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Rev iew A rtic le

Am J C lin N u ir 1987 ;46 :537-56 . Pr in ted in U SA . 1987 Am erican Soc ie ty fo r C lin ica l N u trition 53 7

M e tho ds fo r the assessm en t o f h um an body com pos itio n :trad itio na l and new 13H en ry C Lukask i, P hD

ABSTRACT Ren ew ed in te res t in th e asse ssm en t o f hum an body com po sition h as stim u la tedthe n eed fo r a ba lan ced und erstand ing of ava ilab le m etho do log ies o f est imat ing fat-free massand pe rcen t bo dy fa t. T h is rev iew summ arize s the phy sical base s and assumpt ions , describesapp lica tion s, and d iscusse s the theore t ica l an d prac t ica l lim ita tions o f cu rren tly ava ilab le ind irec tm e thod s. A ltho ugh s tan da rd m eth ods are d iscussed , recen t m od if ica tion s and adap ta tions a reemph as i zed . Am J G un N u ir 1987 ;46 :537-56

KEY WORD S Fat, fa t-free m ass , bo dy w a te r, p ro te in , m in era l

IntroductionR esearch to e stab lish in d ire ct m e th od s o f d ete rm in in g

hum an body com positio n b eg an du ring th e 19 40 s in th elab o ra to ry o f A R B ehnk e (1 ) . S u bsequ en tly a v arie ty o fme thods h as been in tro duced . How ev er, a ttem p ts to d e-sc rib e th e th eo ry and pract ice o find iv idu al m eth od s h av eb een lim ited . E arly rev iew s (2 -7 ) o f body compos i t i ona re de ta iled desc rip tio ns o f few ex isting techn iques w hilem o re recen t e ffo rts (8 , 9 ) h av e been re stric ted to b riefsumm aries o f cu rren t ap p ro ache s . T h e p urp ose o f th isrev iew is to sum m ariz e the back g ro und an d to describethe p recision o r e rro r o fe ach o fthe e s tab lished techn iq ue san d to h ig h lig h t the s tren g ths an d the lim ita tion s o fme thods .

Mo s t body com po sitio n m e tho ds ar e based u pon th em od e l in w h ich th e body con s is ts o f tw o ch em ica lly d is -tin ct com partm en ts , fa t an d fa t-fre e (2 , 1 0 ). T h e ch em icalcom posit ion o fthe fa t-fre e body is assum ed to b e re la tiv elyco ns tan t w ith a d en s ity o f 1 .1 g /ee at 37 #{176 }C2 , 10 , 11 ) , aw ate r co n ten t o f7 2 -7 4% (6 , 12 ), an d a po ta ss ium con ten to f6O -7 0 m m o l/k g in m en and 50-60 mm ol/k g in w om en(13) . F at, o r s to red trig ly cerid e , w h ich is anh yd ro us an dpo tassium free , h as a density o f 0 .90 0 g /cc a t 37 #{176 }C(14 , 15) .

During the de riva tio n o fthe tw o compar tmen t mode l ,K eys and B rozek (2 ) d iv ided th e m am m alian b ody in tofo u r ch em ical g rou ps ; w a te r, p ro te in , as h or bo ne m ine ral ,an d fat. A nd erson (1 6) la te r u sed m easu rem en ts o f bod ypo ta ss ium and w ater to es tim a te the se com pon en ts . O n lyrecen t ly ha s t e chno logy been av ailab le tha t allow s fo r thein v iv o de te rm ina tion o f these fo u r com po sition al va ri-ables .

T h e tw o and fo u r com partm en t m od els served as th ebasis upo n which all b ody com po sit ion m e tho ds w e re d e-

ve lo ped . T h e m e th od s desc ribed in th is p re sen ta tion a rec la ssified a s e ith er trad ition a l o r new . T h e trad it ion alm eth od s rep re sen t th e m ore es tab lish ed ap pro ach es tobody com position a sse ssm en t w h ile th e new m e tho ds re-fle c t co n tem po rary hy po the ses an d techn o lo gy .

T rad itio na l m eth od sTota l b od y w a te r

The find in gs th at w a te r is n o t p resen t in s to red trig lyc -e rid e an d th at w a ter o ccup ies a re la tiv ely fix ed fra ction( 73 .2%) o f the fa t-fre e mass (12 ) have s tim ulated th e de-te rm in ation o f to ta l body w a te r (TBW ) as an index ofhuman body com po sition . In ve s tiga to rs h av e used th ei so topes ofh yd ro gen , deu te rium and tritium , to q uan tita tebody w ate r vo lum es by iso to pe d ilu tio n in hea lth y an dd iseased i nd iv idua l s (6 ) .

Som e g en era l a ssum ptions o fthe iso top e-d ilu tion tech -n ique ar e th a t th e iso top e ha s the sam e d is tribu tio n vo l-um e as w a te r, it is ex ch an ged by the body in a m anne rs im ila r to w ater, and it is n on tox ic in the am oun ts used(17) . B ecause o f th e ease o fliqu id scin til la tion cou n tin g ,

I F rom th e U nited S tate s Dep a r tmen t of A gricu ltu re , A gricu ltu ra lR esearch S e rv ice , G ran d Forks H um an N utr ition R esea rch C en ter , G randForks , N D .

2 Presen ted a t the W ork shop on M etab o lic A d ap ta tion to A lte red En-e rgy In take of the P an Am erican H ealth O rgan iza tion i n W a sh in gt on ,DC o n O cto be r 28-29 , 19 85 .

3 A dd res s repr in t reques ts to H en ry C Luka sk i, P hD , USDA -ARS ,G rand Fork s Hum an Nu tr ition R esearch Center, P 0 Bo x 7166 , U ni-vers ity S ta tion , G rand Forks , N D 58202 .

Rece ived iune 19 , 19 86 .Accepted fo r pub lica tion D ecem ber 2 , 1986 .

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538 L U K A S K Jsom e in v e s tig ato rs re lied upon tritium d ilu tion to m e as ureT B W (18). H ow ev er, th e us e o f a rad io ac tiv e tracer su chas tritium is con train d icated in re search in vo lv ing childrenor w om en o f ch ild -beanng ag e or in app licatio ns u tiliz in grepeated m easurem en ts in a short pe riod o f tim e . E x -pand ed use o f deu te rium o ccurred w hen new er te chn iqu esf o r the assay o f deu terium in aqu eous so lu tion b ecam eav ailab le . For ex am ple , us e o f gas ch rom ato graph y (19 ,20 ), m ass sp ec trom e try (2 1 ), an d f ix ed -f ilte r in f rared ab-sorp tio n (2 2 ) h as f ac ili tat ed the an aly s is o f d eu terium inb io lo g ical f lu id s .

T he typ ica l procedu re f or us in g trit ium or deu teriuminc lud es e ithe r th e ing es tion o r the in trav enous in je c tiono f a spec if ied q uan ti ty o f the trace r, an equ ilib ratio n p e-riod , and a sam p lin g pe rio d . T h e calcu latio n o f T B W v o l-um e is b ased upon th e s im p lif ied re latio nsh ip : C 1V 1= C 2v 2 , w here C 1V 1 is th e am oun t o f trace r g iv en , C 2 isf inal concen tratio n o f trace r in a b io lo g ical f lu id , an d V 2is v o lum e o f T B W . It is im po rtan t th at a co rrec tion b em ade fo r urin ary lo ss o f tracer.T h e quan tity o f trace r g iv en dep ends u pon th e type oftracer adm in is te red , th e analy tical sy s tem used , and th eo b je c tiv e o f th e re search . In g en eral th e sm alle s t d ose th atallow s good analy tical precision and accuracy and pro -v ide s the low est risk to the su b jec t shou ld b e g iv en . Forh ealth y m en and w om en a 10 g do se o f d eu te rium ox ide(9 9 .7% pu rity ) m ix ed w ith 3O O m L o f d is til le d- de io niz edw ate r is u sed rou tin ely in o ur labo rato ry (22 ). T h is p ro -cedure has an analy tical precis io n o f 2 .5% usin g f ix ed -f i lter in f rared abso rp tiom e try (22 ). W hen u s ing gas chro -m atog raph ic m e th od s, som e in v e s tigato rs hav e g iv en anoral dose o f d eu te rium ox ide o f 1 g /k g body w eigh t (19 ,2 0 ). A n oral do se o f 2 g D 20 has b een u sed w ith massspectrometry (2 1 ). A mong researchers us in g tritium as atracer, a dose o f 50 z C i (2 6 m rem rad iation d ose) is corn -m on (2 3 ). W hen repeated b od y -w ater de term in ations areto be p erf o rm ed ov er sh ort pe riod s o f tim e (eg , e v e ry 2w k or less ) o r in w om en o r ch ild ren , th e u se o f d eu te riumas a tracer is sug ge s ted .

T h e len g th o f tim e req u ired f or tracer eq u ilib ration de -pend s upon the ch arac terist ics o f th e s tu dy sam p le . Inhealthy m en and w om en equ ilib ration o f d eu te rium insaliva, p la sma , and urine o ccurs 2 h af te r in gestion o f thetrace r and rem ain s at a co ns tan t con cen tratio n in the sebiological f lu ids f o r the nex t 3 h (19 , 20 , 22) . A m ongin d iv idu als su f f ering f rom w ate r accum ulation , eq u il ib ra-tio n m ay requ ire 4 -6 h (6 ).

A lth ou gh p lasm a o r serum trace r con cen tration s h av ebeen used to calcu late T B W , sa l iva co ncen tration s m aybe use f u l un de r som e cond itio ns (19 , 2 0 , 2 2 ). In f ie ld si t-uatio ns w here o b tain ing pre - an d pos tadm in is tratio n v e-nous b lood sam p le s m ay b e d if f icu lt because o f lo cal be lie f sor lack o f adequate f aci litie s , sal iv a an d u rin e c olle ctio nm ay pe rm it g reate r partic ipatio n by th e g roup und er in -ves t iga t ion .

R ecen tly , th e use o f o x y gen -l8 as a tracer to m easureT B W w as p roposed (2 4 ) because it av o ids the ex ch ang eo f th e labe l w ith nonaqueous hydrogen in th e body thatcan o v e res tim ate bo dy w ate r by 1 -5% (25). I r np lemen-

tation o f th is te ch n iq ue is d if f icu lt o u tsid e o f a specializ edresearch labo rato ry becau se o f the need f o r lab orious pro -cedu re s and sp ec ializ ed equ ipm en t in clud ing a massspectrom ete r. A lso , the cos t o f o x y gen -l8 is pro h ib itiv e($3 00 /trial) f o r rou tine use . C om parativ e cos t f o r the d eu -te rium d ilu t ion pro ced ure u s ing 10 g o f D 20 ($13) andf ix ed -f i lter in f rared sp ec tro pho tom e try ($ 4000 ) is m u chless .Total body potassium

C hem ical an aly ses hav e in d icated th at p o tass ium is es-sen tially an in tracellu lar catio n th at is no t p re sen t in s to redtrig ly ceride . A lso , p o tass ium -40 , w h ich em its a ch arac-te ris tic gam m a ray at I .46 M eV , ex ists in th e b od y at ak n ow n natural ab undance (0 .0 12% ). T hese f ac ts hav e al-low ed inv e s tig ato rs to es tim ate f at-f ree m ass in hum ansand an im als b y ex te rn al co un ting o f po tass ium -4 0 .

Q uan titatio n o f to tal b ody po tassium (T B K ) req u ire sspec ially con struc ted coun ters th at con s is t o f a largesh ie ld ed ro om (to red uce b ack gro und rad iation f romcosm ic and te rres trial source s) con tain in g a gam m a rayde tec tio n sy stem conn ec ted to a su itab le record ing d ev ic e .T h e de tec to rs are o f tw o ty pes: large tha l l ium-ac t iva tedso d ium iod ide cry s tals , o ne or m ore o f w h ich are p os i-t ioned near the su b je ct , and larg e, ho llow cy linde rs or halfcy lind e rs, the w alls o fw h ich con tain liqu id o r p las tic scm -tillatio n m ate rial and in to w h ich th e sub je ct is p laced soas to be com p le te ly o r partially surro unded by the de tec to r.T hese are ref erred to as 4 i o r 2T g eom etrical coun te rs .T he adv an tage s o f the c ry stal sy s tem in clu de v e ry gooden e rg y reso lu tio n and a low back g round rate . T he p las ticor liqu id sy s tem s hav e v ery p oo r re so lu tio n , h ig h back -ground-in te rf e rence rate , an d h ig h coun tin g ef f ic iency .In itial attem p ts to m easu re po tass ium -40 used a sing lecry s tal p laced ad jacen t to th e su b je ct , w ho w as seated ina chair and coun ted f or 3 0 m m (26 , 2 7 ). L ater, a sy s temw as dev e lo ped th at allow ed a s ing le c ry s tal to pass o v e rthe sub je ct w ho w as sup in e o n a co t (2 8 ). O the r inv es ti-gators hav e u tiliz ed 4 r p o tass ium -40 coun tin g (29 ). R e-g ard le ss o f w h ich ty pe o f co un ting sy s tem is used , th erep orted v ariab ility o f cou n tin g p o tassium -40 in an an -th rop om etric ph an tom w as > 5% and th e erro r o f po tas-s ium -4 0 coun tin g in hum ans w as - 5% (30 ). T h ese es ti-m ate s o f co un ting precis io n w ere attribu ted to d if f e rencesin d ete c to rs and in terin d iv id ual v ariation in body g e-ome t r y .

T he m ost s ig n if ican t re cen t im pro v em en t in the w ho le-b od y -coun tin g m etho d is th e dev e lo pm en t o f techn iqu esto p rov ide abso lu te q uan titat ion o f po tass ium -4 0 . C ohnet al (3 1 ) used a un if o rm ly d is trib u ted ces ium -l37 sou rce(0.5 M C i) and a 54 -de te cto r w ho le -b od y co un te r and as-so ciated com pu te r f acil ity to m easu re po tass ium -4 0 ab-so lu te ly . B y coun ting the gam m a ray s em itted f rom th ece s ium sou rce , w h ich is po s itio ned brie f ly un de r the sub -jec t w ho is sup ine on a co t, attenu atio n f acto rs are d ete r-m ined . T h e se f ac tors are used to correct f o r d if f e ren ce sin b od y s iz e an d geom e try an d in g am m a ray self -abso rp -tion f or each sub jec t. T h is m etho d pe rm its an accu -

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BODY COM POSIT ION M ETHODS 539racy and a precision of in v ivo potassium -40 countingw i thin 3% .

A nother recent f i nding that can improve counting ac-curacy is reduction of background bismuth-2 14 contam -ination. L ykken et a! (32) counted endurance cycl i sts be-f ore and af ter prolonged outdoor bicycl ing in N orthD akota and found an apparent increase of 10% inpotassium-40 counts of the legs. I t w as determ ined thatthe increased counts were due to gamma rays em i ttedf rom bismuth-214, w hi ch has one gamma ray in the po-tassium-40 energy level . Radiobismuth is a decay productof atmospheri c radon. Thus, subjects should show er, w ashhai r, and w ear clean clothing to under go w hole-bodycounts.

Once TBK is determ i ned one can estimate lean bodyor f at-f ree mass w i th a f actor dependent upon the potas-si um content of the f at-f ree mass. From chem ical analy sisof a f ew human cadavers, Forbes et al (26) suggested val uesof 2.66 and 2.50 g potassium /kg f at-f ree mass in men andw omen, respecti vely . B ol ing et al (33) deri ved a new setof constants based upon the high correl ati on betw een po-tassium and water. T hey proposed 3.41 and 3.16 g po-tassium/L body w ater in men and women, respecti v el y .A ssum ing a constant l evel of hydration of the f at-f ree body(73.2%) ( 12), these constants became 2.5 and 2.3 1 g po-tassium/kg f at-f ree mass f or men and w omen, respecti v ely .B ehnke (34) cal culated f actors f or converting total bodypotassium to f at-f ree mass to be 2.46 and 2.28 g for menand w omen, respecti vely . U si ng hydrodensi tometryand absolute potassium -40 counting, L ukask i et al (35,36) reported values of 2.46 and 2.50 g/kg f at-f ree massin men.

Therefore, w i th the use ofan accurately cal ibrated, w el l -shielded w hole-body counting system, standard proce-dures f or correction of counts due to body geometry andgamma ray sel f -absorpti on, and good control of bi smuth-214 background interf erence, one can obtain absolutemeasurements of TBK . H ow ever, the cost of such deter-m inat ions, including instrumentation and technical sup-port, may be prohibi t i ve f or some research appl i cations.U rinary crea tin in e excre tion

The origin of endogenous creat inine can be traced tothe synthesis of i ts precursor creat ine in the l i ver and k id-ney . A l though many ti ssues take up creatine, the prepon-derance (98% ) of i t i s located i n skeletal muscle, mostl yin the form of creat ine phosphate (37). Creatinine isf ormed by the nonenzymati c hydrol ysis of f ree creatinel iberated duri ng the dephosphory l ati on of creatine phos-phate (37).

Si nce Fol in (38) f i rst proposed that urinary creati ninegrossly w as associated w i th body composi t i on and H ob-erman et al (39) demonstrated the di rect proportional i tyof body creatine to uri nary creatinine output using ni tro-gen-iS isotopic di l uti on, i t general l y has been acknow l -edged that urinary creatinine excret ion is related to f at-f ree mass and muscle mass (40-43). H ow ever, some fac-tors have been identi f i ed that af f ect the val idi ty of thi smethod.

The greatest draw back to thi s method is the l arge in-traindiv idual variabi l i ty in dai l y uri nary creat inine excre-ti on. The mean i ntraindiv idual coef f i cient of variat ion ofdai ly cr eat ini ne output ranges f rom 1 1 to 20% for mdi -v iduals consuming unrestr i cted, f ree-l i v ing diets (44-47)and can be reduced to 1 1% among people consum ingmeat-f ree diets (48). T his large variabi l i ty has been attr i b-uted to the renal processing of creatinine; i t i s both f I l teredand secreted at the k idney (49).

I n addi ti on to renal handl ing, diet al so can af f ect dai l ycreatinine excretion. Si gni f i cant reductions (10-20% ) inexcreti on occur in heal thy men consum ing meat-f ree dietsf or several w eeks (44, 50). Changes i n creatinine outputw ere related di rectl y to dietary creat ine intake. Crim etal (5 1) consecuti v ely f ed heal thy young men 0.23 g cre-atine/d f or 9 d, 10 g creatine/d f or 10 d, and then a cre-atine-free diet f or 7 1 d. U rinary creatinine excret ion in-creased 10-30% w i th creati ne f eeding and decreased dur-i ng the creatine-f ree diet. N i trogen bal ance was posi ti veduring al l dietary periods. L ykken et al (47) deri ved amathematical model w i th f eedback that describes the crc-atine pool si ze and thus urinary creatinine excretion as afunction of time af ter changes in the amount of creati neand protein consumed. These f indings suggest that thebody creatine pool i s not under str i ct metabol i c controland that urinary creatinine excretion is to some degreei ndependent of body composi ti on.

A nother technical f actor that must be control l ed is ac-curately timed uri ne col lect ions. Forbes (42) indicated thatan error as smal l as 15 mm in a col lecti on period representsan error of 1% in the determ inati on of 24-h uri nary crc-atine excreti on. I t i s general l y adv i sed to make three con-secuti v e 24-h urine col lecti ons to assure representati vecreati ni ne excreti on for an indi v i dual .

The precision of automated procedures usi ng the Jaf l ereacti on to determ ine creati ni ne concentrati on in uri neis 1-2% (43). W hen f at-f ree mass is estimated by creati nineexcretion, error i s high compared w i th ref erence val uesdetermined by densi tometry or potassium-40 counting.A mong men w i th a fat-f ree mass of4O-l00 kg, errors are3-8 kg (43). B y use of total -body -potassium data f at-f reemass can be predicted f rom uri nary creatinine output w i than error of -3 kg i n chi ldren and adul ts (42).

Some i nvestigators have proposed a constant rel ati on-ship betw een creatinine excreti on and muscl e mass. Tal -bot (40) estimated that 1 g of creatinine excreted over a24-h period w as deri ved f rom 17.9 kg muscle mass. Cheeket al (41, 52), however, suggested that each gram of crc-atinine excreted in the 24-h urine sample w as deri vedf rom 20 kg of muscle ti ssue. Thi s apparent di f f erence re-f l eets sampl ing and methodol ogical variation betw eent hese st ud ies.

Forbes (42) addressed the i ssue of a constant ratio ofmuscl e mass or f at-f ree mass to dai l y urinary creati nineexcret ion. H is equation predicting f at-f ree mass f rom en-dogenous creati nine output by chi ldren and adul ts as w el las other publ i shed regression equations (42) possess pos-i t i ve intercepts. T his has been interpreted that excretedcreati nine does not represent a constant f raction of ei ther

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540 LUK A S K Imuscle or f at-f ree mass over the range of subjects studied.Forbes contended that w hen the intercept i s posi t i ve, thef at-f ree mass:creati nine rat io w i l l decl ine hyperbol i cal l yw i th increasing values f or creatinine excretion and w i l lapproach the slope of the regression l ine as an asymptote.Theref ore, i t apparentl y i s inappropriate to use a constantratio of creatini ne:uni t body composi ti on in a populationunl ess f actors such as age, gender, maturi ty , physicaltraining, and metabol i c state are control l ed (43).Dens i tome try

The assessment of human body composi ti on by mea-surement of w hole-body densi ty i s a common methodused for heal thy peopl e. This method assumes that thebody is composed of tw o disti nct components (f at andf at- f ree) and that i t i s possible to determ ine each of thesecomponents f rom the measured whole-body densi ty . T hemathemati cal deri v ati on of thi s relationship was sum-marized by B rozek and K eys (2, 10).

I nherent in the appl i cation of the densi tometri c ap-proach i s the recogni t i on of some fundamental assump-tions. The chem ical composi ti on of the f at-f ree body isassumed to be rel ati v ely constant so that the densi ty ofthe f at-f ree mass di f f ers substant ial l y f rom that of f at(l . 100vs 0.900 g/cc). T hese earl y assumptions f ormulated byB ehnke (1 1) w ere conf i rmed later by di rect chem icalanaly sis of laboratory mammal s (14, 15). O ther assump-tions include a constant level of hydration and a constantproportion of bone mineral (eg, skeleton) to muscle inthe f at-f ree body.

These assumptions were questi oned by Sin (3), w hoemphasized the normal variation in w ater content as thelargest single source of variabi l i ty i n the densi ty of the f at-f ree body and w ho calculated f at content of the body . B yuse of indi rect methods, research in humans revealed avariati on of 1-3% in the w ater content of the f at-f ree mass(53). Sin (3) used thi s estimate to cal culate an error of2.7% body f atness in the general population attr i butableto variabi l i ty in the hydrat ion of the f at-f ree mass.

Si r i (3) also questioned the inf l uence of vari abi l i ty ofbone densi ty on prediction of body f atness using densi -tometri c procedures. A f ter rev iew ing the data presentedby K eys and B rozek (2), Si r i (3) concluded that the van-abi l i ty i n protein:m ineral rati o could lead to a vari ati oni n percent body f at of 2. 1% (0.005 g/cc) in heal thy hu-mans. B akken and Struikenkamp (54) estimated a sim i l arerror in densi ty (0.003 g/cc) caused by variation i n bonedensi ty in the population.B ased upon these estimates of variabi l i ty in bone m in-eral densi ty and hydrati on of the fat-f ree body, Lohman(55) calculated the theoreti cal error of 3-4% for predi cti ngbody f atness in a population using densi tometry . T hi serror i s sim i l ar to that suggested by Sin (3) to be associatedw i th the uncertainty in densi ty and chem ical composi t i onof the f at-f ree body .

T he most w i dely used technique of measuring w hole-body densi ty i s the determ ination of body volume ac-cording to A rchimedes pri ncipl e, w hich states that thevolume of an object submerged i n water equals the volume

ofw ater the object displaced. I f one measures mass in ai rand mass i n w ater, the di f f erence, corrected for the densi tyof the w ater corresponding to the w ater temperature atthe t ime of the underwater w eighing, i s the apparent bodyvolume. W i th thi s technique i t i s mandatory to determ inethe lung volume dur ing submersion (residual l ung vol -ume) w hich makes a sizable (1-2 L ) contr ibuti on in theest imate of total body volume. A second volume, gas-trointestinal gas, i s considerabl y smal ler in magni tudeN 100 mL ), and is never measured (56). B ecause the in-traindiv idual variabi l i ty in gastrointesti nal gas volume canbe qui te large (50-300 mL ), thi s variable can comprom isethe precision of the densi tometri c method (57).

D urni n and Satw anti (58) quanti tated the ef f ects of levelof exhalation, preceding meal si ze, and consumption ofa carbonated drink on estimates of body f atness. For eachtest condi ti on residual lung volume w as measured si -multaneously w i th the determ ination of underw aterw eight. Relati ve to the values obtained at max imal ex-hal ati on, variations in l evel of exhalation and inspi rationand meal si ze caused only 1% absolute di f f erence inthe estimated f at content. Relati v el y l arge volumes of gasin the al imentary tract resul ted in an absolute di f f erenceof 1 .5% i n the estimation of body f atness. These dev ia-ti ons observed in the estimation of body f at by densi tom -etry are w i thin the errors of the method. W hether suchvariat ions could be detected using residual lung vol umemeasurements made apart f rom the underwater w eighingi s d ou bt fu l .

The underw ater w eighing system original l y describedby Gol dman and B usk i rk (59 ) and later modi f i ed by A kersand B usk i rk (60) has gained w idespread use. B rief l y , thi ssystem uses strain gauges mounted under the water onthe f loor of a stai nless steel tank . On these gauges is po-si ti oned a cot on w hich the subject kneels. U se of thesestrai n gauges prov ides a rapid and arti f act-f ree measureof the subject s mass i n w ater. I n addi ti on a pneumati cvalve system is used to f aci l i tate the determ inat ion of re-sidual volume using the ni trogen w ashout of the lungs(61) sim ul taneousl y w i th the underw ater w eighing. T headvantages of thi s system incl ude fast and reproducibledeterm ination of mass i n w ater w hi l e the subject retainscontrol during the submersion procedure w hich reducesapprehensi on and promotes cooperation. The precisionof body densi ty measurements w i th thi s system is 0.00 15 -0.0020 g/ee, or < 1% body f at i n the groups studied (56,60, 62). T hus, the absolute error (eg, kg of f at) of thi stechni que is dependent upon the f atness of the subject.I t i s important to emphasize the need to perf orm re-sidual lung volume determ inations at the time of the un-derwater w eighing. A l though one study (63) show ed nodi f f erence in mean body densi ti es cal culated f rom residuallung vol umes determ ined during submersion, predictedf rom standard tables, or estimated f rom v i tal capaci tymeasurement, al l subjects w ere heal thy , nonsmok ing,physi cal l y acti v e students. I t i s unl i kel y that a sim i l arf i nding w ould be obtained w i th m iddle-aged males andfemales on w hom hydrostati c f orces woul d enhance ex -hal ati on during submersion.

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BODY COM POSIT ION M ETHODS 541A second method of assessing body volume uses the

actual w ater di splacement technique using a body volu-meter (64). T he techni que i s sim i lar to that of hydrostati cw eighing except the actual volume of w ater displaced bythe subject i s measured rather than the loss in w ei ght i nw ater. W ater displacement i s measured w hen the subjectsubmerges under water and the increase in w ater level i smeasured usi ng a prev iously cal ibrated, f i ne bore buretteconnected to the tank . Residual lung volume must alsobe determ ined to calculate whol e-body densi ty . B ecauseof the di f f i cul ti es in distinguishing the changes in volumein the tank necessary to obtain the accuracy associatedw i th the underw ater w eighing method (65) and a lack ofcontrol by the subject during submersion, thi s techniquehas not gained w ide acceptance.

V ery usef ul body composi ti onal data can be gainedf rom the hydrodensi tometri c technique. H ow ever, i ts usein populations unaccustomed to sw imming or apprehen-sive about submersion in w ater may resul t i n longer pe-riods of time requi red to desensi ti ze the subjects to obtainval id and reproducibl e body composi ti on estimates. B e-cause this system (60) includes many components (straingauges, ampl i f i ers, chart recorder, spi rometers, and ni tro-gen analyzer), i ts cost i s $2 5 000-$30 000. Thi s systemcan be adapted f or f i eld use.

A recent development i s the use of a pl ethy sm ograp hw hich el im inates the need for total immersion of the sub-ject (66). T his system is a closed vessel i n w hi ch the subjectstands in w ater up to neck level ; the volume of the subjecti s determ ined by measuri ng the pressure changes producedby a pump of know n stroke volume. T his system requi resan apparatus more compl ex than for hydrodensi tometrybut i t does not requi re the instrumentation f or residualvolume determ inati ons. Thus, total body volume can bemade w i th m inimal subject cooperation and good pre-ci sion (SD < 0.3 kg body f at). T he val idi ty and repro-ducibi l i ty of thi s techni que have been establ i shed in onlyone l aboratory (66).

A common equation f or the calculati on of percent bodyf at (f ) f rom body densi ty (D i ,) i s that proposed by B rozeket al (10): f = (4.570/D) - 4. 142. A nother equati on w asdevel oped by Si r i (3): f = (4.950/D,) - 4.50. W i thin den-si ti es of 1. 1 0-1 .03, the tw o equations give resul ts w i thin1% body f at. For subjects w i th > 30% fat, the Si r i equati ony ields hi gher val ues than the equation of B rozek (55).

Anthropome t r yThe use of anthropometri c data has f aci l i tated the es-timati on of human body composi ti on outside of the lab-

oratory . V alues f rom the determ ination of sk inf old thi ck -ness at various si tes and measurements of bone dimen-sions and l imb ci rcum ferences can be used in mul ti pleregression equations to predict body densi ty and to cal -culate body f atness and fat-f ree mass. D escriptions ofstandard anthropometri c measurements are presentedelsew here (34, 67).

Bone measures . The anthropometri c estimation of f at-f ree mass is based upon the principle that a relati vely con-

slant proportion of the f at-f ree ti ssue is associated w i th agi ven skeletal si ze (68). B ehnke (69) proposed the hy -pothesis that measurement of bone di ameters coul d beused for estimation of skeletal mass and thus f at-f ree mass.U sing a vari ety of bone measurements made w i th a broad-blade anthropometer (head length and w idth; biacrom i al ,bidel toid, bi - i l i ac, and bi trachanteri c diameters; knee, an-ide, elbow , w ri st, and chest w idths), W i lmore and B ehnke(70) developed predi cti on equations f or body densi ty andf at-f ree mass in col lege men. H ow ever, w hen these equa-tions w ere tested in older men, smal ler than expected cor-r el ati on coef fi ci ents (r = 0.73-0.82) w ere f ound betw eenpredicted and densi tometncal l y determ ined body -corn-posi ti on values (7 1). Si mi lar d if feren ces (r = 0.77-0.80)betw een anthropometri cal l y predicted and densi tomet-ri cal l y determ ined densi ty and f at-f ree mass al so w ere oh-served i n w omen (72). T hese f i ndings indi cate that theproposed anthropometnic models w ere val id only f or thesegment of the populati on f rom w hich the model w asderived.

Skinfolds . M ore emphasis has been placed on the useof sk infold thi ckness measurements to estim ate humanbody composi ti on. This approach i s based upon tw o as-sumptions: the thickness of the subcutaneous adipose ti s-sue ref l ects a constant proporti on of the total body f atand the si tes selected f or measurement represent the av-erage thi ckness of the subcutaneous adipose. N ei ther ofthese assumptions have been proven to be true. D espi tethe contenti on that subcutaneous fat makes up about hal fof the total body f at, there are no data to support thi sstatem ent. F urth er mo re, because there is l i ttl e inf orm ationon the distr i but ion of f at in the body of the popul ationat large, the val idi ty of using sk inf old equations to predictbody composi ti on is restr i cted to populations f rom w homthese equati ons w ere deri ved.

The measurement of sk inf old thi ckness is made bygrasping the sk in and adjacent subcutaneous ti ssue be-tw een the thumb and foref inger, shak ing i t gentl y to ex-dude underl y i ng muscle, and pul l i ng i t aw ay f rom thebody just f ar enough to al low the jaw s of the cal iper toimpinge on the sk in. B ecause the jaw s of the cal i per (cal -i brated to exert a constant pressure of 10 g/mm2) compressthe ti ssues, the cal iper reading dim inishes f or a f ew secondsand then the dial i s read. D upl i cate readings are made ateach si te to improve the accuracy and the reproduci bi l i tyof the measurements. I n subjects w i th moderatel y f i rmsubcutaneous ti ssue, the measurement i s easy to perf orm ;i ndi v i dual s w i th f l abby easi l y compressible ti ssue orw i th not easi l y def ormable very f i rm ti ssue pre-sent a problem for obtaining val id measures of sk inf oldthi ckness. B ecause a doubl e f old (tw o layers of sk in andsubcutaneous ti ssue) i s measured, any f actor that af f ectsthe reproducibi l i ty and val idi ty of the sk i nf old thi cknessmeasurement i ncreases the error of the predicted body-composi ti on val ue.

M any equati ons are avai l able f or the prediction of bodydensi ty and thus body f atness f rom sk infold thi cknessmeasurements (55 ) . H owever, in terms of general val idi tyi n the adul t Caucasian populati on, some equati ons have

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542 LUK A S K Ibeen recommended. D urnin and W omersley (73) devel -o_ a regressi on equation to predict body densi ty usingthe logari thm i c transf ormation of the sum of f our sk in-f olds (tr i ceps, biceps, subscapula, and i l i ac crest), age, andgender. Jackson and Pol lock (74) estimated body densi tyi n men using l ogari thm ic or quadrati c transf ormati on ofthe sum of seven sk inf olds (chest, ax i l a tr i ceps, subscapula,abdomen, thigh, and suprai l i ac) , age, and w ri st and fore-arm ci rcum ferences. For w omen, Jackson et al (75) de-ri ved an equation to predi ct body densi ty f rom a quadrati cof the sum of three sk inf olds (tr i ceps, thigh, and suprai l -i ac), age, and gl uteal ci rcumference.

I t i s important to emphasi ze that the use of a mathe-matical transf ormati on (ei ther as l ogari thm ic or quadrat i cf orm ) of the sum of sk inf old thi cknesses is needed becausebody densi ty i s not l i nearl y related to subcutaneous f atmass (73). A l so, the inclusion of age and gender reducesthe error of predi cti on of percent body f at in cnoss-val i -dation tr ial s (74, 75 ) .

The preci si on of a measurement of sk i nf ol d thi cknessis dependent upon the sk i l l of the anthropometri st andthe si te measured. I n general a preci si on ofw i thi n 5% canbe attained easi l y by a properl y trai ned and experiencedindiv i dual (67). T his error can increase sl ightl y i f sk inf oldthi cknesses ei ther get very large (> 15 mm) or smal l (< 5mm) (76). T he error in estimating densi tometri cal l y de-term i ned body composi ti on f rom anthropometry has beenestabl i shed to be 5% body fat (55 ) ; depending on thepredi cti on equati on and the sample of subjects, some re-ported errors have ranged f rom 3 to 9% body fat (55, 77).

A rm circum feren ce . A ssessment of body f atness bystandard techniques prov ides an estimate of nutr i ti onalstatus of an indi v i dual or populati on. B ecause of the im -practi cal i ty of using laboratory methods i n f iel d studi es,upper arm ci rcum ference and tri ceps sk inf old have beenused to assess nutr i ti onal status (78). A rm ci rcum ferencealone, al though being relati v ely age-independent and auseful , though l im i ted, measure (79), does not y ield a pre-ci sc diagnosis of malnutr i ti on. The tr i ceps sk inf old, w hichis relati vely easy to measure, i s an i ndex of subcutaneousadi pose ti ssue. The combi nation of these anthropometnicmeasures have been used as an indicator of protein-energymalnutr i ti on (80).

T he estimated muscl e arm ci rcumference (Cm) gives anindi cation of the body s muscle mass and hence i ts mai nprotei n reserv e. I t (C ,,j can be deri ved f rom the arni C i r-cumference (Ca) and the tr i ceps sk inf old (5) using theequation Cm = Ca - irS.U sing the cross-secti onal f at (F) and muscle (M ) areasis more logical i n the assessment of energy and proteinnutr i ti onal status than using the sk inf old and arm ci r-cum ference. I ndi v idual l y , each of these measurements isa w eak predictor of energy and protein stores part i cularl yin chi ldren. H ow ever, the combinat ion of these variabl esappears to be a more precise indicator of nutri ti onal status(81). T he fat area has the addi ti onal advantage that thestandard in adequately nouri shed chi ldren changes onlysl i ght l y betw een ages 1-7 y , thereby prov idi ng an age-independent assessment of energy reserves (79). Calcu-

lation of these areas uses the f ol l ow ing equations: F = SCJ2 + i rS2/4 and M (Ca

In practi ce the measurements made are arm ci rcum -f erence and tri ceps sk inf old. A rm ci rcum ference is themid-arm ci rcum ference measured to the nearest m i l l i -meter on the right arm midw ay betw een the ti p of theacrom ion and ol ecranon process w i th the arm relaxed.The tri ceps sk inf old i s measured to the nearest 0. 1 mmusing a cal ibrated cal iper at the same level as the m id-arm ci rcum ference on the poster i or aspect of the arm .

The basic assumptions f or the calculation of the armand fat area include: the m id-arm i s ci rcular the tr i cepssk inf old i s tw ice the average fat r im diameter; the m id-arm muscle compartment i s ci rcular; and bone, w hich i sincluded in the anthropometri c arm muscle area, atro-phies in proportion to muscle in protei n-energy malnu-tni t ion. H eymsf ield et al (82) examined the val idi ty of theseassumptions f or adul ts and found that each of these ap-prox imati ons w as in error to some degree. The resul t w asa 15-25% overestimate by anthropometry i n arm muscl earea w hi le m id-arm fat area agreed w i thin 10% to valuesmeasured by computeri zed axial tomography in adul ts.These investigators subsequentl y deri ved gender-speci f i cequati ons to account f or errors in each of the four as-sumptions. These equations reduced the average error f ora given subject to 7-8% for arm muscle area (83). Cor-rected arm muscle (are (cA M A ) equati ons f or men andw omen are [ (M A C - i rS)2/4i r] - 10 and [ (M AC - i rS)2!4ir) ] - 6.5, respect i vely , w here M A C is m id-arm ci rcum -f erence and S is tr i ceps sk i nf ol d (83). U sing estimates ofmuscle mass der i ved f rom urinary creatimne excretion(42), these investigators proposed the f ol l ow ing relation-ship to predict total body muscle mass f rom cA M A : mus-cle mass (kg) = (height, cm ) (0.0264 + 0.0029 (cA M A ).The error of thi s prediction ranges f rom 5 to 9% .

N ew methodsN eutro n a ctiva tio n ana ly sis

The development of in v ivo total -body neutron-acti -v ati on analysis has prov ided the only technique currentl yavai l able for the measurement of the mul ti el ementalcomposi ti on of the human body. A bsolute content of cal -cium , sodi um , chl oride, phosphorus, and ni trogen can bedeterm ined safely .

Total-body neutron-acti v ati on systems designed f or i nv ivo studies del i v er a moderated beam of f ast neutronsto the subject. Capture of these neutrons by atoms of thetarget elements in the body creates unstable i sotopes suchas cal cium-49 and ni trogen-iS. The isotopes revert to astabl e condi ti on by the emi ssion of one or more gammarays of characteri sti c energy . Radiat ion f rom the subjectis determ ined using a recording of the radiospectrum ofthe emissions. The data are obtai ned f rom a subject po-si ti oned caref ul l y w i th respect to a detector array in ahighly shielded f aci l i ty . Standard gamma spectrographi canalysis i s appl ied. The energy level identi f i es the el ementand the level of acti v i ty indicates i ts abundance.

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f lOD Y COM POSIT ION M I T I - IODS 543Neutron acti vati on i s an analy ti cal technique based on

nuclear reacti ons rather than chem ical reacti ons. T he es-sential variables include neutron f lux densi ty , i sotopi cabundance, cross-section of the target element, hal f - l i v esof the product i sotopes, and emission energy of the i n-duced act i vi ty .Tota l b od y ca kium . The f i rst appl i cati on of neutronacti vati on analysi s to the assessment of human bodycomposi ti on w as the determ i nation of total body calcium(TBCa). B ecause calci um is a relat i vely constant f raction(38-39% ) of bone mineral (ash) w eight, est imates of TBCacan be used to quanti tate total -body bone m ineral . H ow -ever, calcium mass i s not alw ays proporti onal to skeletalmass because ex traosseous deposi ts occur in heal th anddisease.

The avai labi l i ty of neutron acti v ati on f aci l i ti es to mea-sure human TBCa is l im i ted to the U niversi ty of B i r-mingham, B i rm ingham, UK (84); B attel l e Paci f i c N orth-w est L aboratories, Ri chland, W A (85 ) ; and B ro ok hav enN ati onal L aboratory , U pton, N Y (86). T he f i rst tw o fa-ci l i ti es ut i l i ze cyclotrons to produce neutrons of 3.5 and8 M eV energy level s, respecti v el y . T he B rookhaven f aci l i tyuses 4.2 M eV neutrons f rom a plutonium -238, bery l l i umsource, w hi ch is better sui ted f or research purposes becausei t gi ves a low er radiation dose ( 280 mrem) to the subjectthan the cyclotron sources ( - --500 m rem). U si ng theBrookhaven method (86), the accuracy and precision ofthe acti v ati on procedure in an anthropometri c phantomis 1% . I n heal thy adul ts measured over a 4- 5 y per iod,the precision of the repeated TBCa determ inat ion w as2.5% (87), w hi ch is sui table f or studies of longi tudinalchanges (88).

T o quanti tate cl i ni cal l y meaningf ul di f f erences in TBCaw i th aging or metabol i c bone disease, speci f i c procedureshave been developed to normal i ze body calcium levels.M ost importantl y , normal i zati on f or anthropometnicvariables, such as age, gender, height, and lean body mass(eg, potassium -40 counting), i s necessary to reduce vari -abi l i ty (f rom 15-18% to 5 - 8% ) i n heterogeneous samples(89). T he abi l i ty of TBCa to discrim inate abnormal i tymay be due in part to thi s normal i zati on procedure.

This experimental approach has been used to estimatemean rates of body calcium loss in a cross-sectional studyof A merican adul ts aged 30-90 y . I n w omen, mean rateof calcium loss up to age 50 y w as 0.37% /y (3.8 g/y ) andaf ter age 50 y i t w as 1. l% /y (7.6 g/y ). T he average loss f ormen is 0.7% /y or 7 g/y af ter age 55 y (90).

T ota l bo dy n itrogen . The abi l i ty to measure total bodyni trogen (TBN ) l evels by neutron acti vation analysis hasprov ided valuable determ inat ions of human body com -posi ti on in heal th and disease. The f i rst nucl ear methods(91, 92) f or di rect determ ination of TBN in humans usedthe 4N (n, 2n) 3N reacti on and suf fered f rom a lack ofspeci f i ci ty because of interf erence f rom posi tron emi ttersf rom other body elements. Recentl y the development ofthe prompt-gamma techni que (93,94), using the reacti on 4N (n , y ) 5N , has led to the recogni ti on of the cl i ni calusefulness of body ni trogen measures in body-composi ti onassessment.

T his prompt-gamma techni que uses a portable pluto-ni um -238 bery l l i um source to prov ide f ast neutrons thatare moderated before contacting the subject w ho is i rra-diated bi lateral l y . T his method quanti tates TBN abso-lutel y by using body hydrogen as an internal standard(93, 94). B y means ofprompt neutron capture, the ni tro-gen and hydrogen nuclei i nteract w i th the moderated orsl ow neutrons to produce transientl y ( l 0_15 s) inducednucl ides, 5N and 2H , em i tti ng gamma rays of character-i sti c energy levels ( 10.83 and 2.23 M eY , respecti vely ) thatare quanti tated simul taneously duri ng the neutron i rra-diation. This procedure requi res a 20-mm neutron ex -posure and counting period and has a cal culated w hole-body radiation dose of 26 mrem. The advantage of thi stechni que over the conventional method of analysis i sthat errors i n counti ng resul ti ng f rom di f f erences in i rra-diat ion and detection condi ti ons and f rom di f f erences insize and shape of subjects are reduced considerabl y . T hisreducti on in error makes sequenti al ni trogen measure-ments consi derably more rel iable, parti cularl y w hen sub-ject w eight has changed signi f i cantl y . T he accuracy andprecision of ni trogen determ inat ion i n a phantom is 3% ;the precision of repeated determ i nations i n heal thy hu-mans i s 2-3% (94).

T he avai labi l i ty of methodology f or absolute determ i -nati ons of TBN values in connection w i th accurate TBNdata al low s the estimation of muscl e and nonmuscle massand thei r respecti v e protein contents using the mathe-mati cal models of B urk inshaw (95 ) . K now ledge of themuscle and nonmuscle components, bone m ineral massf rom TBCa determ inati ons, and body mass al low s thecal culati on ofbody f at by di f f erence. T hi s f ou r- comp ar t-ment model of human body composi ti on (96) w as show nto be useful in evaluati ng large di f f erences i n these corn-partments betw een heal thy and diseased subjects (97).

M ore i nf ormation can be gained f rom total -body neu-tron-act i vation anal ysis than by any other avai lablemethod. H owever, f actors such as the hi gh cost ( i n excessof $400 000), the need for sk i l l ed operators, lack of mo-bi l i ty , and use of i oni zi ng radiati on general l y preclude theroutine appl i cation of thi s method to assess human bodycomposi t ion.M uscle M eta bo lite s

To ta lp la sm a crea tin ine . A s di scussed prev i ously , 24-hurinary creatinine excretion has been used to estimatemuscle mass and fat-f ree mass. Recentl y , the use of totalplasma creati nine w as suggested as an index of total bodyskel etal muscle mass.

Schutte et al (98) ex tended the origi nal w ork of Tal bot(40) and deri ved good rel ati onships (r = 0.82, p < 0.00 1)betw een total plasma creati nine (plasma volume X p lasm acreatini ne concentrati on) and 24-h urinary creatinineoutput by 24 heal thy men. Total skeletal muscle massalso was estimated f rom 24-h uri nary creatinine accordingto Talbot (1 g urinary creatinine = 17.9 k g sk eletal m uscle)(40) and Cheek (1 g urinary creatini ne = 20.0 kg sk el etalmuscle) (41, 52). U sing these est imates of skeletal muscle

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544 LUKASK Imass, the authors calculated that each mi l l i gram of totalpl asma creatini ne w ould account for 0.88 or 0.98 kg skel -etal muscle.

To val idate these calculations determ i nations of plasmacreatinine and di rect dissections of skeletal muscle w eremade in f our mature dogs. The resul ts i ndicated that eachm i l l i gram of total pl asma creatinine is the equivalent of0.88 kg skeletal muscl e, w hich conf i rms the f indi ngs ofT albot (40). A mean error of 3.9% (range: 0.5-10.8% ) w asobserved betw een the predicted and observed skeletalmuscle mass values.

Endogeneou s ur in ary 3 -m ethy lh is tid in e exc re tio n . Theam ino acid 3-methy lhi sti di ne (3-M H ) has been suggestedas a safe, noninvasi ve in v i vo marker of muscle proteinbreakdow n (99). I t i s located principal l y in skeletal musclein w hich i t i s produced by the posttranslational methy l -ation of speci f i c histi dine residues in the acti n of al l muscl ef ibers and in the myosin of w hi te muscl e f ibers. D uringcatabol i sm of the myof i br i l l ar proteins, the released 3-M H is nei ther reuti l i zed f or protein synthesis nor metab-ol i zed ox idat i vely but i s excreted quanti tati vely in theuri ne. T hese characteri sti cs suggested that 3-M H couldbe useful in predicting human body composi ti on.

D ensi tometri cal l y determ ined f at- f ree mass w as show nto be w el l correlated (r = 0.90, p < 0.001) w i th 24-h en-dogenous 3-M H excretion in 16 heal thy men aged 23-52y consum ing a meat- f ree diet (48). U rinary creati ni ne w asa w eak er (p < 0.05) predi ctor (r = 0.67, p < 0.01) of f at-f ree mass in these men.

A fol low -up study w as conducted to ascertain the spec-i f i ci ty of 3-M H as a marker of skeletal m uscle mass (100).M uscl e and nonmuscle mass and thei r protei n contentsfor 14 heal thy men were determ i ned f rom measurementsof TBN and TBK accordi ng to B urk inshaw (95). T hesemen consumed tw o isocalonic i soni trogeneous diets i n thesequence of a 4-d meat diet f ol l ow ed by a 74 meat-f reediet. U rinary 3-M H excret ion during the meat diet (513 2 1 mol /d; mean SE) was signi f i cantl y hi gher thanexcretion on day 3 of the meat-f ree diet (230 10 zmol /d) af ter w hich the mean dai l y output w as relati vel y con-stant w i th a mean coef f i cient of variation of 4.5% . Theendogenous output of 3-M H w as rel ated signi f i cantl y toskeletal muscle mass (r = 0.9 1 ; SEE = 2.0 kg) and w asnot asso ci at ed (r = 0.33) w i th the nonmuscle f racti on ofthe f at-f ree mass. A l though endogenous 3-M H excretionw as a better predictor than urinary creati nine output ofdensi tometr i cal l y determ ined fat- f ree mass and of skeletalmuscle mass, the dai l y excretion of these urinary metab-ol ites w as correl ated (r = 0.87, p < 0.00 1), indicating theval i di ty of these metabol i tes as indi ces of muscle and fat-f ree m ass.

W hen the body composi ti on data f rom these tw o studi es(48, 100) are combined, one can estimate the predicti veval ue of these rel ati onships. The prediction of f at-f ree massf rom endogenous 3-M H excreti on (r = 0.89, p

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BODY COM POSIT ION M ETHODS 54 5S in gle -ph oton a bso rp tio metry . D i rect photon absorp-

tiometry has become w idely used and accepted forbiomedical i nvesti gati on of l ocal or regional bone mea-surement. T his technique involves a transmi ssion scanusing a highl y col l imated beam , usual l y f rom iodine-125(2 7 keY ) or americium -241 (60 keV ), and a col l imatedsodium iodide scinti l l at i on detector (1 10). T he beam ispassed across a l imb bone and changes of beam intensi tyare analyzed; the i ntegral of these changes is proportionalto the bone mineral content i n the beam path. W i th thi stechnique the bone must be enclosed in a constant thi ck -ness of sof t ti ssue. W ater baths, t i ssue-equivalent sub-stances, and local compressi on have been used as controlmateri al s for thi s purpose. D eterm inations of local bonem ineral content can be perf ormed w i th ei ther the ax ialor appendicular skeleton. H owever, most i nvestigatorshave uti l i zed the lower radius at a si te approx imately one-thi rd the di stance f rom the sty loi d process to the ole-crenon.

Single-photon absorpti ometry of the appendicularskel eton usi ng iodi ne-125 w as shown to have a high long-term preci si on (1-2%) in v ivo when care is taken to re-posi ti on the l imb ( 1 1 1 , 1 1 2). I n f i el d studies the preci si onmay reach 5% because of reposi ti oning error. T his errormay be m inim ized by using the ratio of bone mineralcontent to radius w i dth as the index . The accuracy of themeasurement has been 2-4% (1 12, 1 13). A ty pical mea-surement on the distal thi rd of the radial shaf t i s w el lcorrelated (r = 0.9-0.95) w i th the w eight of the radiusand of other long bones and w i th total skel etal m ineraland calcium (1 13-1 16). T his prediction of total skeletalmass has an error of 1 50-250 g (- - 10%), w hich is equiv -alent to an error of 60-100 g i n TBCa determ ined byneutron acti v ati on (1 14-1 16). T hus, f or i ndi v idual as-sessment, an error of 5 -10% i n normal subjects or of 10-1 5% i n osteopenic subjects i s too large and a di rect mea-surement of TBCa is desi rable. I t i s not f easible to scanthe total body w i th the single-photon technique. A lso, theuse of single-photon absorptiometry measurements of theappendicular skeleton may not be as sensi ti v e a predi ctorofosteoporosis as are determ inations, made by dual -pho-ton absorptiometry of the spine, a prime target of dem in-eralization.

D ual-p ho ton a bsorp tiom etry . Total -body bone m ineralcontent and lean body mass are measurable using dual -photon absorptiometry , w hich el im i nates the need forconstant sof t-ti ssue thickness across a scan path and al low smeasurements of hi therto inaccessible body areas. Thismethod uses a whole-body recti l i near scanner and a highact i v i ty (0.5-1.5 Ci ) source of gadol i nium-l53 that em i tsenergy at tw o discrete peaks (44 and 100 keV ). The sourceis mounted beneath a table and is opposi te to a scint i l -l at i on detector above the tabl e. T he source and detectorare passed across the body at a traverse speed of 1 cm /sw i th data col lected at 0.5 cm (0.5 s) intervals (117).

A ttenuation measurements at a given number of energylevels are requi red to analy ze an even number of sub-stances by absorpti ometry . H ow ever, due to exper imenal

uncertainti es and because attenuation coef f i cients arecorrelated, the number of substances that can be deter-m ined w i th preci si on i s l im i ted. T hus, attenuati on mea-surements at tw o discrete photon energies are needed toquanti tate a tw o-component system (bone m ineral andso ft t issu e) .The composi ti on of bone mineral i s essential l y invar-iable; however, sof t ti ssue is composed of variable amountsof f at and lean ti ssue. V ar iati ons in f at-l ean ti ssue corn-posi ti on produce di f f erences in attenuation coef f i cientsf or sof t ti ssue at both energy level s. T hus, one must obtainan estimate of sof t ti ssue composi ti on to obtai n accuratemeasures of bone m ineral and fat or lean ti ssue masses.This i s achi eved by obtaining experimental attenua-tion at the tw o energies as described by Peppler andM azess (117).

B ri ef l y , the ratio of attenuation at 44 keV to that at 100keV gives the quanti ty , R , w hi ch di rectl y indicates the f atcontent of sof t ti ssue (1 18, 1 19). A w eighted average ofthe R value (w eighted f or mass of t i ssue in each pixel ) i scalculated f or the number of pixels containi ng sof t ti ssuealone. The percentage body f at deri v ed f rom the R valuei s used to cal culate f at mass and lean body mass. Total -body bone m ineral i s calculated f rom the bone-containingpixels (1 17).

Estimated precision of total bone mineral contentmeasurements by dual -photon absorpti ometry for skele-tons (1-2% ) and f or humans (2-3% ) are high. A ccuracyof thi s method for skeletons has been reported as a SEEof 36 g, or an error of 1% (1 17). Compari sons of total -body bone mineral measured in v i vo using the dual -pho-ton techni que and TBCa by neutron acti v at ion analysisshowed a higher correlation coef f i cient betw een the meth-od s (r = 0.99) w i th an error of 1 1 3 g for the predictedtotal -body bone m ineral (120).

M azess et al (12 1) compared est imates of total bodycomposi ti on der i ved by dual -photon absorpti ometry anddensi tometry . Sim i l ar estimates f or percent f at and f orl ean body mass were obtained f rom 1 8 volunteers (14f emales, 4 males) aged 23-58 y . Correlation coef f i cientsbetw een the composi ti onal variabl es by the tw o methodsw ere good (r 0.90) and w ere inf l uenced by the total -body bone m ineral mass: lean body mass ratio, w hi ch w asvariable (CV = 17%) and probably inf luenced the v al idi tyof densi tometri c predi ction of body composition.

The advantages of the dual -photon absorpt iometrytechnique includes portabi l i ty , l ow radiation dose (2-10m rem ), and total -body determ ination of bone mineraland lean body mass by relati vel y di rect anal ysi s. I nstru-mentation can be transported saf ely in a van and repeatedmeasures can be made in subj ects w i thout undue radiationri sk . A lso, improved determ i nations of body composi-ti onal v ari abl es can be made because bone mineral , per-haps the most v ar i able component of the lean body , canbe measured. Cost of thi s i nstrumentati on ($65 000) maybe a l im i ti ng f actor f or i ts use. I n addi t i on some theoreti calassumptions, such as the i nf luence of Compton scattering,beam hardening, and attenuati on f actors, requi re addi -

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546 LUKASK It i onal consideration before this method can be recom-mended.E lec tr ica l co ndu ctan ce

B io elec tr ica l im ped an ce . T he method for determ iningbody i mpedance i s based upon the nature of the conduc-ti on of an appl i ed electr i cal current in an organism. I nbiologi cal structures appl i cati on of a constant, l ow - levelal ternati ng current produces an impedance to the spreadof the current that i s f requency dependent. T he l i v ingorganism contains intra- and ex tracel lul ar f l uids that be-have as el ectr i cal conductors and cel l membranes that actas imperf ect reacti ve elements. A t low f requencies ( -1kH z), the current mainly passes through the ex tracel lul arf l uids w hi le at higher f requenci es (500-800 kH z) i t pen-etrates the intra- and ex tracel l ul ar f l ui ds (122, 123). T hus,body f luids and electrol y tes are responsible f or electr i calconductance (eg, 1/resi stance) and cel l membranes arei nvolved in capaci tance.The hypothesis that bioelectri cal impedance measure-ments can be used to determ ine f at-f ree mass is basedupon the pri nciple that the impedance of a geometri calsystem is rel ated to conductor length and conf igurati on,i ts cross-secti onal area, and si gnal f requency . W i th a con-stant signal f requency and a rel ati vely constant conductorconf iguration, the impedance to the f l ow of current canbe related to the f low of current: Z = pL /A , w here Z isimpedance in ohms, p is volume resisti v i ty in ohm X cm ,L i s conductor l ength in cm , and A is conductor cross-secti onal area in cm2. M ul ti pl y ing both sides of the equa-ti on by L /L gives: Z = pL 2/A L , w here A L equal s volume(V ). T hus, Z = pL2/V .

I n l i v ing organi sms electr i cal conduction is related tothe w ater and electrol y te distr i buti on in the biologicalconductor. B ecause fat-f ree mass, including the proteinmatri x of adipose ti ssue, contains v i rtual l y al l the w aterand conducting el ectrol y tes in the body, conduct i v i ty i sf ar greater in the f at-f ree mass than in the f at mass of thebody (124). The hypotheti cal relationship betw eenimpedance and electr i cal volume w as proposed by N yboeret al (125) w ho demonstrated that electr i cal l y determ inedbiologi cal volumes were related inversel y to Z , resi st ance(R), and reactance (X c) w here Z = (R 2 + X c #{ 176} 5Becausethe magni tude of reactance is smal l relati ve to resistanceand resistance is a better predictor of impedance than isreactance (36), volume can be expressed as: V = pL2 /R ,w here L is standing hei ght in cm and R is in ohms. A l -though there are di f f i cul ti es in apply ing this general prin-ciple in a system w i th as compl ex geometry and bioelec-tr i cal characteri sti cs as the human body, thi s relationshi phas been used to deri ve models f or the predi ction of hu-man body composi ti on (36, 126-128) by assum ing thatthe body is a series of connected cy l inders.

D eterm inati ons of resistance and reactance are madeusing a four term i nal impedance plethysmograph (RJLSystems, model 101, D etroi t , M I ). (M ention of a trade-mark , proprietary product, or vendor does not consti tutea guarantee or w arranty by the U S D epartment of A gri -

cul ture and does not imply i ts approval to the excl usionof other products or vendors that al so may be sui table.)T he tetrapol ar method is used to m i nim ize contactimpedance or sk in-electrode interactions (122, 128). A sa general procedure measurements are made - 2 h af tereati ng and w i thin 30 mm ofvoi ding. The subj ect, cl othesbut w i thout shoes or socks, l i es supine on a cot. A lum inumfoi l spot electrodes are posi t i oned in the m iddle of thedorsal surf aces of the hands and feet prox imal to themetacarpal -phalangeal and metatarsal -phalangeal joints,respecti v ely , and al so medial l y betw een the distal prom -inences of the radius and the ulna and betw een the medialand lateral mal leol i at the ank le. A thin layer of electrol y tegel i s appl ied to each electrode before appl i cation to thesk in. A n exci tation current of 800 A at 50 kH z is intro-duced into the volunteer at the distal electrodes of thehand and foot and the vol tage drop is detected by theprox imal electrodes. (A ccording to Ohm s L aw the dee-tr i cal impedance [Z ] to al ternating current of a ci rcui t i smeasured i n terms of vol tage [E] and current, [ I ] as Z= E/I . B y usi ng phase sensi ti ve electronics, one can quan-t i tate the geometri cal components of Z ; resistance ER] isthe sum of i n-phase vectors and reactance [X ci i s the sumof out-of -phase vectors.) T his techni que prov ides a deephomogenous electri cal f iel d in the vari able conductor ofthe body . D eterm inations of resistance and reactance aremade using electrodes placed on the ipsi lateral and con-tral ateral sides of the body . T he low est resistance valuef or an indiv i dual i s used to calculate conductance (ht2/R) and to predict f at-f ree mass. The precision of thi smethod, determ ined in 14 men in w hom impedance w asmeasured on f i v e consecuti v e days, w as < 2% (36).

T he tetrapolar impedance method has been used topredict body composi ti on i n heal thy adul ts. U si ng den-si tometri c data but l ack ing accurate residual volumemeasurements, N yboer et al (129) developed prel im inarystati sti cal relationshi ps betw een conductance and bodycomposi t ion var i ables in col l ege students. Segal et al (130)attempted to veri f y these regression equati ons and foundunsati sfactory predi ctions of f at-f ree mass in men andwomen aged 18-50 y . W e have used standard methodsto establ i sh models to estimate total body w ater and po-tassium and fat-f ree mass in heal thy men (36). T he re-specti v e errors of predicti on of these vari ables w ere 2.1L , 10.7 g, and 2.6 kg. Recentl y w e conducted a study w i th47 men and 67 w omen to val i date the relati onship be-tween conductance and f at-f ree mass ( 13 1). C om pari so nof densi tom etnical l y determ i ned and im pedance-predictedf at-f ree mass y ielded an error of predict ion of 2-2.5 kgand a calcul ated error of relati ve body f atness of 2.7% .Relati v e to densi tometry the observed error of calculatedbody fatness was larger by anthropometry (3.9% ) than byimpedance (2.7% ).

From an analysis of i ntraindiv i dual coef f i cients of vari -ati on in repeated measurements of resistance over f i v econsecuti v e days i n 14 men, i t w as suggested that the ob-served variabi l i ty of 2% probably ref lects smal l changesin body w ater compartments (132). A sample of 33 malesand females aged 19-61 y underw ent determ inati ons of

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BODY COM POSIT ION M ETHODS 547TBW by deutenum di lution and extracel lular f l uid vol -ume by brom ide di l uti on. A s expected, conductance wasthe best single predi ctor of TBW and extracel l ular f l uidspace (r = 0.97 and 0.94, respecti v ely ). B ody hei ght,w eight, and age al so were signi f i cant predictors of TBW(R 2 = # {216 }97# {216 } ;EE = 2.1 L ). W eight and reactance wereaddi ti onal signi f i cant predictors f or ex tracel lular f l uidvo lume (R 2 = 0.930, SEE = 0.9 L ).

K ushner and Schoel ler (133) demonstrated the val i di tyof the impedance method to predi ct TBW in patients w i thei ther inf l ammatory bow el disease and recei v ing totalparenteral nutri t i on or diabetes. I n a sample of 20 non-obese and 20 obese adul ts, an equation w as developed(R 2 0.986, SEE = 1 .7 5 L ) to predict TBW f rom con-ductance, body weight, and gender. I n the prospecti vepatient sampl e, there w as no di f f erence betw een TBWpredi cted by the impedance model (42.9 7.4 L ; mean SD ) and determ ined by deuterium di lution (41.9 7.3L ). T his f i nding indicates the usefulness of the impedancemethod to assess TBW in indiv i duals w i th al tered meta-bol ic f unct i on.

T ota l b od y elec tr ica l condu ctiv ity (TO BEC ). A s w i ththe impedance method, thi s technique rel ies upon thedi f f erences in electr i cal conducti v i ty and dielectr i c prop-erti es of the f at f ree and fat t i ssues to estimate body corn-posi ti on (124). T he instrumentati on used is an adaptationof the commercial dev ice devel oped for determ inat ion oflean ti ssue in meat and l i v e animal s (134).

T he instrument i s a long, uni f orm solenoidal coi l dri v enby a 5 M H z osci l l ati ng radiof requency current (D i ckey-j ohn M edical I nstrument, A uburn, I L ). T he osci l l ati ngf ield w i th i nduce an electr i cal current in any conducti v ematerial placed w i thin the coi l . T he actual mesurementconsists of the di f f erence betw een the coi l impedance w henempty and that w hen the subject i s inserted (135).

D orneruth et al (136) compared conducti v i ty measuresw i th di rect chemi cal anal yses and l i v e potassium -40 datain 45 mature pigs (average weight 100 kg). T he correl ati oncoef f i cient betw een potassiurn-40 and conducti v i ty mea-sures w as 0 .75 . The weights ofcarcass w ater, f at, and pro-tein w ere correlated better w i th l i v e conduct i v i ty measures(r = 0.87, 0.32, 0.83) than w i th l i ve potassium -40 data (r= 0.78, 0.09, 0.69).

B racco et al (137) used a TOBEC instrument designedto est imate the lean component of ground meat (D jM e 100Ground M eat Fat Tester, D ickey -j ohn M edi cal I nstru-ment) to determ ine relationshi ps betw een conducti v i tyvalues (TOBEC scores) and body composi ti on estimatedby densi tometry and carcass analy sis of 30 rats w eighing197-433 g. TOBEC values were correl ated hi ghly w i thlean body mass by densi tometry (r = 0.97, SEE = 13.6g), f at-f ree mass by chemical analy sis (r = 0.97, SEE = 14.2g), total w ater (r = 0.98, SEE = 10.7 g), and total protein(r=0.95,SEE= 5.1 g).

U sing phantoms of inf ants composed of electrol y te so-lutions and corn oi l , K l i sh et al (138) val i dated a TOBECsystem developed for use w i th human inf ants. T he TO-BEC system w as sensi t i ve to changes in total electrol y tecontent and f l uid volume of the phantoms. A strong l inear

relationship (r = 0.97) w as observed betw een f at-f ree vol -ume and the natural l ogari thm (ln) of the T OBEC value.Sim i larl y , strong relati onships were observed betw een theln TOBEC signal s and fat-f ree content of ground meat (r= 0.9 1) and l i ve rabbi ts (r = 0.99).

A method using thi s technique has been devel oped forhuman body composi t i on assessment (139). T he subjectl i es supine on a stretcher on rol lers and is inserted intothe instrument. T en consecuti v e readings are obtainedover a 3-mm interval and the average is displayed. Theamount of electromagneti c radi ation received by the sub-ject during a single measurement period is / io/io lessthan any establ i shed regulatory l im i ts f or cont inuous ex-posure to radiof requency waves (135).

I n a group of 19 adul ts measured on four separate oc-casions, the rel iabi l i ty of the measurements w as high (r= 0.99) and the preci si on of the measurements w as < 1%(139). Raw conducti v i ty scores w ere correlated w i th an-thropometri c est imates of l ean body mass, total body w a-ter, and potassi um (r = 0.69, 0.86, and 0.87, respecti vely ).I n another group of32 adul ts, a good correlation (r = 0.90)w as observed betw een conducti v i ty scores and densi to-metri cal l y determ ined l ean body mass (140). U se of heightx conducti v i ty as the independent vari able improved thecorrelation w i th f at-f ree mass (r = 0.94, SEE = 4.0 kg).H ow ever, mul ti ple regression analy ses indicated that gen-der and height X conducti v i ty y ielded the best predict ionequation (r = 0.95, SEE = 3.8 kg). A study w i th anothergroup of 75 adul ts found that a transf ormati on of theconducti v i ty score, hei ght2 X conducti v i ty , w as a betterpredictor of lean body mass (130). W hen compared w i thdensi tometry , thi s method had a l ow er predicti v e error ofestimating body f atness (SEE = 3.7% ) than did sk inf oldthi ckness measurements (5.8% ).Recentl y , V an L oan and M aycl in (141) described a sec-ond generation instrument (TOBEC I I ) and developedprediction equati ons f or densi tometnical l y determ ined fat-f ree mass, TBW , and TBK in a sample of 40 males andfemales aged 19-35 y . This new instrument al so is a so-lenoidal coi l but i t i s pow ered by a 2. 5 M H z source thatgenerates a magneti c f i el d --79 cm in diameter and 185cm long. I n contrast to TOBEC I , the subject i s passedthrough the magneti c f i el d of TOBEC I I . M ovementthrough the magneti c f i eld y ields a phase curve repre-senting the interact ion of the magneti c f i eld w i th the geo-metri c shape and the distr i bution and amount of con-ducti v e material (eg, f at-f ree t i ssue). T he shape of the phasecurve does not correspond v isual l y to the contour of thesubject or the distr i bution of f at-f ree mass because thecurve represents the sum of tw o components (conducti veand di electr i c masses) passing through the magneti c f i eld.B ecause the phase curve is a periodic f uncti on, Fourieranaly sis, w i th the abi l i ty to represent a complex w aveformw i th simple coef f i cients, has been appl ied to body corn-posi ti on anal ysi s.

M ul ti ple regression analy sis usi ng zero-, and f i rst-, andsecond-order Four ier coef f i ci ents as predi ctor var i ables off at-f ree mass, TBW , TBK y ielded R2 ofO .983, 0.96 1 , and0.89 1 w i th SEE of 1 .43 kg, 1 .57 L , and 1 1 g, respecti v ely .

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54 8 LUK A S K IThese resul ts indicate that TOBEC I I has the potenti alf or increased predicti v e accuracy over the ori ginal TOBECinstrument. H ow ever, cross-val idati on tr ial s are necessaryto prove thi s contention.

The advantages of the bi oelectr i cal impedance methodare portabi l i ty , saf ety , convenience, cost ($2500), and ac-ceptable level s of rel i abi l i ty and accuracy of body com -posi ti on estimates in heal thy adul ts. I t appears to be w el lsui ted f or population or epidem iological surveys. I tsdraw back is the lack of val i dation i n patients undergoingw eight or composi ti onal change and in pati ents w i th ab-normal w ater or electrol y te di str i butions. Sim i lar disad-vantages are associated w i th the total body conduct i v i tytechnique. I n addi ti on, cost ($70 000) may be a l im i t i ngf actor f or TOBEC.Com pu te rized tom ography

Computeri zed tomography (CT ) i s a modern radio-graphic method to determ ine regional body composi ti on.This approach relates smal l di f f erences i n x-ray attenua-t i on to di f f erences in the physi cal densi ty of ti ssues toconstruct a tw o-dimensional image of the underl y i nganatomy in the scan area.

The CT system consists of a col l imated x-ray sourceand detectors al igned at opposi te poles of a ci rcular gantry .L y ing on a movable platf orm , the subject i s advancedthrough the central aperture of the gantry . T he f ield ofv i sion, or the designated area to be scanned, i s a planethrough the m iddle of thi s aperture and paral lel to thegantry . A s the x -ray beam i s rotated around the subject,i nf ormati on about the intensi ty of the attenuated x-raybeams is recorded and stored. The scanner computer thanappl i es complex algori thms to the stored ser i es of prof i lesto reconstruct cross-secti onal images.

Each CT scan image or reconstructi on is a matri x ofpixels or pi cture elements, each about 1 mm X 1 mm,arranged in row s and col umns. B ecause the depth of thescan or sl i ce thickness is know n (i t can vary f rom as l i ttl eas 1 mm to typi cal l y 10-1 3 mm), thi s i s ref erred to as avoxel or vol ume element. For each indiv idual volume ofti ssue, the CT scanner measures the x -ray attenuationw i thin that voxel independentl y of the remainder of thatti ssue. The reconstructed picture represents not the imageat the surf ace of a cut but rather an average representingthe f ul l thi ckness of the sl i ce.

The magni tude of the x -ray beam attenuation is re-f l ected in the degree of pixel shading and is scaled as theCT number in H ounsf ield U ni ts. T he gray scale show non a CT scan uses the same l inear attenuati on coef f i ci entsused for conventi onal radiographi c f i lm . For example,l ow er densi ti es appear black and higher densi ti es are w hi tew i th ai r and bone at the low and high ends of absorption,respecti vely . T hus, high image contrast i s observed be-tw een bone, adipose, and fat-f ree ti ssues.

General l y , the CT scanner requi res about 10 s to com -plete each sl i ce. The number of sl i ces depends upon thepurpose of the CT scan. The typical radiation dose for aCT scan is a peak of 1.5-3.0 rads (142).

D i f f erent approaches have been used w i th the CT scan-ner to anal yze body composi ti on (143). T he structure ofinterest can be traced di rectl y on the v iew ing console w i tha cursor. T he cross-secti onal area ofadipose, bone, muscle,or v isceral organ then can be determ ined for each imageusing sophisti cated sof tw are programs. B ecause sl i cethickness is know n, one can calculate the relati v e surf acearea or volume occupied by each organ or ti ssue in thereconstructed picture. T hese methods have been used toassess changes in muscl e and adipose ti ssue in malnutr i -ti on (144) and to descri be cross-sectional di f f erences i nabdom inal f at di str i buti on during aging (145, 146).

W hen no sharp boundari es betw een structures are ap-parent but the ti ssues di f f er markedly i n radiographicdensi ty , the pi xels in successi ve sl i ces are plotted as a hi s-togram separating the pixels i nto f at-f ree and adipose ti s-sues (147). B ecause the volume of each pixel i s know n,the volume of the adipose and fat-f ree ti ssue in each sl i cecan be determ ined f rom the number of pi xel s f orm ingeach sl ide and added for al l sl i ces perf ormed.A nother method uti l i zes ti ssue matri ces in which thei ndi v idual components are smal ler than one pixel . T hi sapproach has been helpf ul i n di agnosi ng organ tumors(147-149), fatty l i ver (1 50), and ti ssue i ron content (151).

T omographi c pixels deri ved f rom an area of adiposeti ssue represent adipocy tes (tr i gl y ceride and protein ma-tr i x ) and not just neutral f at. T hus, to determ ine f at massone must f i rst assume that a f i xed f ract ion of adipose ti ssuei s tr i gl y cer i de. A n al ternate approach i s to determ ine thevol ume ofadipose ti ssue by addi ng the appropriate num -ber of adi pose voxels and assum ing a constant densi ty f oradipose ti ssue (145, 146). N ei ther of these approacheshas been val idated against standard body-composi ti onmethods.A l though the potenti al of CT scanners for body -corn-posi ti on analysis i s great, practi cal constraints l im i t i tsgeneral use. B ecause of the exposure to ionizing radiation,routine w hole-body scans, mul ti ple scans in the same i n-div idual , and scans of pregnant w omen or chi ldren arenot encouraged. A lso, the cost and general avai labi l i ty ofmodern CT scanners prohibi t the routine use of thi s i n-strumentation f or only body-composi ti on assessment.Subcu ta neou s ad ipose tissue th ickness

I n addi ti on to sk i nf ol d thi ckness measurements, an es-t imate of body f atness using the subcutaneous adipose-t i ssue layer can be made by sof t-ti ssue roentgenograms,the ul trasoni c technique, and i nf rared interactance. Theseal ternati v e approaches w ere developed because investi -gators recognized some l im i tations i n sk inf old thi cknessmeasurements, i ncluding sk inf old compressibi l i ty w i th age(152) and the inabi l i ty to measure sk inf old thi ckness atsome si tes i n obese people (153).

Sof t ti ssue radiography i s more accurate than are sk in-f old measurements of subcutaneous adi pose-ti ssue thick -ness (1 54). H ow ever, thi s technique is cumbersome andonly can be used w i th a l im i ted number of relati vely saf esi tes because of the undesi rable radiati on exposure. A l so,

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BODY COM POSIT ION M ETHODS 54 9

ti ssue magni f i cation and subject posi ti oni ng must bemoni tored caref ul l y .

Ultrasound . This approach uses an instrument i n w hichelectr i cal energy i s converted in a probe to high-f requencyul trasoni c energy , w hich then i s transm i tted into the bodyin the form of short pul ses. A s these ul trasoni c w avesimpinge perpendicularl y upon the i nterf aces betw een ti s-sues which di f f er i n acousti cal properti es, part of the ul -trasonic energy is ref l ected to the receiver in the probeand is transformed to el ectr i cal energy . On an osci l l oscopescreen, thi s echo is v isual i zed as a vert i cal def lecti on ofthe hor i zontal time basel i ne.

Commercial ul trasound instruments prov ide images ofti ssue conf iguration (B -scan mode) or depth readings ofchanges of ti ssue densi ty (A -scan mode). A ssessments ofadipose ti ssue thi ckness are made w i th A -scan mode de-vices.

I ni ti al tri al s establ i shed the val i di ty of the ul trasoundmethod to estimate adipose ti ssue thi ckness in humans.Correlati on coef f i cients ofO .80 and greater have been re-ported betw een ul trasonic measurements and sk inf oldthi ckness assessed by the cal iper technique at the tr i cepsand sub scapu la (155-158) . A lso, ul trasonic data w eref ound to be correl ated highly w i th electr i cal conducti v i tymeasurements of subcutaneous abdom inal adipose ti ssue(1 56) , w i th needle puncture measurements of abdom inalf at (1 55), and w i th sof t ti ssue radiographs over the i l i acc r e s t ( 1 5 7 ) .

U sing ul trasonic and cal i per techniques, H aymes et al(1 59) measured subcutaneous adipose thi ckness in adul tsat the tr i ceps, subscapula, abdomen, and suprai l i ac si tes.Reproducibi l i ty of ul trasound measures (r = 0.87-0.98)w ere marginal l y low er relati ve to cal iper values (r = 0.98-0.99). Correlat ions betw een measures obtai ned by the di f -f erent methods general l y w ere higher among w omen thanmen, probably because of greater adipose depots at eachsi te. In a subsample of subjects f rom w hom sof t ti ssueradiographs were made, di f f erences i n adipose thicknessbetw een ul trasound and roentgenograms of 1 .9 2. 1 m mat the tr i ceps and 4.7 4.9 mm at the w aist w ere attr i butedto distinct di scontinui t i es corresponding to the f ascialmargi ns observed on the roentgenograms.

B orkan et al (160) evaluated ul trasound and sk inf oldmethods to predict body f atness determ ined by potassi um -40 counting in a group of 39 men. A l though measure-ments made w ith the tw o techniques at the same si te typ-i cal l y produced di f f erent mean estimates of adipose ti ssuethickness, the values w ere highl y correlated (r > 0.80)w i th each other, indicati ng sim i l ar rel ati ve rank ing by eachtechni que. Sk inf old thi cknesses w ere correlated morehighly w i th f at w eight than w ere ul trasound measurements(r = 0.5 1 vs 0.39). These data suggest that sk inf ol ds aremore ef f ect i ve than ul trasound in assessing body corn-posi ti on, parti cularl y w hen the large di f f erence in cost i sc o n s i d e r e d .

Recentl y , Fanel l i and K uczmarsk i (161) suggested thatul trasound was as good a predi ctor of body f at as thesk i nf ol d cal iper method. A dipose ti ssue thickness at sevenbody si tes w as measured w i th sk inf old cal ipers and ul tra-

sound in 124 men aged 1 3O y. Body f atness w as deter-mined by densi tometry to be 3.5-32.7% . On average,sl i ghtl y higher correlati on coef f i ci ents w ere observed w i thbody densi ty and sk inf olds than w i th ul trasound rnea-surements. For the cal i per method, the tr i ceps si te w asthe best single predi ctor of body densi ty (r = 0.75) w hi l ef or ul trasound the w aist w as the best predictor (r = 0.74).M ul tiple regressi on analysis show ed that the best predic-t i on equati on of body densi ty using sk infold values hadan r = 0.779 and SEE = 0.0083 g/cc and using ul trasoundmeasures had an r = 0.809 and SEE = 0.0078 g/cc. Thissuggests equal predi cti ve capaci ty of the techniques.

A l though these data suggest a reasonable val i di ty of theul trasound approach, certain l im i tati ons have restr i ctedi ts general use. T he appropr i ate signal f requency of theprobe had not been wel l def ined. The l i terature containsa range of 2.5-7.5 M H z w i th the best predi cti ve accuracyassociated w i th the hi ghest f requency (16 1). A nother di f -f i cul ty i s the need f or uni form and constant pressure ap-pl ied by the probe to the scan si te. Changes in pressureby probe appl i cati on can af fect the di stri buti on of adiposeti ssue and prej udice the ul trasonic determ i nation of adi -pose thickness. A lso, val idation tr ial s should be conductedw i th heterogenous sampl es containing l arge ranges in bodyfatness.

In frared in tera cta nce . I nf rared interactance, a newmethod proposed for the assessment of human bodycomposi ti on, i s based upon the pr inciples of l i ght absorp-ti on and ref l ecti on using near-i nf rared spectroscopy .W hen electromagneti c radiat ion str i k es a material , theenergy i s ref l ected, absorbed, or transm i tted dependingon the scattering and absorption properti es of the sample.Energy transm i tted into the sample is scattered and re-f l ected back out of the sample contains inf ormation aboutthe chem ical composi ti on of the sample. T hi s approachwas developed by Norri s (162, 163) to predict the starch,protein, oi l , and w ater content of grains and oi l seeds.

For est imations of human body composi ti on, a com -puteri zed spectrophotometer i s used w i th a single, rapidscanning monochromator and f iber opt i c probe. The in-strument i s operated in the transmi ttance mode and scansare made over m i drange wavelengths of 700-1 100 nm.The probe em i ts electromagneti c radiat ion f rom themonochromator to a selected si te on the body , col l ectsi nteracti v e energy , w hi ch is the combinat ion of ref lectedand scattered energy , and conducts i t to the detector. T hesignal penetrates the underl y ing ti ssue to a depth of 1 cmand composi ti on is assessed only at the exam ined si te.

Interactance (I ) data are calcul ated by the instrumentas the ratio of the energy received f rom a scan si te to theenergy received f rom a cal ibration standard, a 1 cm thickTef lon block . D ata are transf ormed to log (1/I ) to be sim -i l ar to absorption spectra plotted as log (l /T ) and areshow n to vary l i nearl y w i th concentration of a speci f i cabsorber in a m ix ture of other materi al s i n agri cul turalf ood stuf f s (164).

A nalyses of spectra used the rati o of tw o second deri v -ati ves of l og (1/I ) data measured at tw o di f f erent w ave-lengths. T hi s mathemati cal approach reduces the ef f ects

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5 5 0 L U K A S K Io f tem p eratu re and partic le s iz e , reso lv e s prob lem s o fov erlapp in g abso rp tio n bands , and cancels ou t ligh t-scat-tering e f f e c ts (16 4 ). From em pirical calcu lat ion s , w av e-len g ths o f 91 6 and 1026 nm w ere selec ted fo r u se in cal-cu lating the ratio s .

C onw ay et al (165 ) used th is app roach w ith 5 3 m alesand f em ale s w ho und erw en t d ete rm inatio ns o f T B W byd eu terium d ilu tio n and m easurem en t o f in f rared in te r-actance , sk in f o ld th ick ne sses , and u ltrasou nd at the tri-c ep s , b ic eps , su bscapu la, su prailiac , and th igh . T o ev aluateth e v alid ity o f th e in f rared techn iqu e to pred ic t b od y f at,data f rom 36 sub je cts w ere used to dev e lop a m ode l thatw as tes ted in th e o th er 1 7 sub je cts .

T h e pred ic tio n eq uatio n sh ow ed a good relatio nsh ipbe tw een the ratio o f th e second de riv ativ e in teractan ce sat 916 and 1026 nm and p ercen t b od y f at e st im ated b ydeu te rium space (r = 0 .9 1 , S EE = 3 .2% ) . In g en eral, th ein f rared in terac tan ce m etho d ov ere s tim ated body f atn ess .A m ong all sub je cts s ign if ican t co rrelation co e f f ic ien ts w eref ou nd be tw een p ercen t f at v alue s de riv ed b y in f rared in -te rac tance and deu terium d ilu t ion , sk in f o ld th ic k n esses ,and u ltrasou nd (0 .94 , 0 .90 , 0 .89 , re spect iv e ly ).

T h is m eth od m u st be con side red to b e in th e dev e l-o pm en tal s tage . Q uest ion s arise ab ou t the v alid ity o f e x -trap o latin g com po sition al d ata f rom a lim ited (1 cm ind ep th ) sub cu taneou s depo t to the w ho le b ody .

A m ajor draw back to the u ltrason ic an d in f rared in -te ractan ce app ro ach es is the d ep en dence upon reg io nalad ipo se d is tribu tion to p red ic t to tal b od y f at. A lth ou ghthe se approache s m ay b e u sef u l in h om og enou s sam p les ,the ir ab ili ty to gene raliz e to th e he te rog en ou s p opu latio ni s q u es t io n ab l e.Ma gn e tic r e son a n ce ima g ing

A m ethod wi t h great po ten tial f o r the saf e , n on inv asiv e ,d irec t asse ssm en t o f hum an bod y com po sitio n is m agn e ticreso nance im ag ing (M R I). T h is app roach is b ased on th ef ac t th at atom ic nu c le i, m ade up prin c ipally o f neu tron sand p ro to ns , can behav e lik e m agn ets. W hen an ex te rnalm agn e tic f ie ld is ap p lied acro ss a p art o f the body , eachnuc leu s or m agne tic m om en t at tem p ts to align w ith th eex te rnal m agn etic f ie ld . I f a rad io f req uency w av e is d i-rected in to body tissu es, som e nuc le i ab sorb energ y f romth e rad io w av e and change th eir orien tation in the m ag-n etic f ie ld . W hen th e rad io w av e is tu rn ed o f f , th e ac tiv atedn uc le i em it the rad io signal th at th ey abso rbed . T h isem itted s ig nal is u sed to d ev elo p an im ag e b y a co rn -pu ter (1 66 ).T he m ost f requ en tly s tud ied nu cleu s in b io lo gy is b y -drog en , H , and in p articu lar th e h y d ro gen atom s o f w aterm o lecu le s in ce lls an d tissu es . H y drog en is the m ostabundan t e lem en t in th e b od y w h en it is co ns id ered asn um ber o f atom s (or nu cle i f o r M R I purpo ses) rath e r thanas a p ercen tage o f b od y w eig h t. T he m ajority o f th eseh y d rog en atom s are pre sen t as p arts o f w ater m o lecu les .T he h y d rog en nu cleu s is th e m ost am enab le to M R I d e-tec tio n . N o t on ly is the n atural abundan ce o f H h ig h(99 .9 8% ), bu t th e sens itiv ity o f M RI to th is nu cleus , w h ich

is sim p ly a pro ton , is greate r th an an y o ther atom ic nu-c l e us .

W hereas con v en tion al x -ray rad iog raph ic an d co rn -p u ted tom og raphy im ag es depend on ele c tron d en s ity ,M R I depend s o n the d en s ity o f hy dro gen nu cle i an d th ep hy sical s tate o f th e tissu e as ref le c ted in th e m agne ticrelax ation tim es. A natom ical in f orm ation has been oh -tam ed by com paring M M im ages and co rresp ond in g f ro -z en cro ss sec tio ns o f no rm al an im als (1 67 ). T issu e con trastis h igh b etw een f at an d m u scle and can b e en hanced bych ang in g the m agne tic re lax atio n tim e v ariab le o f them agn etic reso nance in strum en t. A pp licatio n o f M M tod if f e ren tiate m alig nan t f rom ben ign pro ce sses h av e in -d icated d if f e ren ce s in re lax ation tim es b e tw een no rm aland can ce rou s tissue s in rats (1 68 ) an d hum an s (16 9 , 1 70 ).A lthough ex ac t in te rpretation o f th ese ob serv ation s is u n-c lear (1 70 ), th e data app ear to in d icate a corre lat ion w ithdegree o f hy dratio n o f tissue (1 7 1 , 172).

T h ese f ind ing s hav e s tim u lated o the r inv es tig ators toestim ate reg io nal and to tal bod y w ate r u sing M M . H ay ese t al (1 73 ) u sed co nv en tion al M R I to q uan titate the w ate rd is trib u tio n o f saline -f illed and norm al rat lu ng s in iso latedand in s itu preparation s . S tud ies in iso lated lu ng f ragm en tssh ow ed an accu racy o f - 1% , and im ag es o f phan tom shad an erro r o f < 3%.

R ecen tly L ew is et al (17 4 ) used pro ton M R I to de ter-m in e to tal b od y w ate r in b aboons . T he hy dro gen asso -c iated w ith w ate r w as m easured as th e am p litu de o f th ef ree -in du ction d ecay v o ltage . B ody w ate r calcu lated b ym ultip ly ing p eak am p litu de by the ex pe rim en tally d ete r-m ined co ns tan t f o r a w ate r s tan dard w as s im ilar to thatd eterm ined grav im e trical ly in th e sam e baboons .

In con trast to im ages p roduced by x -ray rad iog raphyand com pu ted tom ograph y , M R I doe s n o t u se io n iz ingrad iation . It has th e capab ility to gene rate im ages in re -sp on se to in trin s ic tissu e v ariab le s and to repre sen t g rossch em ical characte ris tic s , su ch as le v e l o f hy dratio n andf at co n ten t. In add itio n to hy drog en , M R I can im agephosphoro us and f u ture pro spects in clude carbon , n itro -gen , so d ium , and ch lorin e (1 75 ). It has th e po ten tial toqu an tiate to tal fat m ass an d to d isc rim inate d if f erence sin reg ion al f at d is tribu tio n (17 6).

T h e o p tim ism o f f u tu re app licatio ns o f M R I f or b