Pou...ais - f 06 an appraisal of trace elements - inorganic and organic

Pou...ais - f 06 an appraisal of trace elements - inorganic and organic

(Parte 1 de 2)

CHAPTER 6 An appraisal of trace elements: inorganic and organic

J.D. van der Klis and P.A. Kemme Division ID TNO Animal Nutrition, DLO Institute for Animal Science and Health, Lelystad, The Netherlands

© CAB International2002. Poultry Feedstuffs: Supply, Composition and Nutritive Value (eds J.M. McNab and K.N. Boorman)9

Trace elements fulfil more or less specific physiological and biochemical functions in the body, as summarized by Underwood and Suttle (1999). They predominantly act as catalysts in many enzyme and hormone systems. Deficiency symptoms include disturbances of many metabolic processes, resulting in reduced production performance, loss of appetite, reproductive disorders, impaired fat and carbohydrate metabolism and immune response.

Much research has been done to determine the bioavailability of inorganic trace element compounds. Most data are expressed relative to a standard source such as mineral sulphates or oxides. Recently, trace minerals have received new attention because:

1.It has been recognized that trace mineral contents in animal manure are far in excess of nitrogen. Mohanna and Nys (1998) calculated that Zn, Cu and Mn in poultry manure are far in excess of crop requirements, causing accumulation in the soil, especially in regions of intensive livestock production. This phenomenon is even more important in pig manure as in some pig diets high Cu and Zn concentrations are used because of growth promoting effects (Meijer and Kröger, 1973; Hahn and Baker, 1993). To avoid further accumulation of these trace elements in the soil, trace mineral compounds with higher bioavailabilities should be used, and/or safety margins lowered. 2.Additional positive effects of specific trace element compounds have been described on immune competence (e.g. Zn-methionine) and on fat and carbohydrate metabolism (e.g. Mn and Cu). Cr sources are also of interest because of their effects on fat and carbohydrate metabolism.

The use of organic sources can potentially improve intestinal absorption of trace elements as they reduce interference from agents that form insoluble complexes with the ionic trace elements. Moreover, positive responses have been described for several organic trace element compounds in various physiological processes. In this chapter, the bioavailabilities of both inorganic and organic sources of trace minerals and the factors affecting them are discussed. In addition, the potentials of different organic compounds are summarized.

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Body retention of most trace elements is very low. Recently, Mohanna and Nys (1998) calculated the retention of Zn, Fe, Mn and Cu in broiler chickens during a complete production period using feed and carcass analysis. They concluded that the retentions (as percentage intake) of Zn, Fe, Mn and Cu were only 6, 10, 0.2 and 6, respectively. These retention values agreed fairly well with calculations done by van der Klis (1991), who concluded that 2, 15, 0.6 and 5%, respectively were retained by broiler chickens during a 6 week growth period. The considerably higher retention value for Zn was primarily caused by the higher Zn concentrations in the French diets. The low retention of trace elements is mainly ascribed to the relatively high dietary intakes, as the body contents of trace elements are rather stable in the second half of the production period (Mohanna and Nys, 1998). Dietary contents of trace elements are far in excess of the animals’ requirements, as published by the NRC (1994), for the following reasons:

1.In general, the physiological requirements as published by the NRC (1994) are determined at low dietary concentrations of potential interacting agents. However, commercial diets contain large amounts of agents such as fibre, phytate and phosphate that may decrease trace element absorption. For this reason, large safety margins are used to avoid deficiencies. Moreover, knowledge of the bioavailability of trace elements in feed ingrediants is lacking and therefore trace elements from this source are neglected in feed formulation. Furthermore, physicochemical conditions in the intestinal tract can adversely affect trace mineral absorption (Mohanna et al., 1999). Finally, mutual interactions between minerals and trace elements for transport proteins in enterocytes and in intermediary metabolism, and the nature of organic molecules in the gastro-intestinal lumen, can affect the efficiency of absorption and utilization. 2.As trace elements play essential roles in many physiological processes, adequate dietary concentrations are crucial. Moreover, positive effects of dietary trace element concentration in excess of the physiological requirements on production performances of the animal have been reported.

The bioavailability of an inorganic nutrient was defined by Fairweather-Tait (1997) as the proportion in the diet that is utilized for normal metabolic functions. The results of bioavailability tests are not only affected by the characteristics of the mineral source, but also by the specific test conditions. Test conditions might affect intestinal absorption and the utilization in the body, such as the composition of the basal diet (practical or synthetic), the age and physiological state of the animals, and the response parameters used.

Before trace elements can be absorbed from the intestinal lumen, they should be in a soluble form, i.e. as ions or bound to low molecular weight lig-

100J.D. van der Klis and P. Kemme TRACE MINERALRETENTION IN BODYTISSUES

Poultry f_s - Chap 06 29/5/02 1:21 AM Page 100 ands. Once absorbed they are generally bound to transport proteins and transported to target organs where they fulfil their metabolic functions.

Many studies to determine the bioavailability of trace elements are based on dose–response curves relating supplementation rate of a trace element to the content of that trace element in specific body tissues or to the production performance of an animal. The experimental diets are formulated to contain adequate levels of all nutrients except the trace mineral to be studied. If necessary, chickens will be depleted during the first week of life by feeding a purified diet deficient in that trace element. The depletion period is followed by a repletion period in which the standard and test diets, containing increasing concentrations of the standard trace element source and the test source to be evaluated, respectively, are fed. The relative biological value (RBV) of the test product is calculated from the ratio of the slopes of the two response curves: ‘slope of the test response’ divided by ‘slope of the standard response’. The RBV of the standard product is set at 100% (Fig. 6.1). Using this slope-ratio technique, a linear response relationship is a prerequisite. It was shown that this assumption is true for a wide range of intakes of Mn (e.g. Black et al., 1985) and Cu (e.g. Miles et al., 1998), using tibia Mn and liver Cu as a response criterion, respectively. Unlike Zn, the estimated bioavailabilities for Mn and Cu were similar at high (up to 1000 mg Mn kg 1) and low supplementation rates (up to 100 mg Mn kg 1) as described by Wedekind and Baker (1990b). For Zn, linearity was only valid up to intakes of about 30 mg kg 1, using tibia Zn as a response parameter (Wedekind and Baker, 1990a). Wedekind et al. (1992) concluded that bioavailability assays involving higher Zn intakes, i.e. where there was a diminishing response per unit of supplementation, are invalid as they would overestimate the RBV of trace element sources with lower availabilities.

A complicating factor in using depletion/repletion techniques for the evaluation of Zn sources is the stimulatory effect of available Zn intake on feed intake in Zn-depleted birds. Under these conditions, Zn intake from the basal diet (mostly with an unknown availability) will confound the assessment of the

Appraisal of inorganic and organic trace elements101

Response Standard source Unknown source

RequirementToxic levelMineral intake AB

Fig. 6.1.Predicted tissue mineral response to dietary addition of a mineral. The biological value of the test product (with an unknown biological value) is expressed relative to the standard source (reference = 100) (from Henry et al., 1986).

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Zn bioavailability of the test source. The accumulation of Zn in the tibia is therefore not only the result of differences in Zn availability, but also of differences in basal diet Zn intake (Wedekind et al., 1992). These problems can be overcome by using either purified basal diets containing no Zn or by feeding a basal diet, which meets the Zn requirements of the animal. However, in the latter case it should be ensured that dose rates do not exceed the linear range.

Black et al. (1985) conducted an experiment to determine which response criterion would be the most discriminative for the evaluation of Mn bioavailability. Based on the highest slope/SD ratio using kidney Mn and tibia Mn contents in 3-week-old chickens as response criteria, they concluded that these were better criteria than plasma, pancreas plus liver, and muscle Mn concentrations. Among others, Henry etal. (1986) have shown that estimation of the RBV of

MnO (reference MnSO4.H2O) using Mn contents in bone, kidney and liver as response parameters resulted in values of 79%, 58% and 64%, respectively.

Nutritional balance methods are not very discriminative for estimating the relative availability of trace element sources, as the proportion of most trace elements absorbed in the intestine is rather low. Moreover, homoeostasis of elements such as Cu, Mn and Zn is regulated via endogenous secretions. This implies that only small differences between intake and excretion of trace elements must be measured. Although the limitations of balance methods can be overcome by the use of isotopes to label either the body pool or the trace elements in the feed, in order to enable calculation of the true absorbability, reports of balance methods on trace elements are as scarce as those of RBV tests. Furthermore, these techniques are only valid if it is assumed that the marker isotopes behave in a similar manner to the normal isotopes of the trace elements (Sandström, 1997). Although this assumption might be correct for inorganic trace element sources, it most likely does not hold for organically bound trace elements.

Animal feed is usually supplemented with trace elements in the form of mineral oxides and sulphates via mineral premixes. Apart from these inorganic sources, organic sources are available. Ammerman et al. (1998) have summarized the biological values of different trace element sources for poultry (Table 6.1).

Since trace elements are absorbed from the intestinal tract as ions or as soluble low molecular weight ligands (Scott et al., 1976), the solubility of a source would directly affect its bioavailability. This was confirmed by Ledoux et al. (1991) who determined the in vitrosolubility of four different copper sources in water, neutral ammonium citrate, 0.4% HCl and 2% citric acid and showed that sulphate and acetate had similar solubilities, oxide a low solubility, while carbonate solubility was intermediate. They showed the same ranking in RBV in vivousing 3-week-old chickens. Ammerman et al. (1998) concluded in their review that the soluble trace mineral compounds, such as sulphates and chlorides, are well utilized by the animal, while less soluble compounds, such as carbonates and oxides, have lower relative biological availabilities.

102J.D. van der Klis and P. Kemme

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In general, basal diets with low concentrations of potential interacting agents are used to determine differences in bioavailabilities of trace element sources, although the dietary content of potential chelating agents can affect the outcome of studies to determine RBV. Fly et al. (1989) observed that the RBV of Mn-methionine was 130% in comparison to MnO when a semi-synthetic diet was used, while the difference between Mn sources was much more pronounced in a practical maize–soy diet (RBV of Mn-met was 174%). These results were confirmed by Wedekind et al. (1992) using a purified basal diet, a basal diet with a soy-isolate and a practical maize–soy diet, for estimation of Zn-methionine. They found RBV estimates of 117%, 177% and 206% for Zn- methionine using ZnSO4.H2O as a reference. These results illustrate the higher potential of organic trace element sources when fed in commercial diets rather than in purified experimental diets. Aoyagi and Baker (1993) suggested that differences in RBV of Zn in Zn-lysine and Zn-methionine, found using a purified diet with isolated soy protein, were possibly due to a higher fraction of Znlysine being dissociated in the upper small intestine compared with Zn-methionine, causing the former to be less protected from potent chelating agents in the gut.

As indicated, the outcome of availability studies is affected by the composition of the basal diet, because of the presence of trace minerals with an unknown availability in the feedstuffs used, and because dietary food components such as phytate and fibre bind trace minerals, inhibiting absorption. Especially in the complex diets used in practical animal feeding, organic trace mineral components might have a higher positive value than established in experiments using

Appraisal of inorganic and organic trace elements103

Table 6.1.The relative biological value (RBV) of different Cu, Mn and Zn sources in poultry as reviewed by Ammerman et al. (1998).

Copper Manganese Zinc Compound RBV n Compound RBV n Compound RBV n nis the number of observations; all values are rounded to the nearest ‘5’.

Poultry f_s - Chap 06 29/5/02 1:21 AM Page 103 semi-synthetic diets. Previously mentioned differences in RBV of organic Mn (Fly et al., 1989) and Zn sources (Wedekind et al., 1992) suggest that, with increasing concentrations of potential interacting agents, the use of trace mineral chelates will be more beneficial than under experimental conditions when semi-synthetic diets are used.

Trace elements are absorbed as ions or soluble compounds, as illustrated for Zn in Fig. 6.2. In the case of practical diets containing high concentrations of potential chelating agents, the use of organically bound trace elements can have benefits, as a higher proportion of the dietary supplement can become available for absorption in the host animal. Therefore, organic trace element compounds should be soluble, with a relatively high stability constant to effect absorption before the element is bound in the intestinal lumen to feed residues that are not absorbed. On the other hand, the stability of such organic complexes should not be too high, as in that case it will prohibit bonding to the endogenous transport proteins in the animal.

The adverse effects of dietary phytic acid, one of the most widespread mineral chelators, on the availability of (trace) minerals have been recognized for decades. It was demonstrated in vitrothat phytic acid binds polyvalent cations, dependent upon the pH and the ratio between phytic acid and the minerals (Nolan et al., 1987). The adverse effects of phytate on trace mineral absorption have been studied by supplementing diets with phytases. Several experiments

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