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Commentaiy

Use and M isuse of Balance StudiesWALTER MERTZU .S . D ep artm en t o f A gric ultu re , A RS B elts ville H um an N utritio n R ese arc h C en te r, B eltsv ille , A ID 2 07 05

ABSTRACT B ala nce stu dies a re in ad eq ua te to ols to d ete rm in e human r equir emen ts fo r m in er als a nd tr ac e e lem ents. A t best, they estim ate th e am oun t of an elem entin a sp ecifie d d iet th at m ain ta in s th e ex istin g p ool siz e o fthe test subject. T he relevan ce of pool size to h ealth rem ains to be established by different and independentmeans. J. Nutr. 117: 1811-1813, 1987.INDEXING KEY WORDS:balance studiespoo l s ize

trace e le me nts diet

T he follow ing discussion of balance studies w ill berestricted to m inerals and trace elem ents. A lthoughsom e of the principles to be discussed should be validfor o th er e lem en ts as w ell, su ch a s n itro gen an d carb on ,the differences betw een inorganic and organic compounds are such that the inclusion of the latter w ouldtranscend not only the available space but also the expertise of the author. Balance studies have been usedfo r two purpose s: to h elp d efin e nutrie nt re qu iremen tsa nd to mea su re th e a vaila bility o f nu trie nts fo r ab so rption. M y attem pt to exam ine the legitim ate uses of theb alan ce ap proa ch w ill e nd w ith th e co nclu sio n th at th edeterm ination of hum an trace elem ent or m ineral requirem ents by this m ethod is neither legitim ate noru se fu l. T ha t c on clu sio n is n ot n ew ; Heg ste d h ad warn edlong ago against deriving m ineral requirem ents frombalance studies (1). T hat his w arning has had little effect m ay serve as one justification for the present discussion.T he te chnic al d iffic ultie s in he re nt in b ala nc e stu die s(e .g ., ens ur ing comp le tene ss o f colle cti on , a ss es smentof cutaneous losses and use of fecal m arkers, etc.) arewell know n and need not be discussed here. Less appreciated is the fact that few analytical m ethods givea coefficient of variation as good as 5% , especially inc omplex matric es su ch as fo od , u rin e o r fec es (2).Mostm ethods, therefore, w ill be unable to detect the sm allbut nutritionally im portant differences that one can

expect in studies with elem ents of low intestinal abso rp tion efficie ncy . T his alo ne elim in ates se veral elem ents from consideration. Y et, the follow ing discussio n wou ld b e v alid ev en in th e ab se nc e of an y te ch nica lproblem.A t first it appears perfectly logical that an intake ofa nutrient less than the sum of the daily losses mustat som e tim e result in deficiency and that such an intake m ust therefore be considered inadequate. T his iso bv io usly n ot th e c ase ; clin ical, p ro no un ce d d eficie nc ies th at wou ld b e ex pe cte d, acc ord in g to tha t assumptio n, a re e xtremely ra re in d ev elo ped countrie s, in sp iteo f in ta ke s well b elow those re su ltin g in z ero o r p ositiv ebalance. The follow ing brief review of a few sim ple,w ell-established facts w ill try to delineate the properin te rp re ta tio n o f re su lts from bala nc e stu die s.C otz ia s (3 ) a nd colla bo ra to rs in p io ne erin g stu die s o fm anganese m etabolism dem onstrated in hum ans andanimals that the loss of manganese from the intacto rgan ism can be des cr ibed by a s er ie s o f f ir st- order equations. Expressed in the sim plest term s, the am ount ofd aily lo ss o f mangane se from th e o rg an ism is a fu nc tio no f t he existin g body sto re s a nd c an b e d esc rib ed in te rmso f p erce ntag e o f th e e xistin g sto res. In re ality, b ec au seth e ex cre tio n k in etics are multicomp artm en tal, w e ared ea lin g w ith a number o f d i ffe re nt ra te s o f l oss o f whic hthe first is usually fast and of short duration, w hereassubsequent ones are slow er and of longer duration. Itis not know n whether any of the com partm ents identified by kinetic studies correspond w ith any anatomical or biochem ical entity in the organism , but it canbe assum ed that the first tw o (perhaps also the third)com partm ents are m etabolically m ore active and possib ly nutritio na lly mo re sig nific an t th an th e la te r o ne s,w hich m ay function as sinks and participate little inme tabo lism . Co tz ia s and collabo ra to rs r epor ted s evera lobservations that proved the validity of their m odel.Amon g th em was th e d emon stra tio n th at the e xcretiono f p revious ly admin is te red, r ad ioac ti ve manganes e wasstrongly accelerated by the adm inistration of stablemanganese.Since then sim ilar excretion kinetics have been established for zinc (4) and chrom ium [5], calcium (6)

0022-3166/87 $3.00 1987 American Institute of N utrition. R eceived 24 July 1987. A ccepted 27 July 1987.1811

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1812 MERTZm agnesium (7), selenium (8) and possibly others. T hism odel m ay not be fully applicable to iron for w hich theam ount of losses appears to be m uch less subject toregulation and to depend largely on the am ount of ironsequestered in the intestinal m ucosa.N otw ithstanding the m ulticom partm ental nature ofex cretion k inetics a sim plif ied statem ent can be m ade,probably applicable to m any trace elem ents, that theam ount of daily loss f rom the body pool is som e function of the siz e of that pool. Intak es that result in anex cess of absorbed trace elem ent ov er the prev ailingdaily loss w ill increase the pool until the daily lossequals the daily input. A t that tim e a steady state isreached. S hould the intak e now be dim inished, the dailyloss w ould initially ex ceed the inf low , resulting in adim inution of the pool until at som e tim e the sm allerdaily outf low w ould again equal the inf low and a newequilibrium w ould be established. T hus, ex pressed inpractical term s, the am ount of av ailable nutrient in thediet determ ines the pool siz e w ith tim e, but does notpredict a priori def iciency or tox icity . It m ust be understood, of course, that the regulation of body pools oftrace elem ents is m uch m ore com plicated than suggested by this sim ple m odel (9). T here are ex ogenousand endogenous factors af fecting the rate of inf low intothe pool and endogenous controls of rates of daily loss(T ab le 1 ).

PHY S IOL OGICA L IN TERPRETA TION OFB A L A NC E S TUDIEST im e is a decisiv e factor in the interpretation of datafrom balance studies. W hen the daily intak e of an ele

m ent is sm all relativ e to the total pool siz e, it w ill tak em ore tim e to establish a new equilibrium af ter changesof intak e than for an elem ent w hose intak e is largerelativ e to the body pool. T he daily turnov er of iron isonly 1/5000 of the body content; thus, changes in thepool siz e and establishm ent of a new equilibrium w ouldbe ex pected to tak e m any m onths. In contrast, the dailyturnov er of se lenium in N orth A mericans could be asm uch as 1/200 of the body content, resulting in a fasterrate of changes and of attainm ent of a new equilibrium .T hese ex am ples dem onstrate how som e elem ents areTAB LE 1

Factors af fect ing m ineral balanceInflow OutflowA m o unt eatenGas tro in te st in al l um i nal e ve nt s:pH , d ie tary i nt erac ti on s,ch em ic al f orm o f n utrien tR at e o f tran sitMucosal re gu lat io n: ab so rp ti onef ficiency depending on pool siz eH orm o nes ; ce ph alic inf lu en ces (?]

Po ol s iz eHormonesK idn ey func ti onInt es ti nal f u nc ti on

w e ll bu ff ered against the consequ ences of intak e c hanges,w hereas others are subject to m ore rapid changes inpool siz e. T he im portant phy siological conclusion isthat negativ e balances do not persist (prov ided the intak e is abov e the m inim um obligatory loss), and thatthe equilibrium w ill establish itself , ev en if it tak esy ears. T he pool siz e at w hich the new equilibrium isestablished depends on the intak e.A lthough the f oregoing discussion has postulated thata trace elem ent balance can be achiev ed w ith a v arietyof intak es and pool siz es, there are lim its to balance onboth ex trem es of intak e. H igh ex posures m ay ex ceedthe ex cretion capacity of the organism and result in apersistent positiv e balance and a continuing accum ulation. T hat is the concern w ith certain heav y m etalsin the env ironm ent, such as lead and cadm ium , andev en w ith the essential elem ent, iron. On the otherhand, ex trem ely low intak es m ay not com pensate form inim al obligatory losses and result in def iciency .(M inim al obligatory losses are those persisting in subjects w ho are adapted to a diet sev erely def icient in the

e le m en t u nd er in ve stig atio n.)W ith intak es betw een those ex trem es, a positiv e ornegativ e balance w ould indicate that the ex perim entaldiet fed is dif ferent in am ount, com position and bio-av ailability f rom the subject's habitual diet. A positiv e(or negativ e) balance indicates that the intak e and/orbioav ailability of the elem ent under study from the testdiet is greater (sm aller) than the habitual intak e andthat the pool siz e is increasing (decreasing) until a newequilibrium is reached. A zero balance indicates thatthe intak e during the study of the av ailable elem ent isequal to the habitual intak e, nam ely , that am ount necessary to m aintain the ex isting pool siz e. T hus, a balance study does not determine " the" requirement fora mi ner al el ement, but the i ntake r equi red to mai ntai nthe exi sti ng pool si ze.

P RAC TIC AL A S P ECTS

A lthough balance studies hav e been used w idely inthe past to estim ate requirem ents for m inerals and traceelem ents, their suitability for that purpose has beenquestioned long ago, w hen H egsted discussed the w idelydif ferent calcium intak es required to m aintain balanceand bone health in A m ericans v ersus those in Peruv ians (1). T he situation w ith selenium is sim ilar: L ev -ander has show n that on av erage 1 jig of selenium /k gbody w eight m aintains a zero balance and seleniumpools in A m ericans. A pprox im ately one-tenth of thatintak e m aintains a balance in subjects liv ing in low -selenium areas of China (10). W hereas in these latterpeople the risk of selenium def iciency and the resultingK eshan disease is signif icant, the slightly larger selenium stores of N ew Z ealand w om en (11), m aintained byan intak e of approx im ately 20 M -g/d/hav e not been associated w ith increased health risk .

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USE A ND M ISUSE OF B ALA NCE STUDIES 1813T hus, w e m ust conclude that the results of balancestu die s b ear no d ire ct re latio nsh ip to c rite ria o f h ealth .T hey do relate to the pool siz e of the nutrient understu dy (and th at siz e c an b e calc ulate d f rom balan ce dataif th e tu rn ov er rate is k now n), b ut th e re lev an ce o f p oo lsiz e to hum an and anim al health rem ains to be determ in ed in div idu ally f or e ac h e lem en t.LEGITIMATE USES O F BALANCE STUDIE S

The re su lts o f b alan ce stu die s c an b e u se d to e stimatethe "habitual" nutrient intak e of people. It can be assum e d that a person's pool of a nutrient ref lects, nex tto body siz e, the am o unt and bioav ailability of the nutrient consum ed ov er ex tended periods of tim e . A dietthat results in a zero balance in a person and, by def inition, m aintains the ex isting pool siz e, theref ore approx im ates closely the diet history of that person forthe am ount and bioav ailability of the nutrient understudy . O nly in that connection, as an estim ator of habitual dietary intak es, and only if the status of the nutrient in question has been prov ed adequate by independent m ethods, can the results of balance studies beused as a basis for dietary recomm endations. In thatcase, adding an increm ent as a safety factor to the estim ated requirem ent, as is usually done f or R DA s, isu nn ec essary . It w ould re su lt in u nre alistic ally h ig h re comm e ndations that w ould be dif ficult to im p lem e nt.B alan ce stu die s c an b e u se d to c ompare th e b io lo gic alav ailability of dif ferent elem e ntal species w ith eacho th er. A h ig hly availab le sp ec ie s, su ch as th e h eme iro n,w ill produce a m o re positiv e or less negativ e balancethan one poorly av ailable, such as the non-hem e ironcategory . B alance studies can also be helpful in thequan tif ic atio n o f c omp le x nutrie nt in te rac tio ns as th eyin flu en ce b io availab ility . S andste ad has f ed d ie ts w i thd if f ere nt c on te nts o f p ro te in , p ho spho ru s and calc ium ,e tc., an d ex pre ssed th e e ff ec ts o f th ose n utrien ts o n th ere qu irem en t f or trac e e lem e nts to estab lish a z ero b alan ce in re gre ssio n equatio ns (1 2). S u ch stu die s are b estperf orm ed w ith low or interm ediate intak es of the test

substance and in subjects w ith a low or interm e diatepool siz e to m agnify the ef f ect on absorption. T heyshould be of relativ ely short duration and should bete rm i nate d b ef ore a balan ce is e stab lish ed e v en tu allyf or su bstan ce s both o f h ig h and low b io availab ility .

LITERATURECITED1 . HEGS TED,D. M . (1973) Calcium and phosphorus. In: M odernNut ri ti on i n Heal th and D i sease, 5t h ed. (Goodhar t, R. S. & Shi l s,M . E ., e ds.), p p. 2 68 -2 86 , L ea an d Fe big er, Philad elp hia.2. HOR W ITZ ,W ., K AM PS ,L . R . & BOY ER ,K . W . (1980) Qualityass uran ce in th e analy sis of f ood s f or trace c ons titu en ts. /. A s so c.O ff . Anal . Chem. 63: 1344- 1354.3. CO TZ IA S ,G . C . (1962) M anganese. In: M ineral M etabolism :An Advanced T reat ise, vol . 2B (Comar , C . L . & B ronner , F ., e ds.),pp. 403-442, A cadem ic, N ew Y ork .4. A A M OD T,R . L ., R UM B LE,W . F., JOHN S TON ,G . S ., FO ST ER ,D . &HEN KIN ,R . I. (1979) Z inc m etabolism in hum ans af ter oraland intrav enous adm inistration of Z n-69m . A m . J. C lin. ut32: 559 -569 .5. S A RG EN T,T ., m , L IM , T. H. & JEN SON ,R . L . (1979) R educedchrom ium retention in patients w ith hem ochrom atosis, a possible basis of hem ochrom atotic diabetes. M etabolism 28: 70-79 .6. B RO NN ER ,F. (1974) Dynam ics and functions of calcium . In:M i ner al Metabol ism, vol . 2A (Comar , C. L . & Br onner , F ., eds.),pp. 342-444, A cadem ic, N ew Y ork .7. A V IOL I,L . V . & B ER MA N ,M . (1966) M g28 k inetics in m an. /.

Appi Physi ol 21: 1688-1694.8. G RIFFIT H, . M ., S TEW A R T ,. D .H . & R OB IN SO N, . F. (1976) T hem e tab olism o f 7 5S e-se le nom eth ion in e in f ou r w om en . B r. f . N u tr.35 : 373-382 .9. SOLOM ON ,A . K . (1960) Com partm ental m ethods of k ineticanaly sis. In: M ineral M etabolism , v ol. 1A (C om ar, C . L . & B ronner, F., eds.), pp. 119-167, A cadem ic, N ew Y o rk .10. LEVANDER ,. A . (1986) Selenium . In: Trace Elem ents in H uman and Ani mal Nutr iti on, 5th ed. (M er tz, W., ed.), pp. 209-279, A cadem ic, N ew Y o rk .1 1. STEWART ,. D . H ., GRIFFITH ,. M ., THOMSON , . D . & ROBINSONM . F. (1978) Q uantitativ e selenium m etabolism in norm al N ewZ ealand w om en. B r. J. N utr. 40: 45-54.12. SANDSTEA D, . H. (1985) Are estimates of trace elem ent requirem ents m eeting the needs of the user? In: T race Elem entsi n M an and Ani mal s (TEMA-5) (M il ls, C. F ., B remner , I . & Ches-ters, f . K ., eds.), pp. 875-878, C omm onw ealth A g ricultural B ureaux , Farnham R oy al, U .K .