Soluble non-starch polysaccharides affect net utilisation of energy by chickens.

Abstract

31 Soluble non_starch polysaccharides affect net utilisation of energy by chickens M. Choct School of Rural Science and Natural Resources, University of New England, Armidale NSW 2351 Summary Feed constituents such as the nonstarch polysaccharides (NSPs) not only affect nutrient digestion in general, but also influence gut dynamics through changing the types and numbers of the microflora and the secretory response of the gut. The use of nutrients such as starch through microbial fermentation is not energy efficient; increased endogenous secretion of water, protein, lipids and minerals can affect postdigestive and, perhaps, post absorptive processes because the gut secretes some 20 hormones or regulatory peptides. This paper discusses the importance of evaluating feed constituents from the standpoint of efficiency and presents some experimental data on the effect of soluble NSPs on the net energy value and losses of energy as heat and as volatile fatty acids in the excreta of broiler chickens. One constituent that has negative effect on the nutritive value of grains in poultry is the content of soluble nonstarch polysaccharides (NSPs) (Burnett 1966; Antoniu et al. 1981; Annison 1991). This paper discusses how soluble NSPs affect energy utilisation beyond digestive processes in poultry. Effect of NSPs on post_digestive processes The effect of soluble NSPs on the apparent digestibility of nutrients in poultry has been studied extensively during the past ten years (Bedford et al. 1991; Choct and Annison 1992a). The negative correlation between NSPs in the diet and its nutritive value has been demonstrated in poultry (Choct and Annison 1990; Annison 1991), in pigs (King and Taverner 1975) and in dogs and cats (Earle et al. 1998). A general inhibition of the digestibility of nutrients occurs when diets with high levels of soluble NSPs are fed to chickens. Thus, Choct and Annison (1992a) demonstrated that addition of a soluble NSP isolate from wheat to a broiler diet depressed the ileal digestibilities of starch, protein and lipid by 14.6, 18.7 and 25.8%, respectively. It is believed that viscous gut contents impede nutrient digestion by reducing the mixing of digestive enzymes with their substrates, but the effect of NSPs on the gut is not limited to their physical characteristics. They can induce changes to the microflora of the gut and modify endogenous secretion. These effects of NSPs on nutrient digestion are interrelated and depend on the solubility, molecular weights, and solution configurations of the polymers. Solubility of NSPs, in turn, depends on their chemical structures and association with other cell wall components. The role of viscosity in the antinutritive effect of NSPs has been demonstrated by numerous workers (Bedford et al. 1991; Bedford and Classen 1992; Choct and Annison 1992b; Dusel et al. 1998; Steenfeldt et al. 1998). Choct and Annison (1992b) depolymerised an Introduction The measures used to assess the nutritive value of poultry diets to date are metabolisable energy and amino acid contents, which also are the bases for practical diet formulations. The use of a net energy system for energy and digestible amino acid values for diet formulations has been strongly advocated in recent years (Farrell 1996; Ravindran et al. 1998; Ravindran and Bryden 1999). The rationale is that the efficiency of energy utilisation and the digestibility of amino acids are affected by diet constituents and that it is logical to consider the amounts of nutrients available to the animal when evaluating feed. However, gut dynamics in relation to absorption of nutrients and their regulation and energetic costs have been given little attention in nutritional research. Considering the fact that 20% of whole animal energy expenditures can be accounted for by the gut (McBride and Kelly 1990; Cant et al. 1996), it is essential to investigate the way feed constituents behave in the gut and their effect on energetic efficiency of feed utilisation. Recent Advances in Animal Nutrition in Australia, Volume 12 (1999) 32 Mingan, M. isolate of wheat arabinoxylan (MW 758,000 Daltons) using a xylanase in vitro so that its viscosity was reduced fourfold. When these depolymerised NSPs were included in broiler diets, they did not exhibit strong antinutritive effects on nutrient digestion compared with intact NSPs included at the same level. NSPs can also bind nutrients and form complexes with digestive enzymes and some regulatory proteins in the gut. Angkanaporn et al. (1994) showed that addition of soluble arabinoxylans to a broiler diet markedly increased endogenous losses of amino acids, leading to a significant decrease in the apparent digestibility of protein in the ileum. The gut secretes some 20 hormones or regulatory peptides (Unv�sMoberg 1992), some enhance nutrient absorption, and others depress it. Feed components that have effects on endogenous protein secretion can be supposed to have an effect on hormonal secretions. Furthermore, viscous NSPs can enhance bile acid secretion and subsequently result in significant loss of these acids in the faeces (Ide et al. 1989; Ikegami et al. 1990). In addition to the modification of the gut physiology, certain NSPs can also bind bile salts, lipids and cholesterol (Vahouny et al. 1980; Vahouny et al. 1981). The net effect may be an altered lipid metabolism in the intestine, resulting in increased hepatic synthesis of bile acids from cholesterol to re establish the composite pool of these metabolites in the enterohepatic circulation. The continued drain of bile acids and lipids by sequestration, and increased elimination as faecal acidic and neutral sterols, may ultimately influence the absorption of lipids and cholesterol in the intestine. These effects could lead to major changes in the digestive and absorptive dynamics of the gut, with consequent poor overall efficiency in nutrient assimilation by the animal. NSPs on energy losses via heat increment and as volatile substances in excreta The energy derived from microbial fermentation in the chicken is small in quantity and inefficient as a metabolic fuel (Bolton and Dewar 1965). NSPs can bring about significant changes to the ecology of the gut, e.g. proliferation of fermentative microbes in the small intestine which ferment digestible nutrients such as starch to VFA (Choct et al. 1996). The amount of these VFA absorbed from the gut is not known. It is thought that the effect of soluble NSPs on the net utilisation of energy of feedstuffs for poultry may be more pronounced than that on the apparent metabolisable energy (AME) value because: (1) the digestive system of the bird would have to work harder than usual to cope with a highly viscous gut environment, and (2) the proliferation of fermentative organisms in the small intestine of chickens is energyinefficient and detrimental to the bird. In a recent study using a close circuit calorimetric system, we (Tukei, Thomson and Choct, unpublished data) examined the effect of NSPs on energy utilisation in broiler chickens. In the first experiment, the loss of energy as VFA in the excreta of birds fed maize and barley was compared because these ingredients represent the two extremes amongst cereals in terms of their soluble NSP levels. As shown in Table 1, there was a large difference between birds fed maize and barley in the energy loss as VFA in the excreta, apparently indicating that elevated levels of soluble NSPs affected the way nutrients were digested. To test this finding further, we determined the net energy (NE) value of an NSPenriched diet with or without enzyme supplementation in a second experiment. As the level of NSPs increased in the diet, the energy losses as heat increased and a considerable amount of energy was lost as VFA in the excreta. Thus when the antinutritive properties of the NSPs were removed by use of an exogenous enzyme, the AME and NE were increased by 29.1% and 37.3%, and heat production and energy loss as VFA were decreased by 11% and 61%, respectively. The data are shown in Table 2. The increases in AME and NE were not proportional, indicating that NSPs not only interfere with digestive processes, but also have strong negative effects on net utilisation of energ y. Part of this discrepancy may be explained by the loss of energy as VFA in the excreta. The use of nutrients through microbial conversion of digestible carbohydrates, such as starch, to VFA is not efficient compared to a direct absorption of glucose released from enzymatic digestion (M�ller et al. 1989; Furuse and Okumura 1989; Carr� et al. 1995). The production of VFA is energetically costly due to their costs of production and the energy losses as VFA in the excreta. It is apparent that nutritionists should start to pay more attention to postdigestive and postabsorptive processes when evaluating nutritive quality. These include gut microbial balance, changes in gut dynamics such as secretory regulations and repartitioning of nutrients. Muramatsu and colleagues (Muramatsu et al. 1983; 1987; 1993) clearly demonstrated that gut microflora increase energy costs by modifying the rate of energyconsuming reactions such as protein turnover within the chicken body. One such example is the gut cell turnover. According to LeBond and Walker (1956), a 100 g rat gaining 5 g/d synthesises 1 g mucosal cells daily, which represents a 20% additional tissue synthesis not manifest as weight gain. Extrapolating this to a 2 kg bird gaining 60 g daily, the bird would synthesise 12 g of mucosal tissue to maintain the integrity of its small intestine. Increased microbial load can exacerbate this loss (Abrams et al. 1963; Lesher et al. 1964) since some of its fermentation products, e.g. putrescine, have been shown to significantly enhance small intestinal and colonic mucosal growth rates (Osborne and Seidel 1989; Seidel et al. 1985). The indirect evidence of such costs is the often significant improvement in bird performance resulting from the inclusion of antibiotics in highNSP diets (MacAuliffe Soluble non_starch polysaccharides affect net utilisation of energy by chickens 33 Table 1 Apparent metabolisable energy (AME) and loss of energy as VFA in the excreta of broilers fed maize or barley. Maize Barley 13.4 31.6 433 AME (MJ/kg dry matter) Excreta VFA energy (kJ/bird/day) Excreta VFA energy (kJ/kg DM intake/bird) 16.5 11.4 159 Table 2 AME, net energy and loss of energy as VFA in the excreta of broilers fed a NSP enriched wheat based diet with or without enzymes. Wheat + NSPs (_ enzyme) Wheat + NSPs (+ enzyme) 14.8 1.006 0.81 7.5 17 145 AME (MJ/kg DM) Respiratory quotient Heat production (MJ/kg liveweight/day) Net energy (MJ/kg DM) Excreta VFA energy (kJ/bird/day) Excreta VFA energy (kJ/kg DM intake/bird) 10.5 0.995 0.91 4.7 56 371 and McGinnis 1971; Misir and Marquardt 1978a,b). 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