PEDIATRIC HIGHLIGHT. Body fat reference curves for children. HD McCarthy 1, TJ Cole 2, T Fry 3, SA Jebb 4 and AM Prentice 5.

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1 (06), & 06 Nture Publishing Group All rights reserved 07-06/06 $.00 PEDIATRIC HIGHLIGHT Body ft reference curves for children HD McCrthy 1, TJ Cole 2, T Fry 3, SA Jebb 4 nd AM Prentice 1 Institute for Helth Reserch & Policy, London Metropolitn University, London, UK; 2 Centre for Peditric Epidemiology nd Biosttistics, Institute of Child Helth, London, UK; 3 Child Growth Foundtion, 2 Myfield Avenue, London, UK; 4 MRC Humn Nutrition Reserch, Elsie Widdowson Lbortory, Cmbridge, UK nd MRC Interntionl Nutrition Group, London School of Hygiene nd Tropicl Medicine, London, UK Objective: To refine the dignosis of childhood obesity by creting new sex-specific centile curves for body ft nd to bse these references on simple nd ffordble method tht could be widely dopted in clinicl prctice nd surveys. Design: Body ft ws mesured by bio-impednce in 198 Cucsin children ged 18 yers from schools in Southern Englnd. Smoothed centile chrts were derived using the LMS method. Results: The new body ft curves reflect the known differences in the development of diposity between boys nd girls. The curves re similr by sex until puberty but then diverge mrkedly, with mles proportiontely decresing body ft nd femles continuing to gin. These sex differences re not reveled by existing curves bsed on body mss index. We present chrts in which cutoffs to define regions of underft, norml, overft nd obese re set t the 2nd, 8th nd 9th centiles. These hve been designed to yield similr proportions of overweight/overft nd obese children to the IOTF body mss index cutoffs. Conclusions: Direct ssessment of diposity, the component of overweight tht leds to pthology, represents significnt dvnce over body mss index. Our new chrts will be published by the Child Growth Foundtion for clinicl monitoring of body ft, long with the softwre to convert individul mesurements to Z-scores. (06), doi:.38/sj.ijo ; published online 14 Februry 06 Keywords: children; centiles; body ftness; bioelectricl impednce; Cucsin Introduction The obesity epidemic, t one time confined to dults, hs now penetrted the peditric ge rnge nd shows every sign of rpid escltion. 1,2 This hs led to clls for better ssessment tools both for longitudinl nd cross-sectionl surveillnce of popultions, nd for clinicl mngement of individuls. 1,3 Body mss index (weight (kg)/height 2 (m)) is widely used to ssess overweight nd obesity, nd stndrd cutoff vlues re now widely ccepted for dults. In children BMI chnges considerbly during growth nd development. This necessittes the use of centile curves with vrible cutoff vlues for different ges, for exmple, the British 1990 reference 4 published by the Child Growth Foundtion, the US Centers for Disese Control (CDC) 00 chrts, nd the curves Correspondence: Dr D McCrthy, Institute of Helth Reserch & Policy, London Metropolitn University, Hollowy Rod, London N7 8DB, UK. E-mil: d.mccrthy@londonmet.c.uk Received 21 April 0; revised 27 September 0; ccepted 14 December 0; published online 14 Februry 06 currently dopted by the Interntionl Obesity Tsk Force (IOTF). 6 Although body mss index is simple to mesure nd hs been vluble tool in monitoring trends in obesity, it lso hs numerous disdvntges. 7 Principlly, it does not distinguish between incresed mss in the form of ft, len tissue or bone, nd hence cn led to significnt misclssifiction. Since the pthology ssocited with obesity is driven by the excess ft mss 8 the idel monitoring tool should directly ssess diposity. Mny tools re vilble to do this but re complex, time consuming nd expensive. Considerble reserch hs gone into developing bio-impednce monitors tht cn distinguish between len nd ft tissue on the bsis of their differentil conductnce nd impednce chrcteristics. 9 These techniques re slightly less ccurte thn the more sophisticted reserch tools, but offer n importnt prcticl dvntge in being simple nd chep to use. We used the totl body ft results from bioimpednce segmentl body composition nlyser to develop reference centile curves in 198 Cucsin children ged 18 yers in the UK. These curves my be used to ssess children s diposity in both clinicl nd survey settings.

2 Subjects nd methods Subjects The study popultion consisted of n opportunistic smple of school children, recruited into the study following initil contct with the school/college. A totl of 21 schools nd colleges locted in Hertfordshire, Cmbridgeshire nd West London greed to prticipte in the survey. Prents/crers were sent letter explining the ims of the study nd requesting permission for their child to tke prt. Only children for whom signed prentl consent ws obtined were mesured in this study. No informtion bout current mediction use or whether the child ws following ny weight mngement diet ws collected. The smple is not therefore bised by such exclusions. Dt on dte of birth, gender nd ethnicity were collected together with nthropometry. Children were individully coded nd the dt nonymised. The nlysis ws restricted to 198 Cucsin children (1116 boys nd 869 girls) ged between.0 nd 18. yers. Anthropometric nd body ft mesurements Mesurements were conducted on school premises by two field workers. Height ws mesured to the nerest 0.1 cm with portble stdiometer (Sec, Mrsden, UK) with children stnding in bre feet. Body mss nd totl body ftness were mesured using the Tnit BC-418MA Segmentl Body Composition Anlyser (Tnit Corportion, Tokyo, Jpn) with correction for light indoor clothing. The mesurement procedure required the subject to stnd in bre feet on the nlyser nd to hold pir of hndgrips, one in ech hnd. The bio-impednce component of the mesurements took pproximtely s per subject. Body mss index ws clculted s weight (kg)/height 2 (m). Although the body ft monitor used for this study provides seprte mesures of ft in the trunk nd limbs, only the whole-body percentge ft ws used to construct the centiles. The prediction equtions used in this model re bsed on bio-impednce, weight, height nd ge nd were derived from clibrtion studies ginst whole-body dul X- ry bsorptiometry (DXA). The s.e. of the estimte for boys ws 2.7% nd for girls ws 2.8% body ft (dt provided by mnufcturer). The impednce scles used in this study hs been vlidted ginst DXA in mixed popultions of children nd dults nd found to be superior to previous BIA methods. More recently peditric vlidtion of the BC-418MA model ginst DXA nd ir-displcement plethysmogrphy (BodPod) hs been performed. 11 In smples of 4 boys (ge yer) nd 34 girls (ge yer) results were highly correlted with DXA (r ¼ 0.91, SEE ¼ 4.46%) nd men vlues did not differ significntly. In the current study, the within-dy coefficient of vrition for percentge body ft ws 1.3%. Children s body ft curves Ethicl pprovl This study ws pproved by the London Metropolitn University Ethics Committee. Sttisticl nlysis nd centile curves Centile curves for body ft percentge were constructed for boys nd girls seprtely using the LMS method, which summrises the dt in terms of three smooth ge-specific curves, nmely L (lmbd), M (mu), nd S (sigm). The M nd S curves correspond to the medin nd coefficient of vrition of body ft percentge t ech ge wheres the L curve llows for the ge dependent skewness in the distribution of body ft percentge. For the construction of the percentile curves, dt were imported into the LMS softwre (version 1.) nd the L, M nd S curves estimted. Seven centile curves were clculted, from the 2nd to the 98th, spced two-thirds of n s.d. score prt, in the formt used for other British growth reference chrts. 12 We then selected the 2nd centile to define the upper limit of underft, nd the 8th nd 9th centiles to define the lower limits of overft nd obese. The rtionle behind these cutoffs is discussed below. 1 b Figure 1 Body ft centile curves for Cucsin boys nd girls. Dt from 1116 boys nd 869 girls ged 18 yers smoothed by the LMS method. Numbers on right-hnd side represent centiles. 99

3 600 Tble 1 Tbulted body ft % centile vlues by exct ge Children s body ft curves Yers The 2nd, 8th nd 9th centiles define the cutoffs for underft, overft nd obese. Centile Results Preliminry nlysis of the children in this study showed tht they were generlly similr in height nd BMI compred with the UK 1990 nd US CDC 00 references. Figure 1 illustrtes the full set of centile curves for the boys nd girls. The tbulted dt re listed in Tble 1. The boys show reltively flt 0th centile vrying between 1 nd 18% body ft over the entire ge rnge, with pek t ge 11 yer. Vribility increses up to ge 11 yer with mrked increse in positive skewness. Both skewness nd vribility fll fter ge 11 yer, but the lower centiles diverge slightly from the 0th centile. The girls centiles show similr pttern to the boys up to ge yer but re then strikingly different in shpe. The 0th centile continues to rise slightly while the other centiles diverge from the 0th centile. At ge 18 yer the girls hve proportiontely 60% more body ft thn the boys; the medin percent body ft vlues re 24.6 nd 1.4%, respectively. The 98th centiles re 37.2 nd.9%, nd the 2nd centiles 14.7 nd 9.6%. In order to define cliniclly nd epidemiologiclly useful cutoffs tht re brodly consistent with the body mss index cutoffs currently dopted by the Interntionl Obesity Tsk Force (IOTF) we pplied the IOTF cutoffs to the current dt set nd selected the nerest body ft centile cutoffs. The 8th nd 9th centile lines provided close pproximtion to the overweight nd obese boundries of the IOTF curves, while the 2nd centile ws chosen (firly rbitrrily) to form the underweight boundry. These centiles re illustrted in Figure 2 nd tbulted in the Tble 1. The new boundries define underft, norml, overft nd obese children. Discussion The shpes of the body ft curves produced by this study mtch the expected chnges in ft ptterning during humn growth. 13 Following n erly decrese in body ft during infncy (not seen in these dt since they strt only t ge of yers), body ft increses until puberty. At puberty sex hormones induce pronounced sexul dimorphism: mles gin proportiontely more muscle nd len tissue compred to ft, nd femles ly down ft s nturl prt of the ontogeny of their sexul nd reproductive physiology. Note tht these norml ntomicl differences re not reflected in

4 Children s body ft curves b 1 Obese Overft Norml Underft Obese Overft Norml Underft Body ft (%) b Body ft (%) Age (y) UK 0th US 0th UK 9th US 9th UK 0th US 0th UK 9th US 9th Figure 2 Recommended cutoffs for defining underft, norml, overft nd obese children. Dt s in Figure 1 Chrts pply to Cucsin children. the corresponding body mss index curves, which show remrkbly similr ptterns for boys nd girls. 4 Body ft curves hve recently been published from Project Hertbet in the United Sttes using n lterntive bioimpednce system (RJL Systems, model not stted). 14 These were derived from smller smple (278 boys, 263 girls) nd over nrrower ge rnge ( yer). The 0th centile curves from the two smples gree quite closely with mximum difference of 3.4% body ft in boys nd 3.% in girls (see Figure 3). However, the 8th nd 9th centiles from the US dt re much higher in the ge rnge 8 12 yers with mximum devition of 12.7% body ft boys (US boys 9th centile ¼ 39.7% nd UK boys 9th centile ¼ 27.0% t 9. yers). This my reflect imprecision in estimting the 9th centile from the smller US smple, but more likely reflects greter prevlence of obesity in the US. The fct tht body mss index represents only crude proxy for body ft nd my produce significnt level of misclssifiction is universlly ccepted but widely ignored. This is becuse, in the bsence of lterntive mesures, the dvntges of body mss index hve outweighed its disdvntges. 1 However, bio-impednce offers the opportunity to move beyond body mss index. 7 Its dvntges re Age (y) Figure 3 Comprison of current reference curves for body ft ginst dt from similr smple from the United Sttes. US dt from Mueller et l. 14. tht it is reltively inexpensive, portble, simple nd rpid to use. Its disdvntges re tht it is less ccurte thn more sophisticted methods. We propose the body ft centile curves presented here s n lterntive or ddition to using body mss index curves. The chief merit of the new curves is tht they ssess dipose tissue mss, the component of excess weight tht is ssocited with comorbidities. 8 They will lso reduce misclssifiction in lrge-frmed nd/or musculr children who re rted s overweight or obese by body mss index. Additionlly the new curves will help focus medicl ttention on excess diposity s distinct from overweight. To further emphsise this distinction we propose tht the four ctegories identified in the proposed clinicl cutoff chrts in Figure 2 should be termed underft, norml, overft nd obese. As with ll nthropometric reference curves it is necessry to use representtive smple of the wider popultion of interest. Geogrphicl, ethnic, socio-economic nd nutritionl considertions hve lwys been importnt in design-

5 602 ing the smpling frme. In the cse of body ft, timing hs lso become vitl considertion in the light of the rpid seculr increse in obesity. We, therefore, resoned tht it would be optiml to try to mtch our smple with the body mss index chrts, nd especilly the British 1990 growth reference smple. This would void the confusion tht would rise if, in future nlyses, the existing Child Growth Foundtion or IOTF body mss index chrts nd our new body ft chrts hd generted widely divergent estimtes of overweight/overft nd obese children. To this end, we intentionlly pproched schools in more ffluent res in the expecttion of finding obesity rtes lower thn the current ntionl verge 16 nd close to the 1990 smple. This strtegy ws successful. The children in our new smple were slightly tller ( þ 0.41 Z-scores) nd slightly hevier ( þ 0.14 Z-scores) thn British However, their BMI Z-score ws close to zero ( 0.13 Z-scores) with s.d. of 1.19 (i.e., close to the expected 1.00). Although we believe tht reference curves bsed on ctul body ft re n dvnce over other mesures, the definition of cutoffs shres common problem with ll previous chrts; nmely tht we lck clinicl correltes on which to bse such cutoffs. There is n urgent need for lrge-scle surveys tht could relte body ft in children (using curves such s those presented here) nd future risk fctors for obesity-relted ill helth. The body ft chrts will be published by the Child Growth Foundtion to dd to their series on weight, height, wist nd body mss index (Avilble from Hrlow Printing, Mxwell St, South Shields, NE33 4PU. Tel: þ E-mil: sles@hrlowprinting.co.uk.). They cn be used in clinicl setting to trck body ft within individuls in n exctly nlogous wy to current weight nd height chrts. Similrly the Child Growth Foundtion s nthropometry softwre to convert individul mesurements to Z-scores hs been extended to include percent body ft. It is importnt to emphsise tht these chrts were derived using the BC- 418MA Segmentl Body Composition Anlyser. The chrts should not be used in conjunction with other mkes nd models of bio-impednce monitors until cross-clibrtion studies hve been performed in children. Finlly, these curves should only be used for Cucsin children. We hope to develop dditionl sets for Afro-Cribben nd Asin children in the future. Acknowledgements Children s body ft curves The uthors thnk Srh Perrin, Dimple Smni nd Gerldine Murphy who conducted the fieldwork mesurements nd collted the dt. The uthors lso thnk ll the prticipting schools nd children. This study ws funded by grnt from Tnit UK to London Metropolitn University. Conflict of interest Dvid McCrthy hs received reserch funding from Tnit UK (this study). AMP nd SAJ hve received pst reserch funding from Tnit UK nd re members of the Tnit Medicl Advisory Bord. References 1 Lobstein T, Bur L, Uuy R. IASO interntionl obesity tsk force. Obesity in children nd young people: crisis in public helth. Obes Rev 04; (Suppl 1): Royl College of Physicins. Storing Up Problems: The Medicl Cse for Slimmer Ntion 04. Royl College of Physicins: London. 3 Pietrobelli A, Peroni DG, Fith MS. Peditric body composition in clinicl studies: which methods in which situtions? Act Dibetol 03; (Suppl 1): S270 S Cole TJ, Freemn JV, Preece MA. Body mss index reference curves for the UK, Arch Dis Child 199; 73: 29. Ntionl Center for Helth Sttistics. Peditric Growth Chrts, 00. Avilble t 6 Cole TJ, Bellizzi MC, Flegl KM, Dietz WH. Estblishing stndrd definition for child overweight nd obesity: interntionl survey. Br M J 00; 3: Prentice AM, Jebb SA. Beyond body mss index. Obes Rev 01; 2: Fortuno A, Rodriguez A, Gomez-Ambrosi J, Fruhbeck G, Diez J. Adipose tissue s n endocrine orgn: role of leptin nd diponectin in the pthogenesis of crdiovsculr diseses. J Physiol Biochem 03; 9: Chumle WC, Guo S. Bioelectricl impednce nd body composition: present sttus nd future directions. Nutr Rev 1994; 2: Pietrobelli A, Rubino F, St-Onge MP, Heymsfield SB. New bioimpednce nlysis system: improved phenotyping with whole-body nlysis. Eur J Clin Nutr 04; 8: Pietrobelli A, Rubino F, Wng J, Wng Z, Heymsfield SM. Vlidtion of contct electrode bioimpednce nlysis in peditric popultion. Eur Congress Obes (Athens) 0 (bstrct in press). 12 Cole TJ. Do growth chrt centiles need fce-lift? Br M J 1994; 8: Forbes GB. Body composition in dolescence. In: Flkner F nd Tnner JM (eds). Humn Growth: 2: Postntl Growth. Billiere Tindll: London, 1978, pp Mueller WH, Hrris RB, Lbther DR. Percentiles of body composition from bioelectricl impednce nd body mesurements in US dolescents 8 17 yers old: Project Hertbet!. Am J Humn Biol 04; 16: 1. 1 Ellis KJ, Abrms SA, Wong WW. Monitoring childhood obesity: ssessment of the weight/height(2) index. Am J Epidemiol 1999; : Lobstein TJ, Jmes WPT, Cole TJ. Incresing levels of excess weight mong children in Englnd. Int J Obes 03; 27: