significance of different fat sources for growing broilers. Poultry Sci. 64: 1602- 104. Anderson-Hefermann, J.C., Y. Zhang and C.M. Parsons, 1993. Effects of processing on the nutritional quality of canola meal. Poultry Sci. 72:326-333. Araba, M., and N.M. Dale, 1990. Evaluation of protein solubility as an indicator of overprocessing soybean meal. Poultry Sci. 69: 1749-1752. Araba, M., J. Gos, P. Kerr and D. Dyer, 1994. Identity pre-served varieties: High oil corn and low stachyose soybean, pages 135 142. In: Proc. Arkansas Nutr. 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