am j clin nutr-1992-wang-19-28

7/27/2019 Am J Clin Nutr-1992-Wang-19-28

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.1 ,71 i C /u i Nu i r 1992 :56 : 19-28 . P r in ted in U SA . 1992 Am erican Soc ie ty fo r C lin ica l N utrition 19

The five -leve l m ode l: a new app roach to o rgan iz ingbody -com pos itio n resea rch1 2Z i- fia n W an g , R ichard N P ie rson Jr. a nd S teven B H eym s/leld

AB STRA C T Body -com po sition resea rch is a b ranch of hu -m an b io logy tha t has th ree in terco nnec tin g areas: b ody-com -pos itio n leve ls and the ir o rgan izationa l ru les, m easurem en ttechn iques , and b io lo g ica l fac to rs tha t in fluence b ody com po-s ition . In the f ir st a rea , w hich is inadequ a te ly fo rm ula ted a t p res-en t, five leve ls o finc reasing com plex ity a re p ro posed : I, a tom ic :II. m olecu la r: I II, ce llu la r; IV , tissue-system : and V . whole body .A ltho ugh each leve l and its m ultip le com partm en ts a re d istinc t,b iochem ica l an d physio log ica l connec tio ns ex is t su ch tha t th em ode l is con sisten t and func tions as a w hole . T he m ode l alsoprov id es the opportun ity to c lea rly define the concep t o fa b odycom po sitio n steady sta te in w h ich quan tita tiv e assoc ia tion s ex is tover a spec ified tim e in te rv al b etw een com partm en ts a t the sam eor d if fe ren t leve ls . F ina lly , the fiv e -lev el m ode l p rov ides a m atrixfo r c rea ting exp lic it bod y-com po sitio n equ a tion s, revea ls gapsin the study ofhum an body com po sition , and sug gests im po rtan tnew research areas. .1 ,n J C /in Nutr 1992 :56 :19-28 .

K EY W O RD S Body com pos itio n , nu tritio na l assessm en t.s teady s tate

IntroductionThe study of hum an body com pos itio n sp an s > 100 y and

con tinu es to b e an ac tiv e a rea o f bas ic science and c lin ica l re -search . N early ev ery aspect o f clin ica l nu tr ition , selec ted areasw ith in m any m edica l spec ia ltie s , an d com ponen ts o f exerc isesc ience are touched on by th e s tudy of body com pos ition .

In fo rm ation re la ted to body com position is accum ula tingrap id ly and is ex ten d ing our kn ow ledge ofhum an b io log y . M osto f th is in fo rm ation is now ca tegorized as techn ica l o r b io log ica l.T he techn ica l ca teg ory in clud es the m any c lass ic and con tin ua llyem erg ing n ew body -com pos itio n m eth ods . A ltho ugh no sys-tem atic c lass if ica tion fo r bo dy-com po sition m ethodo logy h asbeen propo sed . in fo rm al g rou p ings are o f ten pub lished . su ch asd ilu tion techn iq ues and neu tron -activa tion ana lys is . w h ich arebased on a p hys ica l p rinc ip le o r o ther charac teris tic s o fthe tech-n iqu es inv o lved . T he b io log ica l ca tegory inc ludes in fo rm ationon the study ofhow grow th , deve lopm ent, p regn an cy . lac ta tion ,ag in g . ex ercise , and d isease in fluence body com po sition .

A ltho ugh the techn ica l and b io lo g ica l catego ries w ou ld appearto encom pass m o st b ody-com position in fo rm atio n , a recen tstudy ( 1 ) led u s to app recia te a se rious lim ita tio n of the fie ld asit is now organ ized . W e recogn ized th at no t all o f the rap id lyaccum u la ting in fo rm ation em erg in g from body-com positionresea rch cou ld be sa tisfac to rily inc luded in to th e techn ica l an d

b io lo g ical ca teg ories. F or ex am ple, the re a re m any m ath em atica lm od els th at desc rib e th e re la tio ns be tw een d iffe ren t com pon en tsin hea lthy sub jec ts [eg , to ta l bo dy w ate r (TBW )/fa t-free bodyma s s = 0 .732 ] (2 ) . T h is fo rm ula tion ind ica tes tha t som e quan -tita tive assoc iations ex ist tha t desc ribe the re lationsh ips am ongcom partm en ts tha t a re in equ ilib rium . A nother exam ple is p ro -v id ed by the reco nstruc tion of hum an chem ica l com partm en tsan d body w eigh t (Bw t) from elem en ts estim ated in v ivo by neu-tron-ac tiv a tion ana lys is ( 1 ). This sug gests tha t re lationsh ips ex is tn o t on ly b etw een ind iv idua l com ponen ts bu t be tw een d iffe ren tleve ls o f bo dy com pos ition as w ell.

A no ther p rob lem is tha t inv es tig a to rs a re frequen tly con-fron ted w ith q uestions abo u t te rm ino log y . F or exam ple : A relip id - free body m ass , fa t-free body m ass , and lean body m ass(LBM ) the sam e or d iffe ren t com partm en ts? T he lack of c lea rdefin itions fo r bod y-com pos itio n com ponen ts has a sub tle b u tse riou s consequence : m any erro rs a re ev iden t in p ub lished body-com pos itio n equa tio ns and m ode ls because ofov er lap or om is-sion ofcom ponen ts. In fac t, w e cou ld find no c lea r approach todefin ing com ponen ts and bu ild ing m ulticom partm en t body -com pos itio n m ode ls in ex ten sive rev iew s of p rev io us lite ra tu re .

G row ing from th ese observa tion s is the hypo thesis tha t a th irdcen tra l ca tegory of body -com pos ition resea rch ex ists tha t un tilnow has n o t been ad eq ua te ly fo rm ulated : the lev els o f bod ycom position and th eir o rgan iza tio na l ru les . T h is report p resen tsa com prehens ive m ode l o fhum an body com po sition con sistingof five d istinc t lev els o f inc reasing com plex ity in w hich eachleve l has c lea rly defined com ponen ts th a t com p rise to ta l Bw t.T he fiv e lev els a re I. a tom ic : II, m o lecu la r: III , cellu la r: IV . tis sue-system : and V . w hole b ody (F ig I).

The fo llow ing sec tio n presen ts a de tailed desc rip tio n o f eachleve l and its asso cia ted com pon en ts . In the nex t sec tion the fea -tu res o r o rgan iza tio na l ru les o f the m ode l as a w hole a re d e-sc ribed . Im po rtan t con cep ts re la ted to d ev e lo pm ent o f bod y-com position m ode ls an d equa tions a re p resen ted in th is po rtio nof th e paper, and th e w id e ly apprec ia ted bu t never fo rm ally d e-fined co ncep t o fa steady sta te o fbody com pos ition is in trod uced .

I F rom the Obes ity R esearch C en te r. S t L ukes-R ooseve lt H osp ita l,C o lum bia U nivers ity . C o llege of P hys ic ians and Su rgeons . N ew Yo rk ,N Y .

2 A ddress rep rin t reques ts to Z -M W ang , W eigh t C on tro l U nit. 4 I 1W est I 1 4 th S tree t, N ew Y ork , NY 10025 .

R ece ived S ep tem ber 1 7 , 199 1 .A ccep ted fo r pub lica tio n D ecem ber 12 . 1991 .


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FIG 1. T he f i ve level s of human body composi ti on. ECF and ECS,ex tracel lul ar f lu i d and sol ids, respecti vel y.

TA BL E 1B ody composi ti on on the atom ic lev el (I ) f or the 70-kg ReferenceM an

Element A mount Percent of body weightkg %

Oxygen 43 61Carbon 16 23H ydrogen 7 10N i trogen 1.8 2.6Cal cium 1.0 1.4Phosphorus 0.58 0.83Sulfer 0. 14 0.20Potassium 0. 14 0.20Sodium 0.1 0.14Chlori ne 0.095 0.14M agnesium 0.019 0.027Total 69.874 99.537

* I nf ormati on based on reference 2 (modi f ied).

Fiv e -lev el m od e lA tom ic (I)

The fundamental bui l di ng block s of the human body are atomsor elements. O f the 106 elements, 50 are found in the humanbody and thei r di str ibutions in the vari ous tissues and organsare w el l documented (2). Si x el ements (ox ygen, carbon, hydro-gen, ni trogen, cal ci um , and phosphorus) account f or > 98% ofBw t, and one element, oxy gen, consti tutes > 60% of total bodymass in the Reference M an (T able 1) (2). T he remaini ng 44elements make up < 2% of Bw t.

T he equati on for Bw t, as def ined in the atom ic l evel of bodycomposi tion i sBwt=O+C+H+N+Ca+P

+S+K+Na+Cl+Mg+R (1 )w here R is the resi dual mass ofal l el ements present in amounts< 0.2% percent of Bw t (1).

E lem ental anal ysi s of humans i s tradi ti onal l y carri ed out i ncadavers or in biopsy specimens f rom selected ti ssues and organs.I n addi ti on, the whole-body content of most major el ementscan now be measured di rectl y i n v i vo: potassi um by w hole-bodycounti ng: sodium , chlori ne, and cal cium by delay ed--y neutron-acti vati on anal ysis (3); ni trogen by prompt- y neutron-acti vati onanal ysi s (1 , 3): and carbon by inelasti c neutron scatteri ng (4).M ore than 98% of Bw t can now be reconstructed f rom elementsthat can be estimated in v i vo, l argel y by neutron-acti vationtechniques. T he atom ic l evel i s the foundation of body -corn-posi ti on anal ysi s and is the starting point f or the f i ve l evels w epropose.P110/ce l l /ar ( I I )

The 1 1 pri ncipal el ements are incorporated into moleculesthat f orm > 100 000 chem ical compounds found in the humanbody . T hese molecules range in complex i ty and molecular w eightf rom water to deoxy ri bonuclei c acid. I t i s nei ther useful norpossibl e to measure al l of these chem ical compounds indi v i dual l yi n l i v i ng humans. T he al ternati ve used in body-composi ti on re-search is to consider chem ical compounds in categori es of cl osely

related molecular species. T he major components i n present useare w ater, or aqueous (A ) : l i pi d (L ) ; protei n (Pro): m ineral (M ):and gl ycogen (G ) (T able 2). B ecause some confusi on ex i sts inthese di f f erent categori es. w e now rev iew the f i ve chem ical corn-ponents i n detai l .

Water . The most abundant chem ical compound in the humanbody i s w ater , w hi ch compri ses 60% of B w t in the ReferenceM an (2).

Pro te in . The term protei n i n body -composi ti on research usu-al l y i ncl udes almost al l compounds contain ing ni trogen, rangingf rom simple am ino acids to complex nucleoproteins. T he mostw idel y used representati ve stoichi ometry f or protei n i sCH 59N 26O32S07 . w i th an average molecular w eight of 2257.4and densi ty of 1.34 g/cm3 at 37 # {1 76 }C1, 5).

G/icogen . The primary storage form of carbohydrate i s gly -cogen. w hi ch is f ound in the cy toplasm of most cel l s. T he prim -cipal di str i buti on i s i n skeletal muscle and l i ver, w hich contai n 1% and 2.2% of thei r respecti ve w et w eights i n the form of

T A B L E 2Body composi ti on on the molecular l ev el (I I ) f or the 70-kg ReferenceMan*

Component A mount Percent of body w eight1c v %

WaterEx tracel l ul ar 18 26Intracel l ul ar 24 34

L ip i dN onessenti al (f at) 12 17Essential 1. 5 2.1

Protei n 10.6 15M ineral 3.7 5.3Total 69.8 99.4

* Glycogen. normal ly 400 g. i s not i ncl uded in the Reference M an.I nf ormati on based on reference 2.


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FIV E-L EV EL BODY COM POSIT ION M ODEL 21gl ycogen ( I . 2) . The stoi chi ometry of gl ycogen i s (C6H10O5)w i th an average densi ty of 1 .52 g/cm3 at 37 #{ 176} C1 , 2).

.tIiThl(ll. The term m ineral descri bes a category of i norganiccompounds contai ni ng an abundance of metal elements (eg.cal ci um . sodium . and potassium ) and nonmetal el ements (eg.oxygen. phosphorus. and chlori ne). A sh, a term sim i lar to mm-eral . i s the resi due of a biol ogical sample heated for a prol ongedper iod to > 500 # {1 76 }C ,nd consi sts of the nonvolati l e por ti on ofm ineral compounds. T otal body ash i s sl ightly l ow er i n w eightthan m ineral mass because of the loss of carbon diox i de f romsome carbonate groups and the rel ease of ti ghtl y bound w aterduri ng the heati ng peri od ( 1 . 2). M ineral i s usual l y div ided intotw o subcategori es: osseous and ex traosseous. Osseous m ineral .the l argest component of w hi ch is cal ci um hydroxyapati te( [Ca3(PO4)2]3 Ca(OH )2). contai ns > 99% of total body cal ci um(TBCa) and 86% of total body phosphorus in the ReferenceM an (2). Other el ements, such as potassium , sodium . and chlo-r ime. are primari ly f ound in ex traosseous m ineral .

L ip id . A mong the f i ve pri ncipal chem ical components on themolecular level , l i pi d i s the most confusi ng because the termsl i pi d and fat are used interchangeabl y . ev en though stri ctl yspeak ing they refer to di f f erent compartments. T he tradi ti onaldef in i t i on of l i pi d ref ers to a group of chem ical compounds thatare i nsoluble in w ater and very soluble i n organic sol vents suchas diethy l ether, benzene, and chlorof orm (6, 7). A bout 50 di f -f erent l ip i ds are recogni zed in humans, and these are div ided byorgani c chem ists i nto f i ve subcategori es: 1 ) simple l i pi ds (i n-cluding tri gl ycerides and w axes): 2) compound l i pi ds (eg. phos-phol ipi ds and sphi ngol i pi ds): 3 ) steroi ds: 4) f atty aci ds: and 5)terpenes (6).

T he simple l i pi d, tr ig ly cer ide, contai ns three fatty aci ds ester-i f l ed to gl ycerol . T he term fat i s synonymous w i th tri gl yceri deand therefore f at is cl earl y a subcategory of total l ip id (6. 7). Acommon error i s to conf use the terms fat and l i pi d, w hich canlead to er rors i n constructing model s of body composi tion. I nthe adul t. 90% of total body l i pi d i s f at (2).

L i pi ds can al so be classi f i ed physiol ogi cal l y i nto tw o groups:essenti al (L e) and nonessenti al (L n) (2). Essential l i pi ds. such assphingomyel in and phosphol i pi ds, serv e important f uncti onssuch as f orm ing cel l membranes. The nonessenti al l i pi ds, l argel yin the form of tr i gl y ceri de, prov i de thermal i nsulati on and astorage depot ofmobi l i zable f uel . A bout 10% of total body l i p idis essenti al and 90% is nonessenti al i n the Reference M an (2).

A lthough essenti al and nonessenti al l i pi ds are structural l y andphysiologi cal l y di f f erent. thei r sol ubi l i t ies i n organi c sol vents aresim i l ar and i t i s di f f i cul t to cl ear ly separate them even in v i tro(6. 7). A n approx imate separati on can be accompl ished by carefulsel ecti on of the type of ti ssue anal yzed, the ex tracti on tim e andtemperature, and parti cul ar ly the type of sol vent used (6). Sol -v ents such as petrol eum or ethy l ether are usual l y used alone toex tract nonessenti al l i pi ds, mainl y the neutral f at or tr i gl y ceri de.T he remaini ng l i pi ds. w hi ch are primar i l y essenti al . can be ex -tracted by using binary or ternary sol vent m ix tures such as 45%chlorof orm . 10% methanol , and 45% heptane (8).

T he fatty aci d prof i l e of tr i gl ycer ide vari es w i th diet. anatom icsi te. and other f actors. but the general l y accepted representativ estoi chi ometry f ound in adul t humans i s C51H98O6, w i th an av -erage molecular w eight of 806 and a densi ty of 0.900 g/cm3 at37#{ 176} C2). T he stoi chi ometry of total l i pi d i n humans could notbe found in a rev i ew ofprev ious studies.

The equati on f or Bw t as def i ned by the molecular l evel ofboth composi ti on i s

Bw t= L +A + P r o +M+G +R (2 )w here R represents resi dual chem ical compounds not i ncludedin the f i ve main categories and that occur i n quanti t i es of < I %of total Bw t (1).

On the molecular l evel , three rel ated equati ons can al so bedef i ned as fol l ow s. D ry Bw t consi sts of the anhydrous chem icalcomponents (Fig 2, l ef t), and equati on 2 can therefore be re-w ri tten as

Bw t = A + dry Bw t (3 )D ry Bw t accordi ng to thi s equati on i s the sum of L + Pro + M+ G + R.

B w t = L + l i pi d-f ree body mass (4 )I n equati on 4 (Fig 2, ri ght). l i pi d-f ree body mass i s the materi alremaini ng af ter ex tracti on of a w hole-body homogenate w i thappropr iate organi c solv ents and optimum condi ti ons. T husl i pi d-f ree body mass can be expressed as the combined w eightof A + Pro + M + G + R.

A s fat accounts almost enti rel y f or total body nonessenti allipid, then

Bw t = f at + FFM = L n + FFM (5 )where FFM is f at-f ree body m ass, w hich represents the combinedweights of L e + A + Pro + M + G + R.

A sim i l ar term to fat-f ree body mass i s L BM . T he earl y def -in i t i on of l ean body mass suggested incl uded at l east f i ve corn-ponents: w ater, protei n. m ineral . gl ycogen, and an unspeci f i edamount of essenti al l i pi d (9. 10) . M ore recently , most i nvesti -gators have used the terms L BM and FFM interchangeably . al -though some debate sti l l prevai l s about w hether or not these arethe same or di f f erent compartments. Our suggesti on is that L BMand FFM henceforth be considered synonymous on the basi s ofthe fol l ow ing reasoning.

I n equati on 4 w e clearl y def i ne tw o f racti ons ofB w t, l i pi d andl i pi d- f ree body mass. T he l i pi d f racti on consi sts of tw o portions.essential and nonessenti al or f at. A ccordi ngly ,

FIG 2. B ody-composi tion model on the molecular level (I I ). FFM .f at-f ree body mass: L FM . l i pi d-f ree body mass: and L e and L essenti aland nonessenti al l ipi ds. respecti vel y.


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* L n, nonessenti al l ip id or f at; and L e, essential l i pi d.

B ody w eight L n + L BM = f at + FFMin w hich both L BM and FFM are the sum of essenti al l ip id pl usl i pi d-f ree body mass and the remaini ng Bw t i s nonessenti al l i pi dor f at. A l l of the terms of the molecular l ev el are consistent w i theach other w hen def i ned accordi ng to these guidel i nes and asshow n in T able 3.

A t present the main di rect techniques avai labl e f or estimati ngcomponents on the molecular l evel are f or w ater and m ineral .T BW can be measured by several w el l -establ i shed isotope-di -luti on techniques ( 1 0. 1 1), and osseous m ineral can be estimatedby whole-body dual -photon absorpti ometry (12). The remaini ngcomponents of the model must be estimated indi rectl y by usingmeasurements i ncl uded in one of the other f our l evel s. For ex -ample, protei n can be determ ined f rom total body ni trogen atthe atom ic lev el by mak ing tw o assumpti ons: that al l of bodyni trogen i s i n protei n and that 16% of protein i s ni trogen (1).A nother example, total body fat can be cal culated f rom bodydensi ty , w hich is a dimension at the whole-body lev el , by as-sum ing that f at and FFM have respecti ve densi ties of0.900 and1.100 g/cm3 (9, 10).

T he molecular l evel of body composi ti on i s the conceptualf oundati on for the higher level s that f ol l ow . A l so, the m olecularl evel connects the study of body composi ti on to other researchareas, notabl y bi ochem istry .

C e llu la r (II I )A l though the human body can be di v i ded into di f f erent corn-

ponents at the molecular l evel , i t i s the assembly of these corn-ponents i nto cel l s that creates the l iv ing organi sm . T he coordi -nated functi ons and interacti ons betw een cel l s are central to thestudy of human phy siol ogy in heal th and di sease. T he cel lu larlevel i s therefore an important area of body -composi tion re-search.

T he human body i s composed of three main compartmentson the cel lu lar l evel : cel l s, ex tracel l ul ar f l ui d, and ex tracel l ul arsol i ds. Each of these compartments is now descri bed in addi ti onaldetail.

Cells . The cel l s possess the characteri sti cs of l i f e i ncl udingmetabol i sm , grow th, and reproduction. A l though the l018 cel l sof the adul t human body share m any properti es in common,there are great v ari ati ons in si ze, shape, el emental and molecularcomposi ti on, metabol i sm , and di str i buti on. Cel l s are adapted tospeci f ic f uncti ons, such as suppor t, el ectri cal conducti on, andcontracti on. B ased on these di f f erences, f our categori es of cel l scan be def i ned: connectiv e, epi thel i al , nervous, and muscular (13) .

(6 ) Connecti ve cel ls include three groups: l oose, dense. and spe-cialized ( 1 3). A dipocy tes. or f at cel l s, are a type of l oose con-necti ve cel l i n w hich f at i s stored. B one cel l s. the osteoclasts andosteoblasts. and blood cel l s are representati ons of special i zedconnecti ve cel l s.

M uscle cel l s i nclude striated skeletal , smooth. and cardiac.T he stri ated skeletal muscle cel l s are the foundati on of humanmovement and account f or a l arge f racti on ofbody weight. Cel l sconsist of f l ui d and sol id components, the intracel lu lar f l ui d andsolids.

E .v trace l/u /a r flu id . The nonmetabol izi ng f l ui d surroundingcel l s that prov ides a medium for gas ex change, transfer of nu-tr ients, and ex creti on of metabol ic end products i s ref erred toas the ex tracel l ul ar f l ui d.

Ex tracel l ul ar f l ui d, w hi ch i s 94% w ater by volume, i s di s-tr ibuted into tw o main compartments: plasma in the intravas-cular space and intersti t i al f l u id i n the ex trav ascular space.Plasma and intersti t i al f l ui d account f or 5% and 20% of Bw ti n the Reference M an (2), respecti vely .

E xtrace /lu la r so lid s. Extracel l ul ar sol i ds are al so a nonmetab-ol i zi ng porti on of the human body that consi sts of organi c andinorgani c chem ical compounds. T he organic ex tracel lu lar sol idsi ncl ude three types of f i ber: col l agen, reticular, and elasti c (13).B oth col lagen and reti cul ar f i bers are composed of col l agen pro-tei n w hereas elasti c f i bers are formed f rom the protei n elasti n.

T he inorgani c ex tracel l ul ar sol i ds represent 65% of the drybone matri x i n the Reference M an (2). C al ci um . phosphorus,and ox ygen in bone are the main elements of the inorgani c ex -tracel l ul ar sol ids that are incorporated into cal ci um hydroxy -apatite ( 1 ). Other inorgani c components are al so present i n ex -tracel l ul ar sol i ds. i ncl uding bicarbonate, ci trate, magnesium , andsodium (1. 2).

From the prev ious di scussion, the cel l ul ar l evel of body corn-posi tion can be accuratel y descr ibed by the equati ons

B w t = CM + ECF + ECSCM = muscle cel l s + connecti ve cel l s

+ epi thel i al cel l s + nervous cel l sEC F = plasma + 1SFEC S = organi c ECS + inorgani c ECS

(7 )

(8 )(9 )

(10)

w here CM is cel l mass, ECF is ex tracel lu lar f l ui d, ECS is ex tra-cel lu lar sol i ds. and 1SF is i ntersti t ial f l ui d. H owever. becausemost components i n equati ons 7-10 cannot be measured in

TA BL E 3D i f f erent body -composi ti on terms on the molecular level (I I )

L ip i ds*L n L e W ater Protei n M ineral G l ycogen

B odyw eight x X X X X xD ry body w eight x X x x xL ipi d-f ree body mass x x x xFat-f ree body mass x x x x xL ean body mass x x x x x


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Ino r g an ic C ell R es id ua lIno rg an ic E CI R esId ual

I norgani c ECSTotalBody

Mine ra l

FIG 4. Relati onship betw een total body m ineral and inorgani c sol i ds.ECF. ex tracel lu lar f lu i d: ECS, ex tracel l ul ar sol ids.

FIG 3. Relati onship betw een body f l u ids. ECF. ex tracel l ul ar f l ui d:ECW . ex tracel l ul ar w ater: I CF. i ntracel lu lar f l ui d: ICW . intracel l ul ar w a-ter: R, and R, . ex tracel lu lar and intracel l ul ar resi dual : and TBW . totalbody w ater .

FIV E-L EV EL BODY COM POSIT ION M ODEL 2 3v iv o at present. the fol l ow ing equati on is suggested as a practi calal ternati ve f or Bw t at the cel l ul ar l ev el

Bw t = f at cel l s + BCM + ECF + ECS ( Ii )where BCM is body cel l mass. 13CM is a por ti on of total cel lmass that accordi ng to M oore et al ( 1 1 ) i s the w ork i ng. energy -metabol i zi ng porti on of the human body in relati on to i ts sup-porting structure . H ence, B CM includes the protoplasm in fatcel l s hut does not i nclude the stored fat, w hi ch occupies 85% to90 of f at cel l w eight. A l though no present method can di rectl ymeasure B CM . i t i s a w idely used term and i s assumed to berepresented by ex changeable or total body potassium (TBK ) ( I 1) .A dev iati on must be noted in equati on 1 1 i n that BCM and fatcel ls share i n common the nonfat porti on of adipoces andtherefore overl ap by 1 kg in the Ref erence M an (2).

T he f lu id compartments at thi s l evel can al so be related toTBW as show n in Figure 3. A ccording to thi s model . ECW andICW are ex tracel l ul ar and intracel l ul ar w ater. and Re and Riare nonaqueous residual ex tracel l ul ar and intracel l ul ar sol i ds.

A nother relati on at the cel l ul ar l evel i s betw een total bodym ineral and inorgani c sol i ds (Fig 4). Each of the three compo-nents i n equati on 7 contri bute to total m ineral , i norganic cel land ex tracel l ul ar f lu id residual , and the inorgani c porti on ofex tracel lul ar sol ids.

Of the three pr imary compar tments at the cel l ul ar lev el . thevolume of ex tracel l ul ar f l ui d and i ts plasma subcompartmentcan be quanti f i ed di rectl y by di l uti on methods ( 1 0). I n contrast.no di rect methods are yet av ai l abl e f or estimati ng ei ther cel lmass or ex tracel l ul ar sol i ds. I ndi rect methods ofevaluati ng somecompartments are avai l able, such as ex tracel l ul ar sol i ds estimatedf rom TBCa measured by neutron-acti vation anal ysi s (ECS= TBCa/0. 1 77) ( 1 0) . A nother example i s the cal culati on of BCMf rom TBK [BCM (in kg) = 0.00833 X TB K (i n mmol )] ( 1 1)

B ecause the cel l ul ar l ev el i s the f i rst l evel at w hich character-i stics of the l i v i ng organi sm appear. i t occupies a central posi ti oni n connecti ng the inanimate features of body composi ti on atthe low er l evel s w i th those of the animate features of tissues.organs. and intact humans at the higher l evel s. D espi te i ts im -portance in the study of human body composi ti on. very l i tt leresearch has been di rected at thi s level , perhaps because of thedi f f i cul ty i n quanti f y ing some of the compartments.

Tissue-System (I J A t the cel l ul ar l evel the human body i s composed of cel l s.

ex tracel lu l ar f l ui d. and ex tracel l ul ar sol i ds. T hese three corn-ponents are f urther organi zed into ti ssues. organs. and systems-the four th l evel of body composi ti on.

i s s l1L s . General ly . t i ssues contai n cel l s that are sim i l ar inappearance. f uncti on. and embryoni c orig in. A l l of the di verseti ssues of the body can be grouped into f our categori es: muscular.connecti ve. epi thel ial . and nervous ( 1 3).

B w t at the ti ssue level of hody composi ti on i s def i ned asBw t = muscular ti ssue + connecti v e ti ssue

+ epi thel ial t issue + nerv ous tissue (/2)Three speci f i c ti ssues are par ti cul arl y impor tant i n body -corn-

posi ti on research: hone. adipose. and muscular, w hich togethercompri se 75 ofB w t in the Reference M an (2).

B one is a special ized form of connecti ve ti ssue that consistsof bone cel l s surrounded by a matri x of f i bers and ground sub-stance. T he disti ngui shing feature of bone is that the groundsubstance i s cal ci f ied and accounts f or 65 of dry bone weight(2). The cal ci f i ed ground substance i s mainl y hydroxyapati te([Ca3(P04)2]3Ca(OH ),) and a smal l amount of cal ci um car-bonate (14).

A dipose ti ssue i s another type of connecti ve ti ssue made upof f at cel l s (adipocv tes) w i th col lagenous and elasti c f ibers. f i -broblasts. and capi l lari es. A dipose ti ssue can be di v i ded intof our ty pes accordi ng to i ts di str i buti on: subcutaneous. v i sceral( i e. l oosel y surrounds organs and v i scera). i ntersti t i al ( i e. i nti -matel y i nterspersed among the cel l s of organs) . and yel l ow mar-row (2). M uscle ti ssue can be subdi v i ded into stri ated skeletal ,smooth. and cardiac ti ssues (2).

Oq.a i i s . The organs consi st of tw o or more tissues combinedto f orm large functi onal uni ts such as sk in, k i dney , and bloodvessels.

Siste ,ns. Several organs w hose functi ons are i nterrel ated con-sti tute an organ system . For example. the digesti ve system iscomposed of many organs. i ncl uding the esophagus. stomach,i ntesti ne. l i ver. and pancreas. Each organ. such as the stomach.


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24 W A NG ET A Lcontai ns several k i nds of ti ssue (muscular. connecti ve. epi thel i al .and nervous) and each tissue is made up of many cel ls andex tracel lul ar m ateri al .

T here are nine main systems in the human body . hence Bw tat the sy stem lev el of body composi ti on can be def i ned asBw t = musculosk eletal + sk i n + nerv ous

+ ci rcul atory + respi ratory + digesti ve + uri nary+ endocri ne + reproducti ve systems (13 )

A lthough Bw t can be expressed accuratel y on the ti ssue-systemlevel , most components in equati ons 1 2 and 1 3 cannot be mea-sured in v i v o at present. T he fol l ow ing equation i s suggested asa practi cal al ternati veBw t = adipose tissue + skeletal muscle

+ bone + v i scera + blood + R ( 14 )w here the f iv e components account f or 85% and R accounts f orthe remaini ng 1 5% of Bw t i n the Ref erence M an (T able 4) (2).

T he ti ssue-sy stem level i s complex and interf aces w i th severalbranches ofhuman biol ogy , i ncl uding hi stol ogy and hi stochem -i stry at the ti ssue lev el and anatomy and physiol ogy at the organand system level . Physi ci ans, nutri ti oni sts, and exerci se physi -ol ogists f ocus much of thei r i nterest in body composi tion at theti ssue-system lev el .

A lthough a great deal of inf ormati on is avai l abl e at thi s l evel ,most of i t comes f rom cadaver studies or ti ssue biopsies. T hereare onl y a few in v i vo di rect methods that can be used to estimatethe major compartments at the ti ssue-system level . A n exampleis com pu t er iz ed ax ia l tomography , w hich can di rectl y determ inethe volume of subcutaneous and v i sceral adipose ti ssue (15).Some indi rect techniques are also avai l abl e at thi s l evel . such asestimation of skeletal muscle mass f rom 24-h urinary creati ni neexcretion or f rom TBK and ni trogen content by neutron-acti -vati on anal ysi s (16, 17) .i/ i Ito /c b ody (I )

Both humans and some primates have sim i l ar body compo-si t i ons at the atom ic, molecular, cel lu lar, and ti ssue-system level s.I t is at the whole-body level , how ever, w i th i ts complex char-acteri stics that di sti ngui shes humans f rom al l other primates. I naddi ti on, many biol ogi cal , geneti c, and pathologi cal processeshave an impact not onl y at the f i rst f our l evel s but al so on thehuman body as a w hole.

T he w hole-body level of body composi ti on concerns bodysi ze, shape, and ex teri or and physical characteri sti cs. T here are 10 suggested dimensions at the whole-body level (18).

I) Stature: T his i s a major i ndicator of general body si ze andsk el etal l ength.

2) Segment l engths: M any segment l engths are used in thestudy ofbody composi ti on, the most common ofw hi ch are l ow erex trem i ty l ength, thi gh length, cal f l ength, shoulder-el bow length,and elbow -w ri st length.

3) Body breadths: B ody breadths are a measure ofbody shape,skeletal mass, and f rame si ze. T he si tes most w idel y used arethe w r ist, el bow , ank le, knee, and bi i l i ac.

4 ) Ci rcum f erences: The ci rcum ferences are useful i ndi catorsof body densi ty , FFM , adipose ti ssue mass, total body protei nmass. and energy stores. The most w idel y used ci rcum ferencesare upper arm , w ai st (abdom inal ), and thi gh.

TA B L E 4Body composi ti on on the ti ssue-system leveand organs of the 70-kg Reference M an*

I (IV ) f or principal ti ssues

T i ssue or organ A mount Percent of body w eight&v %

Skeletal muscle 28A di pose ti ssue

Subcutaneous 7.5V i sceral 5I ntersti t i al 1Y el l ow marrow I .5

B one 5B lood 5.5Sk in 2.6Liver 1.8Central nervous sy stem I .4Gastrointestinal tract 1.2L ung I

401 17.11.42.17.17.93.72. 621. 71.4

* I nf ormation based on reference 2 (modi f i ed).

5) Sk inf old thi cknesses: Sk inf olds represent a double lay er ofadipose tissue and sk in at speci f i c anatom ic l ocations. T ri ceps.subscapular, cal f (medial ), and abdom inal are the most corn-monl y used si tes. Sk inf ol d thickness prov ides a simple methodof estimating fatness and the distr i buti on of subcutaneous adi -pose ti ssue. N umerous equati ons for the predi cti on of body fathave been developed that make use of sk i nf ol d thi cknesses.

6 ) Body surf ace area (B SA ): T he total BSA is an ex teri orcharacteri sti c that i s of ten used to estimate basal metabol i c rateand FFM .

7) Body volume: T he total body volume is an important i n-dicator of body size and is used to cal culate body densi ty .

8) Bw t: One of the simplest and most impor tant morphologi ci ndi cators. Bw t i s used in screening f or grow th rate, obesi ty . andundernutri t i on. T he B w t equati on that def i nes the w hole-bodylevel i sBw t = head w eight + neck w eight + trunk weight

+ l ow er ex trem i ty w eights + upper ex trem i ty w eights (15 )9) B ody mass index : Bw t and stature can be combined to

f orm indi ces that correlate w i th total body fat. T he best k now nof the indi ces i s body mass index (body w eight/stature2. i n kg!m2), w hi ch is of ten used in obesi ty studies as a measure of f atness( 1 9). H ow ever, more complex and population-speci f i c i ndi ces,such as the Fel s i ndex (B w t 2/stature33), of ten cor relate betterw i th total body f at ( 18).

10 ) Body densi ty : T he densi ty of the human body , der iv edf rom Bw t and volume, i s w idel y used to i ndi rectl y estimate totalbody fat and FFM (9, 10) and i s def i ned at the molecular l ev elas

1 /Db = f Fat/D F + fFFM /D M ( 16 )w here D b. D F, and DFFM are the densi ti es (in g/cm3) of the totalbody , f at, and f at-f ree body , respecti vely , and f represents thef racti ons of Bw t as fat and FFM , respectiv el y (20). Sim i l ar equa-ti ons for total body densi ty based on indi v i dual components atthe cel l ul ar, t i ssue-system , and w hole-body levels can al so bewritten.


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FIV E-L EV EL BODY COM POSIT ION M ODEL 25T A B L E 5Some related but di sti nct components on di f f erent l evel s

A tom ic l ev el M olecular l ev el Cel lul ar l evelT issue- sy ste m l ev el

T i ssue level O rgan lev elT otal hod calci um

an d p ho sp ho ru sM ineral Ex tracel lu lar sol i ds B one Skeleton

Total body carbon L ipi d and fat Fat cel l s A di pose ti ssueSkeletal muscle cel l s Skeletal muscle ti ssue Skeletal muscle

I t i s cl ear that any major changes in body composi ti on on theother f our l ev el s w i l l mani f est themsel ves on the whole-bodylevel . Conversel y . most di f f erences at the w hole-body level arerel ated to changes in composi ti on on the other f our l evel s. T hi sl atter rel ati on i s the foundati on for estimati ng the componentsof the other f our l ev els by using measurements at the w hole-body level . M ost i ndi cators at the w hole-body level are simplerand easier to perf orm than are m easurements at the other f ourl evel s. thus the techniques at thi s l evel are of ten w el l sui ted forl arge-scale studies or f or f i el d w ork .

Featu r es o f t h e m od e lThe f i ve-l evel model prov i des a structural f ramework for

study ing human body composi ti on that goes beyond an mdi -v i dual compartment or level . I n thi s secti on w e descri be someof the features of the f i ve-l ev el model as a whole.D istinc tio ns a nd con nection s b e ttieen d ifliren t leve ls

A n essenti al aspect of the model i s that the lev el s themsel vesare di sti nct and have unique properties that should not be con-f used w i th one another .

I) On the atom ic l evel , there are no special elements or anyfundamental di f f erences betw een the human body and the in-organi c w orl d. al though the rati o ofel ements to each other vari es.

2) On the molecular l evel , the human body i s di f f erenti atedf rom the inorgani c w or ld because of the appearance of complexorgani c compounds such as l i pi d and protei n.

3) On the cel l ul ar l evel , the human body i s di sti nct f rom thenonl i v i ng world because of the appearance of cel l s that have thecharacteri sti cs of l i v i ng organi sms.

4) On the ti ssue-system level , the human body is di f f erentf rom the low er animal w orl d because of the appearance of ti ssues,organs. and systems hav ing complex structures and functi ons.

5) On the w hole-body level , the human body is di f f erenti atedf rom al l other primates because of the presence ofdisti nct mor-phol ogi cal f eatures.A l though these di sti nct properties ex i st f or each of the f i vel evel s, l i nk ages are also present that are clearl y recogni zable i nthe contex t of the f i ve-l evel model . A n example i s that cel ls thatappear f i rst on the cel l ul ar l evel have many of the characteri sticsof l i v i ng organi sms such as membrane transport, energy metab-ol ism . and enzymati c processes. T hese character istics of the cel lare sti l l maintained at the ti ssue-sy stem and w hole-body level s.Each higher l evel i s thus unique but maintai ns some of the char-acteri sti cs of the level below i t.

Recogni ti on ofdi sti nct l ev el s and thei r connecti ons can revealgaps in present body -composi ti on i nf ormati on and suggest a

di recti on for f uture research ef f orts. For example. i t i s knownthat many biol ogi cal f actors i ncl uding grow th, development, se-nescence. race, sex , nutri t i onal status, exerci se lev el , and thepresence of di sease al l have important ef f ects on body compo-sit ion . H ow ev er , m ost studies ofbody composi ti on i n these areasare l im i ted in scope, f ocusing on onl y a few components at oneor tw o level s and thereby fai l i ng to appreciate the connectionsbetw een levels. For example, most prev i ous obesi ty studies w erel im i ted to anthropometri c changes (at the w hole-body level ) andal terations in f at mass (at the molecular l evel ). V ery f ew studieshave investi gated how obesi ty i nf l uences the other l evel s of bodycomposi ti on or more importantl y the coordinated changes thatoccur throughout al l f i ve l ev el s w i th i ncreasing B w t.D istinc tio ns a nd conn ec tions be tw een d iffe ren t com ponen ts

A n important f eature of the model i s that every major corn-ponent has a clear def i ni t i on and can be incl uded in one of thef i ve l ev els. Each of these components has unique properti es andyet maintai ns rel ati onships w i th other components at the sameand di f f erent l evel s.

I t w as not unusual in ear l i er studies f or rel ated componentsto be confused w i th each other, parti cul arl y i f they w ere on di f -f erent l ev els. A n example of three sets of commonl y confusedcomponents i s presented in T able 5. I n the f i rs t set, TB Ca andphosphorus. m ineral , ex tracel l ul ar sol i ds, bone ti ssue, and skel -eton, are related compartments but belong to di f f erent l evelsand have di sti nct di f f erences f rom each other:

1) Calci um and phosphorus, and m ineral : M ost ofTBCa andphosphorus ex i st i n m ineral al though there i s some phosphorusin protei n and l i pi d (eg, D N A , RN A , and phosphol i pi d). On theother hand, i n addi ti on to calci um and phosphorus, m ineralcontai ns other el ements (eg, carbon, oxy gen, hydrogen, mag-nesium , and sodium ).

2) M ineral and ex tracel l ul ar sol i ds: M ost of total body m inerali s in ex tracel l ul ar sol i ds al though there sti l l is a smal l amountof m ineral i n cel l s and ex tracel l ul ar f l ui d. On the other hand,in addi ti on to the m ineral i n the form of i norganic materi al .ex tracel l ul ar sol i ds contai n organi c sol i ds such as col lagen, re-ti cul ar f i bers, and elasti c f i bers.

3) Extracel l ul ar sol ids and bone ti ssue: M ost of total bodyex tracel l ul ar sol ids are i n the form of bone ti ssue al though theresti l l i s a smal l amount ofex tracel l ul ar sol i ds i n other ti ssues (eg,i n skeletal muscle). On the other hand, i n addi ti on to ex tracel l ul arsol i ds. bone ti ssue contai ns bone cel l s and ex tracel lu l ar f l ui d.

4) Bone tissue and skeleton: B one ti ssue consti tutes the ma-j ori ty of the skeleton al though the latter al so i ncl udes skeletalcar ti l age, peri ar ti cul ar ti ssue adher ing toj oi nts, and red and yel -l ow m arrow .


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26 W A NG ET A LA nother example of rel ated but di sti nct components is total

body carbon (l evel I ). l i pi d and fat (l evel I I ), f at cel l s (l evel I I I ),and adipose ti ssue (l ev el IV ) (T able 5). T hese terms are of tenconfused w i th each other, a problem that the f i ve-l evel modelhel ps to resol ve.

T he thi rd and f i nal ex ample i n the table i s the di sti nctl y di f -f erent but related components skeletal muscle cel ls, skeletalmuscle ti ssue, and intact w hole skeletal muscles. T he modelthus demonstrates that di f f erences and relations ex i st betw eencomponents on each of the f i ve l evel s. I t i s therefore adv i sableto develop equati ons for body w eight, vol ume, or densi ty thati ncl ude components f rom the same level in order to avoid overl apor om issi on of some components.S teady -sta te O fbO dI com pos ition

The concept of a steady state i s important not onl y in bio-chem istry , physiol ogy , and other cl assi c sci enti f i c di scip l i nes butalso in body -composi ti on research. T he meaning of a steadystate of body composi ti on can be def i ned in the contex t of thef iv e- level model : A steady -state or dynam ic homeostasi s ex i stsduri ng a speci f ied time peri od i f Bw t and the mass of v ariouscomponents on di f f erent l evel s i s maintai ned rel ativ el y constant.

T he important impl i cation of a steady state i s that there arestable proportions among the di f f erent components on the samelevel . For example, on the molecular lev el the average ratio oftotal body w ater content to FFM is rel ativ el y constant i n heal thysubjects (ie, total body w ater/FFM = 0.732) (2). On the atom icl evel the correl ation betw een TBK and TBCa is reproducibl ef or males [ i e, TBK (g) 0. 1 383 X TBCa(g) - 1 7. 1 ] (2 1 ). On thewhole-body level the rel ati on betw een BSA (BSA , i n m2) andBwt (kg) and stature (i n rn) i s al so rel ati v el y constant such thatBSA = 0.007 1 84 x st at ur e# { 1 76 } 7 25 X b od y we ig ht #{ 1 7 6} 4 2 5( 22 ).

T here are also rel ati vel y constant propor ti ons among the rel -evant components on di f f erent level s w hen body composi ti oni s i n a steady state. For example, total body protei n/total body

nitrogen = 6.25 ( 1 ) : BCM ( k g) /TBK (mmol ) = 0.00833 ( 1 1 ), andfat(kg) = [(4.95/Db) - 4.50] x Bw t (20).

T he steady state of body composi tion indi cates that al thoughthere are so many components i n the human body , and al l ofthese components di f f er f rom each other, they are w el l organi zedaccordi ng to def i nable quanti tati ve relati ons.

Quan tita tive h od i COlf l / )Os i t iO f l relat ionsA primary aim of body -composi ti on research i s to estimate

the si ze of each compartment. al though there are numerous in-di v i dual compartments of cl i ni cal rel evance that have not beenmeasured di rectly . A n al ternati ve i s to estimate the unknow ncomponents by establ i shing relati onships to measurable corn-ponents. B ody composi ti on i s rel ati vely stable i n heal thy adul ts.and i t i s thi s property that enables i nvesti gators to establ i sh thesereproducibl e rel ations or rul es. The f i ve-l ev el model of bodycomposi ti on af f ords a logi cal matri x w i thi n w hi ch to establ i shthe quanti tati ve steady -state rel ations betw een known measur-able components and presentl y unmeasurable compartments.

Present research in developing body -composi ti on equati onsprimari l y invol ves estimati ng one unknow n component f rom ameasurable component. T he f i ve-l evel model suggests the pos-si bi l i ty of reconstructing Bw t and volume by w ri t i ng simul ta-neous equati ons that exploi t steady -state rel ati ons betw een sev -eral measurable and unknown components. A n example is thecal culati on of the f i ve major chem ical components and Bwt atthe molecular level f rom si x elements (carbon, ni trogen, sodium .potassium . chlori ne, and cal ci um ) measured by in v i vo neutron-acti vation anal ysi s ( 1). U nti l recentl y the concept of recon-structi ng w hole l evels ofbody composi ti on f rom mul ti pl e com -ponents w as l im i ted and the studies w ere f ragmentary . T he f i ve-l evel model def i nes expl i ci tl y the equati ons for Bw t at each leveland presents the chal l enge of developing more complex andcomprehensiv e body-composi ti on equati ons.

TA BL E 6The relati on betw een direct and indi rect body -composi ti on measurements organi zed by the f i ve-l evel model

DirectIndirect

A tomic level M olecular level Cel lular level T issue-system l evel W hole-body level

A tomic level TBP = (0 .456 X TBCa) Pro = 6.25 X TBN BCM = 0 . 0 0 8 3 3 X TBK SM = 0.0196 X TB K Bw t 0 + C + H + NTBO. TBC. TB H , TB N . + ( 0 . 5 5 5 x T B K ) FFM = TBK /68.l ECS = TBCa/0.l77 - 0 . 0 2 6 1 X TBN + Ca + P + K + N a

T BK . T BCa. T BN a. ECF = (0.9 X TBCI )/ + Cl + RTBP . TBCI . Nae. K e Plasma Cl

M olecular level FFM = TBW /O.732 SM = 1 1 .8 X Cr Bw t = L + A + ProT BW , m in er al . c reat in in e. FFM = 24.1 X Cr + 10.1 + M + G + R

3 -MH + 20.7C el lul ar l ev el Bw t = CM + ECF

ECF. plasma v olum e + ECST issue-system lev el Bw t = ad ip ose t issu eV ol um e of subcutaneous + skeletal muscle

and v isceral adi pose + bone + vi5Cerstissue + blood + R

W hole-body level TBK = (27.3 X Bw t) Fat% = (4 .95 X B V / ECF = 0 . 1 3 5 X Bw t SM = 5(0.0553 Body surface = 0.007184Bw t , S. B y, circumference. + (11.5 X 5) - (21.9 Bw t - 4 .5 ) X 100 + 7 . 3 5 X CTG2 + 0. 0987 X 50725 x BW# {176 }4 25

skinfold x A ge) + 77.8 x FG2 + 0.0 331x CC G2) - 2445

* A . w ater (kg): BCM . body cel l mass (kg): BV . body volume (L ): Bw t. body weight (kg): CCG. corrected medial cal f gi rth (cm): CM . cel l mass (kg): Cr, 24-h urinecreatinine (g): CTG. corrected thigh gi rth (cm): ECF. extracel lular f luid (kg): ECS. extracel lular sol ids (kg): FFM . f at-f ree body mass (kg): FG. f orearm gi rth (cm): G .glycogen(kg): K e. exchangeable potassium: L . l ipid(kg): M . M ineral (kg): 3-M H. 24-h urine 3-methy lhistidine: Nae. exchangeable sodium : plasma Cl . plasma concentrationof chlorine (mmol /L ): Pro. protein (kg): R. residual (kg): S. stature (cm): SM . skeletal muscle (kg): TB . total body el ement (k g): and TBW . total body w ater (kg).


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FIV E-L EV EL BODY COM POSIT ION M ODEL 27R e/a tio mi to 1 flC t/lO dO l()t

A t present. body -composi ti on methods are primari l y cate-gori zed into technique-speci f i c groupings such as di l uti on meth-ods and neutron-acti vati on anal ysi s. A ccordi ng to the f i ve-l ev elmodel , how ever, the methods can be organized in a more sys-tem ati c f ashion.

D irec t m easu rem en t ! fitt/lO ds . There are some di rect methods,such as anthropometri c, bi ochem ical , and radioi sotopi c tech-niques that can be used to estimate components of body com -posi ti on. D irect methods can be organi zed accordi ng to the f i ve-lev el model as fol l ow s:

1 ) On the atom ic level , TBK can be di rectly determ ined bywhole-body # {176 }Koun t ing ( 1 2): total body sodium . chlori ne.phosphorus, and cal cium by delayed- y neutron acti vati on (3):total body ni trogen by prompt--) neutron acti vation (23): andtotal body carbon by inelasti c neutron scatteri ng (4).

2) On the molecular l evel , TBW can be di rectl y estimated byseveral i sotope-di l ution techniques (10), and osseous m ineralcan be quanti f i ed by dual -photon absorptiometry (24).

3) On the cel l ul ar l evel , ex tracel l u lar f lu i d (and plasma vol -ume) can be di rectly determ ined by several i sotope-di l utiontechniques (25 ) .

4 ) On the ti ssue-system level , the volumes of subcutaneousand v i sceral adipose ti ssue can be di rectly determ ined by com -puteri zed ax ial tomography and by magnetic resonance imagingtechniques (26) .

5) On the whole-body level , anthropometr ic i ndi ces such asBw t, body volume, stature, ci rcum ferences, and sk inf ol d thi ck -nesses can be estimated di rectl y (18).

A ccording to thi s anal ysis there are not many di rect methodsused in the study ofhurnan body composi tion. M oreover, mostof the di rect methods are concentrated on the atom ic and whole-body lev el s. T here are only a f ew direct techniques on the mo-l ecular, cel lu lar, and ti ssue-system level s.

Ind irect n ea surem nen t m e thod s. These estimate unknowncomponents of body composi ti on by combini ng di rect mea-surernent techniques w i th the establ i shed steady -state relati on-ship betw een the di rectl y measurable and unknow n components.I ndi rect methods greatl y expand the number of body compart-rnents that can be evaluated. A t present, some important corn-partments can be assessed onl y by indi rect methods. For ex -ample, al though total body fat i s a major compartment of in-terest. there are no practi cal methods ofdi rectl y ev aluati ng thefat compartment i n v iv o. A l l of the presentl y used methods areindi rect and based on di rect measurements at di f f erent l evel s asfo l lows :

1) f rom di rect method on the atom ic l evel (10) , f at = Bwt- TBK (mmol )/68. 1 : 2) f rom di rect method on the molecularl evel (2, 10), f at = Bw t - TBW/0 .732 : 3 ) f rom di rect methodon the molecular and whole-body level s, f at = 2.057 X BV- 0.786 x TBW - 1 .286 X Bw t, where BV is body volum e inl i ters (20); and 4) f rom direct methods on the whole-body level ,f at=4.95X BV -4.5OX Bw t,andfat=0.7l 5X Bw t- 12.1x stature2 (i n m ) (19, 20) .T hus i t can be seen that indi rect methods are not only based onthe di rect methods, but al so are dependent on the steady -stateproporti ons betw een know n and unknow n components as de-term ined in sample populati ons.

D i rect and indi rect body -composi ti on methodology can beoutl i ned accordi ng to the f i ve-l evel model as show n by the ex -

The Study of B ody Composi ti onC om p onent s on L ev els I , II , III , IV , and V

Bod y com p osit ion r u le sI _ IL Methodo lo g y j I B io log ica l effec t s

FIG 5. T he three areas ofbody -composi ti on research.

am ples presented in T ab le 6 . T h e ta b le d em on st r a t e s t h a t m os tof the pri nci pal el ements and anthropometri c i ndices can bedi rectl y measured and that many of the indi rect methods havebeen developed f rom the di rect methods on the atom ic andw hole-body level s. respecti vel y ( I 0, 1 7, 22, 25 , 27-29). Con-v ersel y , the table show s that there are only a few di rect methodson the cel l ul ar and ti ssue-sy stem level s, so the relevant i ndi rectmethods are also very l im i ted. T hi s i s one of the w eak areas inbody-composi ti on m ethodology and could consti tute an im -portant topi c f or f uture research.

D efin it ion of b od y com p osit ion r esea r chThe study of body composi ti on spans > 100 y , and the term

body composi ti on i s w idel y used. H ow ever, i t i s unclear w hatthi s branch of science represents and what exactly i s meant bythe term body composi tion. T he f i ve-l evel model presented inthi s paper not onl y bui l ds an appropri ate structure f or body -composi ti on research. but i s conduciv e to cl early def i ne humanbody composi tion as a branch of human biology that studiesvari ous body compartments and thei r quanti tati ve steady -staterel ati ons or rul es. B ody-composi ti on research incl udes three in-terconnecti ng areas: study ing the proportions of vari ous corn-ponents and thei r steady -state associati ons among the atom ic,molecular. cel l ul ar, t i ssue-system , and w hole-body levels: study -i ng the methods of measuri ng var ious components i n v iv o; andstudy ing the inf l uences of bi ol ogi cal f actors on var ious level sand components (Fig 5) .

Conc lu s ionThe f i ve-l ev el model grow s f rom a need to organi ze both the

rapidl y developing methodologies and physiologi cal conceptsthat relate to the study ofhuman body composi ti on. T he modeli s intended to be a foundati on on whi ch future studies can ref i neor ex pand selected def i ni t i ons or equati ons. T he f i ve-l evel modelserves in thi s organi zati onal capaci ty and al so stimulates abroader v i ew of body -composi ti on research as a w hole. C ]

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