Measuring trace mineral bioavailability key - The Poultry Site

Feedstuffs, January 18, 2010Feedstuffs ReprintMeasuring trace mineralbioavailability keyIt is important to measure several indicators of trace mineralbioavailability in order to predict the outcome of feeding aparticular mineral source.By JAMES D. RICHARDS*ZINC, copper, manganese and anumber of other essential traceminerals are required for theproper development, maintenance andhealth of animals (Underwood andSuttle, 1999).Collectively, these minerals supportsuch diverse functions as immunedevelopment and function, structuralintegrity of bones and tissues,reproduction and defense againstoxidative stress. As such, these traceminerals are supplemented in virtuallyall animal diets either as inorganic tracemineral (ITM) salts — such as sulfatesand oxides — and/or as organic traceminerals (OTM).While ITMs are relatively inexpensive,it is generally accepted that they sufferfrom relatively poor bioavailabilitycompared to some chelated minerals,primarily due to the numerousantagonisms and interactions amongITMs and other components of thedigesta (Leeson and Summers, 2001;O’Dell, 1989; Underwood and Suttle,1999).Phytic acid, for example, binds totrace minerals and precipitates at theneutral pH of the small intestine, therebyrendering the minerals unavailable forabsorption (Cheryan, 1980; Leeson andSummers, 2001). Many other types ofantagonisms have been reported (O’Dell,1989; Underwood and Suttle, 1999).It has been accepted that a higher planeof nutrition is required to achieve the fullgenetic and economic potential of today’sanimals. Optimizing mineral nutrition ispart of this package.So, how is this done, given thelimitations in bioavailability of ITMs?The solution is not simply to feed higherlevels of minerals, as this in itself canlead to even lower mineral bioavailability,increased environmental burden andpotentially even decreased animalperformance. The key is to feed a morebioavailable source of trace minerals.What does bioavailability mean?Mineral bioavailability is simply thedegree to which an animal can absorband utilize a mineral (for example, zinc)from one particular source (such as achelate). In practice, bioavailability ofa mineral from a given source is usuallymeasured relative to the same mineral ina second, or “standard,” source.In bioavailability experiments withOTMs, the standard source is typically anITM salt like zinc sulfate or manganousoxide. Chelated minerals are widelyreported to be more bioavailable thanITMs, presumably due to their ability toavoid feed ingredient antagonisms anddeliver the trace mineral to the smallintestine for absorption (Cao et al., 2000;Fly et al., 1989; Guo et al., 2001; Predieriet al., 2005; Wang et al., 2007; Yan andWaldroup, 2006).Still, not all OTMs are more bioavailablethan ITMs. While chelation is important,just because a mineral is chelated doesnot guarantee that it is more absorbablethan an ITM salt. For this reason, it isimportant to examine how bioavailabilityis estimated.Tibia zinc (ppm)Measuring bioavailabilityPeople have tried to measure or predictmineral bioavailability with a widevariety of assays, only some of which arerelevant or useful.Furthermore, while it is tempting tofocus on only one or two particularmeasures of bioavailability, it isimportant to realize that this would giveonly a partial understanding of OTMbioavailability and function.Rather, it is important to measureseveral indicators of mineralbioavailability in order to be able tounderstand and predict the outcome offeeding a particular mineral source.Several labs have run chemistry-basedexperiments that attempt to define thephysical characteristics that lead to highmineral bioavailability or have developedelaborate in vitro model systems ofabsorption to predict bioavailability invivo, most of which have proven to berelatively uninformative.For example, one test that has beenproposed is solubility of the mineralsource in water or buffered solution.Initially, the notion that solubility wouldpredict bioavailability seemed to makesense in that in order for a mineral (orany nutrient) to be absorbed, it has tobe in a soluble form at the absorptivesurface of the small intestine. Inaddition, there are data to indicate thatbioavailability and degree of solubility ofOTMs at specific pH levels can be related(Guo et al., 2001).However, if solubility were the key1. Relative bioavailability of zinc sources*Dr. James D. Richards is with Novus International.BasalZincsulfateZincmethionineZinc(HMTBa) 2© 2010 Feedstuffs. Reprinted with permission from Vol. 82, No. 03, January 18, 2010.

2Feedstuffs, January 18, 2010Reprintto bioavailability, then ITM salts wouldbe extremely bioavailable. Copper,manganese and zinc sulfates are allhighly soluble in water (CRC, 1981-82),yet they are not as bioavailable as ahigh-quality OTM. In fact, the majority ofdata show that there is little relationshipbetween mineral solubility and mineralbioavailability (Guo et al., 2001; Ledoux etal., 1995; Miles et al., 1998).Some peer-reviewed data even suggestthat for zinc chelates, high solubility isinversely related to bioavailability in bothpoultry and ruminants (Cao et al., 2000).Thus, one should be cautious about usingsolubility — or any in vitro test — topredict the relative bioavailability of atrace mineral source.Measuring the deposition or storage ofminerals into selected tissues is the mostcommon output in trace mineral relativebioavailability experiments (Underwoodand Suttle, 1999).Figures 1 and 2 are from experimentsthat compared the relative bioavailabilityof zinc sources and manganese sources,respectively, by measuring tibia zincor manganese content. Figure 1 showsthat while all supplemented treatments(added at 40 parts per million of zinc)increased tibia zinc, zinc (HMTBa) 2(zinc chelated by two molecules of themethionine hydroxy analogue) was morebioavailable than the other two sources.Figure 2, which is from a slope-ratiodesign, demonstrates that manganese(HMTBa) 2 was about 150% as availableas the manganese from manganous oxide(Dibner, et al., 2004; Yan and Waldroup,2006).These experiments support the notionthat certain chelated minerals can bemore bioavailable than ITMs.Still, tissue mineral experimentshave some limitations. First, theseexperiments measure only a fraction ofthe mineral that is absorbed. Minerals areabsorbed by the small intestine and thendistributed via the bloodstream to othertissues. Therefore, tissue mineral levelsonly measure the mineral distributed tothose particular tissues and, as such, maynot reflect total mineral uptake.A second limitation is that tissuemineral levels actually represent astorage pool of mineral rather thanthe total amount of mineral deliveredto that particular tissue. In general,tissue mineral stores can be altered as aconsequence of mineral excretion, useor stress (Underwood and Suttle, 1999).Therefore, measuring tissue minerals tellsonly part of the story. To truly measurebioavailability, it would be useful tomeasure mineral absorption in the smallintestine where absorption occurs.BiomarkersAnother method for estimatingbioavailability is to measure mineralresponsivebiomarkers, such as changesTibia manganese (ppm)Metallothionein mRNA (relative units)2. Relative bioavailability of manganese sourcesManganese oxideManganese (HMTBa) 2Supplemental manganese (ppm)3. Small intestinal metallothionein mRNA expressionas an indicator of zinc bioavailabilityControlZincsulfatein gene expression, or the activity of amineral-dependent enzyme. Biomarkersare particularly informative whenmeasured in the small intestine.Metallothionein is one such biomarker,because its expression is regulatedby zinc status; the magnitude ofmetallothionein messenger RNA (mRNA)and protein expression depends onthe amount of zinc absorbed (Davisand Cousins, 2000). Therefore,metallothionein mRNA or proteinexpression is often used as an indicatorof the zinc status of humans and animalsand to evaluate the bioavailability ofdifferent zinc sources (Blanchard et al.,2001; Cao et al., 2002; Huang et al., 2009;Lu et al., 1990; Martinez et al., 2004;McCormick et al., 1981; Reeves, 1995;Rojas et al., 1995; Sullivan et al., 1998).Figure 3 shows an example of usingsmall intestinal metallothionein mRNAexpression as an indicator of zincbioavailability (Richards et al., 2007;Richards et al., 2008b). In this experiment,broilers were fed control diets or dietsZincproteinateZinc aminoacid complexZinc(HMTBa) 2supplemented with 70 ppm zinc fromthe indicated sources. Because zincabsorption occurs in the small intestine,differences in metallothionein expressionhere would be expected to more closelyrepresent relative bioavailability thantissue zinc levels would.These data are consistent withthe data in Figures 1 and 2 in thatthey demonstrate that some, but notall, chelated zinc sources are morebioavailable than inorganic zinc.Consistent with the other experiments,Zn(HMTBa) 2 was the most availablesource.Measuring tissue minerals or mineraldependentbiomarkers can be the easiestand most straightforward measuresto generate a quantitative estimate ofmineral bioavailability.Fundamental rolesWith that said, it is important not to stopthere. Zinc, copper and manganese playfundamental roles in the biochemistry

ReprintFeedstuffs, January 18, 20103of the cells of the animal, primarilyby serving as essential componentsof hundreds of cellular enzymes andtranscription factors (Underwood andSuttle, 1999).If trace minerals are limiting in the diet,any of these processes can suffer.So, by supplementing diets with morehighly available forms of trace minerals,a producer will more effectively “feed”these enzyme systems, which shouldtranslate into a wide variety of biologicalbenefits.Indeed, recent results with traceminerals chelated with HMTBa havedemonstrated benefits such as enhancedimmune function (Dibner, 2005), reducedincidence of tibial dyschondroplasia(Dibner et al., 2007; Richards et al., 2006)and footpad lesions (Richards et al.,2006), reduced leg abnormalities (varus,valgus, shaky leg) and increased bonebreaking strength (Ferket et al., 2009),reduced oxidative stress (Richards etal., 2008a) and improved performance(Ferket et al., 2009) even at lower levelsof trace mineral inclusion (manuscriptsubmitted).While measures such as these are nottraditionally considered to be indicatorsof trace mineral bioavailability, theycan and should be used to confirm theinformation gained from assessing morecustomary measures, such as tissueminerals and genetic biomarkers.ConclusionsFeeding high-quality, high-bioavailabilitytrace minerals is an important key tomaximizing the growth potential and wellbeingof production animals. Chelatedminerals have the potential to delivertrace minerals more effectively to thetissues of the animal and, thus, to bettersupport the biochemical functions of theanimal’s cells and tissues.This, in turn, leads to a wide varietyof biological and performance benefits.Unfortunately, not all OTMs can dothis. Only by truly understandingthe structure and consistency of agiven OTM source and by rigorouslyinvestigating its bioavailability througha variety of methods can one be assuredof the predictability and consistencyof the animal’s responses to OTMsupplementation.Mineral bioavailability has beenestimated in a wide variety of ways thatvary greatly in their effectiveness. Ingeneral, techniques that rely on chemicalassays and in vitro methodology are oflimited utility and can be misleading.In contrast, in vivo methods can bevery reliable methods on which to basenutritional decisions.Finally, and perhaps most importantlyfor decision-making, we need to rely onnot only one but many different in vivotechniques for assessing trace mineralquality and bioavailability. Only when weexamine a variety of different readouts,including tissue mineral levels, mineraldependentbiomarkers and functionalassays, can we make well-informed,confident decisions about trace mineralsupplementation.ReferencesBlanchard, R.K., J.B. Moore, C.L. Green and R.J.Cousins. 2001. Modulation of intestinal gene expressionby dietary zinc status: Effectiveness of cDNAarrays for expression profiling of a single nutrientdeficiency. Proc. Natl. Acad. Sci. 98:13507-13513.Cao, J., P.R. Henry, S.R. Davis, R.J. Cousins, R.D.Miles, R.C. Littell and C.B. Ammerman. 2002. 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