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What are the reasons for the variability of pig responses to dietary tryptophan supply?
Nathalie Le Floc’H, Cornelis F.M. de Lange, Aude Simongiovanni
To cite this version:
Nathalie Le Floc’H, Cornelis F.M. de Lange, Aude Simongiovanni. What are the reasons for the vari- ability of pig responses to dietary tryptophan supply?. CLANA, Mexican Animal Nutrition Specialists Association (AMENA)., Oct 2012, Puerto Vallarta, Mexico. 6 p. �hal-01455866�
What are the reasons for the variability of pig responses to dietary tryptophan supply?
Nathalie Le Floc’h
1,2, Cornelis F.M. de Lange
3and Aude Simongiovanni
41 INRA, UMR1348 Physiologie, Environnement et Génétique pour l'Animal et les Systèmes d'Elevage, F-35590 Saint-Gilles, France et 2 Agrocampus Ouest, UMR1348 Physiologie, Environnement et Génétique pour l'Animal et les Systèmes d'Elevage, F-35000 Rennes, France,
3 Department of Animal and Poultry Science, University of Guelph, 50 Stone Road East, Guelph, Ontario, Canada N1G 2W1
4 AJINOMOTO EUROLYSINE S.A.S., 153 rue de Courcelles, F-75817 Paris cedex 17, France
Keywords : Pigs, Tryptophan, Requirement INTRODUCTION
Besides being a constituent of body protein, tryptophan (Trp) plays other important roles in metabolism (Henry et al., 1996; Melchior et al., 2004 and 2005; Zhang et al., 2007). Increasing the dietary Trp content has also been shown to limit the impact of an unfavourable sanitary environment on performance in pigs (Le Floc’h et al., 2008 and 2010; Trevisi et al., 2009; de Ridder et al., 2012). Being an indispensable amino acid (AA) for pigs, Trp has to be supplied by the diet in sufficient quantities to cover the animal’s requirement. However, Trp requirement reported in the literature can be quite variable. For instance, the reported standardized ileal digestible (SID) Trp:Lys requirement in piglets varies between 17% and 23% (Lewis et al., 1977; Susenbeth and Lucanus, 2005; Jansman et al., 2010). The objective of this paper is to to give an overview of the variability of Trp requirement in growing pigs and to discuss the potential reasons for the variability that is reported in the literature.
I. ESTIMATION OF THE TRYPTOPHAN REQUIREMENT
In animal nutrition, it is usual to perform dose-response trials to estimate AA requirements. A part of the variability of pig response to Trp comes from the fact that the objective and the protocol of the trial are not always in line with the method used to interpret experimental results.
Trp requirement in the literature is reported in a wide range of units: amount in the feed (% of the feed or g per unit of energy), ingested quantity relative to weight gain (mg per g of daily gain), relative to Lys (% of Lys), and daily intake (g/d). In dose- response studies, the basal diet must be designed according to the way in which the requirement will be expressed (Petersen, 2011). For example, to express the Trp requirement as a ratio of Lys, Lys
must be the second limiting AA in the diet after Trp while the supply of the other AA should meet or slightly exceed animals’ requirements (Boisen, 2003).Flaws in experimental design are responsible for a part of the variability of the Trp requirement.
A principal component analysis (PCA) was performed on a database comprising 26
trials that study the Trp requirement of piglets (7-25kg). A sub-set of 26 dose-
responses (testing at least four levels of Trp) was selected from 116 studies because
they were designed to express the Trp requirement relative to Lys. The set of
variables used for the PCA included, among others, the net energy dietary level and
the piglets responses to Trp as average daily gain (ADG), average daily feed intake
(ADFI) and gain to feed ratio (G:F). The net energy appeared to be correlated at 34,
31 and 51% to response for ADG, ADFI and G:F respectively, meaning that the dietary energy level can be a factor affecting the piglets response to Trp.
The PCA study also revealed a negative correlation between the duration of a trial and the response to Trp at 37, 20 and 35% for ADG, ADFI and G:F respectively.
However, if the duration is too short, the requirement will be underestimated; this is more likely to occur when growth is used as the response criteria, since a deficiency of some nutrients will have an effect on growth only after a certain period (Pomar, 1995; Haushild et al., 2010). To estimate the Trp requirement for a growing population, the duration of the experiment has therefore to be taken into account.
II. THE CONTRIBUTION OF A META-ANALYSIS
The important variability that exists between Trp dose-responses illustrates that a single study may be insufficient to provide a “reliable” estimate of the Trp requirement. Meta-analyses are often used to combine results of several studies and allow identification of sources that contribute to variation within an experiment and between experiments (Sauvant et al., 2008). The results of the meta-analysis performed by Simongiovanni et al. (2012) about the SID Trp:Lys requirement in piglets highlighted some factors of the variability that is reported in the literature about the pig response to Trp.
First of all, the statistical method used to interpret data from dose-response studies will affect the reported animal response. The means comparisons (ANOVA) are not an adequate method to analyse data from doses-response experiments (Morris, 1983; Shearer, 2000) but they are still misused by biologists (Dawkins, 1983). A response-curve that represents animal response to Trp has to be chosen to estimate the Trp requirement (Pesti et al., 2009). Response-curves of populations of piglets to an increased level of SID Trp:Lys have been generated using linear-plateau (LP or broken-line), curvilinear-plateau (CLP or quadratic-plateau) and asymptotic (ASY) models by Simongiovanni et al. (2012). The results indicated that the choice of the statistical model has an important impact on estimated Trp requirements (the mean SID Trp:Lys requirement was estimated at 17% with the LP model, at 22% with the CLP model and at 26% with the ASY model) and this observation is in line with those of Baker (1986), Shearer (2000), Barea et al. (2009) and Petersen (2011). The comparison of models as discussed in Simongiovanni et al. (2012), concluded that the CLP model is the best descriptor of the effect of a nutrient on growth performance for growing populations and so, gives an adequate estimate of the SID Trp:Lys requirement for practical applications.
The results of the meta-analysis also revealed that t
he response criterion optimised is a factor explaining a part of the variability in the reported Trp requirement. For models CLP and ASY, requirement estimates were greater for ADG and ADFI than for G:F, which is in agreement with the results reported by Baker et al. (1971) and Han et al. (1993). The specific effect of Trp on feed intake (see below) could explain the slightly lower requirement estimate for G:F.In this meta-analysis, the trial selection was done on the nutritional values recalculated with the INRA tables (Sauvant et al., 2004); however, the recalculated Trp values were significantly lower than the values reported in the original experiments. A selection based on the same database but using the published nutritional values could therefore be performed, leading to a dataset comprising 26 dose-responses. Compared to the results published by Simongiovanni et al. (2012), the piglet response to Trp will be increased and the SID Trp:Lys requirement be
estimated between 1 and 3 points higher depending on the criteria optimised (ADG, ADFI or G:F). The usage of table values or analysed values will therefore also have an effect on the Trp requirement estimate.
Moreover,
e
stimates of the SID Trp:Lys requirement were slightly higher when only peer- reviewed publications were used. The reason for this difference is not known; however, it underlines the fact that there is a potential bias depending on the source of data used.Finally, the meta-analysis also demonstrated that the SID Trp:Lys requirement estimate was not affected by the source of cereals in the diet (corn-based diets vs. diets based on a mix of cereals), and this result agrees with experimental results reported by Jansman et al. (2010) and Naatjes et al. (2010).
III. PHYSIOLOGICAL ROLES OF TRYPTOPHAN
The potential factors of variation of the piglet response to Trp have been described in the literature. They are associated with the complexity of Trp metabolism as well as with the numerous biological functions involving Trp or some of its metabolites (Le Floc’h et al., 2011). One important physiological function of Trp is to be a constituent of body protein. Otherwise, Trp is the precursor of serotonin (5-HT or 5- hydroxytryptamine), an important neuromediator regulating gastrointestinal functions, mood, appetite and hemodynamics. Lastly, Trp is degraded into kynurenine, an intermediary metabolite of a complex metabolic pathway ending in quinolinic acid, niacin, kynurenic and xanthurenic acid. Two enzymes degrade Trp into kynurenine, tryptophan dioxygenase or TDO (EC 1.13.11.11) and indoleamine dioxygenase or IDO (EC 1.13.11.42). While the first enzyme degrades Trp in excess, the second is associated with body defenses and immune response regulation.
From a quantitative point of view, Trp incorporation into proteins represents the most important fate of Trp utilization by the animals. In fact, the amount of Trp daily involved in protein turnover, i.e. both protein synthesis and breakdown, largely exceeds the amount of Trp supplied by the diet. In a growing pig, it could be estimated that around 50 to 60 % of ingested Trp are retained in body protein when Trp was supplied below the requirement. This proportion decreases when Trp supply exceeds the requirement because catabolism increases. The proportion of dietary Trp that follows the other metabolic pathways is probably low. However, these metabolic pathways are closely related to major biological functions that could require Trp repartitioning away from growth.
Trp is the precursor for niacin synthesis. It could be assumed that dietary niacin level could influence the conversion of Trp into niacin especially when niacin supply was low. Old data suggested that Trp was a substantial contributor for niacin synthesis but such results had been obtained with large Trp supplies that forced Trp catabolism and conversion into niacin (Leklem, 1971). Recently, Matte et al. (2008) clearly showed that the growth response of postweaning pigs to dietary Trp was not influenced by dietary niacin supply.
Protein deposition and growth rate are closely dependent on feed intake.
Consequently, factors influencing feed intake
can impact on estimates of requirements as well.In pigs, the effect of Trp deficiency on growth is mainly caused by a reduction of
appetite and feed intake (Eder et al., 2001; Henry et al., 1992, 1996), which is not a
common response of other limiting AA except for valine. The mechanism whereby Trp
interacts with feed intake is not fully known. The synthesis of serotonin in the brain is
impaired in pigs fed a low Trp diet (Henry et al., 1996) while large excess of Trp
increased brain serotonin concentration (Adeola and Ball, 1992). However, the link
between serotonin and the regulation of feed intake is not so clear because serotonin is known to reduce appetite. Depressed feed intake caused by low Trp can be explained by a phenomenon of imbalance detected at the brain level because of a relative excess of LNAA compared to Trp. Another potential mechanism could be linked to ghrelin secretion, a gastrointestinal hormone regulating appetite, by the gastrointestinal tract (Zhang et al., 2007).
Variability of the response to Trp can also be associated with the pig sensitivity and response to different kind of stress. Indeed, the effect of Trp on stress sensitivity has been demonstrated when pigs were fed large doses of Trp (Koopmans et al., 2005;
Guzik et al., 2006). However, these effects remained much more discrete or inexistent for dietary supplies within nutritional ranges (Meunier-Salaun et al., 1991).
At last, the effects of sanitary and health statuses are now clearly associated with modifications of Trp metabolism. Plasma Trp was dramatically reduced during an inflammatory response (Melchior et al., 2004; Le Floc’h et al., 2008). Two metabolic pathways could be involved in the disappearance of plasma Trp: the first one is the incorporation of Trp in acute phase protein synthesised by the liver during the inflammatory response (Reeds et al., 1994); the second pathway corresponds to Trp catabolism into kynurenine by the enzyme IDO (Melchior et al., 2005). The induction of IDO pathway has been proposed to be a mechanism that limits the availability of Trp during an inflammatory process and may play crucial roles in the regulation of the immune and inflammatory responses (Pfefferkorn, 1984; Mellor and Munn, 2001).
However, it could be also responsible for reduced Trp availability for growth and protein deposition during systemic (Le Floc’h et al., 2010; de Ridder et al., 2012) and digestive (
Capozzalo et al., 2012)inflammations.
CONCLUSIONS
The variability of pig responses to dietary Trp level can be explained at different levels. First, based on the same data, the reported response to Trp can vary with the way they are interpreted: unit used to express the requirement, model, criteria that are optimized, usage of tables vs. analyzed AA values. Second, the protocol of an experiment will influence the response: the dietary levels of energy and Lys, the experimental conditions (duration, sanitary and health statuses, stress and interaction with other feed component). Finally, the major part of this apparent variability can be explained and each nutritionist can make his own choice on the Trp requirement knowing the implications on performance and taking into account the economic constraints.
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