DXA: Can It Be Used as a Criterion Reference for Body Fat Measurements in Children?

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1 nture publishing group rticles methods AND techniques DXA: Cn It Be Used s Criterion Reference for Body Ft Mesurements in Children? Romn J. Shypilo 1, Nncy F. Butte 1 nd Kenneth J. Ellis 1 Objective: Dul-energy X-ry bsorptiometry (DXA) is often cited s criterion method for body composition mesurements. We hve previously shown tht new DXA softwre version (Hologic Discovery V12.1) will ffect whole-body bone minerl results for subjects weighing <40 kg. We wished to renlyze peditric whole-body scns in order to ssess the impct of the new softwre on peditric soft-tissue body composition estimtes. Methods nd Procedures: We renlyzed 1,384 peditric scns (for ges yers) using Hologic softwre V12.1, previously nlyzed using V11.2. Regression nlysis nd ANCOVA were used to compre body ft (totl body ft (TBF), percentge ft (%BF)), nd non-bone len body mss (LBM) for the two versions, djusting for gender, ge nd weight. Results: Softwre V12.1 yielded vlues tht were higher for TBF, lower for LBM, nd unchnged for DXA-derived weight in subjects weighing <40 kg. Body composition vlues for younger, smller subjects were most ffected, nd girls were more ffected thn boys. Using the new softwre, 14% of the girls nd 10% of the boys were reclssified from the norml %BF rnge to t risk of obesity, while 7 nd 5%, respectively, were reclssified s obese. Discussion: Hologic s newest DXA softwre hs significnt effect on soft-tissue results for children weighing <40 kg. The effect is greter for girls thn boys. Comprison of TBF estimtes with previous studies tht use older DXA instruments nd softwre should be done with cution. DXA hs not yet chieved sufficient relibility to be considered gold stndrd for body composition ssessment in peditric studies. Obesity (2008) 16, doi: /oby Introduction Rising helth cre costs in the Unites Sttes in recent yers re prtly ttributble to n incresed prevlence of obesity (1). Increses in overweight nd obese sttus re evident in children nd dults, with little or no distinction between gender, ethnicity, or socil sttus (2 4). Recent epidemiologicl dt indicte threefold increse in the number of overweight children ged 6 19 yers from 1980 to 2002 (5 7). Overweight nd obesity tend to persist from childhood to dulthood (8 10), hence it is dvntgeous to direct more ttention to the prevention nd detection of peditric obesity (1,11). As there re no ccepted rnges of body ft defining obesity in children (12,13), indirect nthropometric indictors, such s BMI nd skinfold mesurements, hve been dopted for this purpose (14 17). Low cost nd portbility, mong other fctors, support the ppliction of these indices to epidemiologicl studies, where mesurement errors nd model ssumptions re offset by lrge smple sizes. Indeed, given the sensitivity nd specificity of these indices, their utility my be limited to initil screening of weight sttus, to be followed by more sophisticted methods of overweight or obesity ssessment (15,18). In order to better evlute nd detect childhood obesity, investigtors nd clinicins my need to employ some of the more ccurte methodologies vilble tody. These include ir displcement plethysmogrphy, dul-energy X-ry bsorptiometry (DXA), nd possibly bioelectricl impednce nlysis. DXA is incresingly being used s criterion method for body composition ssessment in peditric popultions (19 22). This methodology hs chieved reference sttus for bone minerl ssessment, nd similr use for soft tissue ssessment of ft mss nd percentge body ft hs been implied, lthough not fully tested. This hs occurred despite substntil chnges in hrdwre nd softwre tht hve produced somewht vrible DXA frme of reference (23 26). The most recent significnt modifiction, involving fnbem DXA instruments mnufctured by Hologic (Wlthm, MA), uses body weight-bsed djustment for bone minerl vlues for subjects weighing <40 kg. We hve shown tht this chnge in softwre ltered bone minerl results for most children below 14 yers of ge, nd for older children with certin diseses (23). In this pper we hve exmined the effects of this softwre chnge on the estimtes of soft tissue composition in children, with focus on body ft nd percentge ft. 1 USDA/ARS Children s Nutrition Reserch Center, Deprtment of Peditrics, Bylor College of Medicine, Houston, Texs, USA. Correspondence: Romn J. Shypilo (shypilo@bcm.edu) Received 10 April 2007; ccepted 10 July doi: /oby obesity VOLUME 16 NUMBER 2 FEBRUARY

2 rticles Methods nd Techniques We investigted how these new results ffected the interprettion of body ft levels with respect to ge, body weight, nd gender. Methods nd Procedures A dtset, consisting of 609 boys nd 775 girls who hd prticipted in severl reserch protocols in our center, hd been obtined using Hologic QDR-4500A nd Delphi-A DXA instruments, nd originlly nlyzed using softwre versions 8.25 nd 11.2, respectively. All of the studies were pproved by the Institutionl Review Bord for Humn Reserch for Bylor College of Medicine nd Affilited Hospitls. Body weights rnged from 8.3 to 48.0 kg; height from 67.7 to cm; nd ge from 1.7 to 17.2 yers. Correspondence with Hologic (T. Kelly, personl communiction) hd indicted tht bone results for subjects with DXA-derived body weight (WT DXA ) >40 kg were unffected by the new softwre. To llow us to confirm this weight-bsed threshold, 224 subjects in our dtset hd WT DXA between 40 nd 45 kg. For comprison, the mximum scle weight in our smple ws 48.0 kg. Of the 1,384 study subjects, 1,160 hd WT DXA below 40 kg (502 boys, 658 girls). Whole-body scns were renlyzed using worksttion running Windows XP with Hologic Discovery QDR softwre version 12.1:3. All scns cquired on the QDR-4500A (version 8.26) hd lredy been renlyzed using Delphi-A softwre version In order to ensure tht the body imge ws segmented in exctly the sme wy for ech nlysis version, the originl 10 regions of interest for ech whole body scn were preserved. All sttisticl nlyses were performed using XLSTAT Version (Addinsoft, New York, NY), nd were pplied to the full dtset (n = 1,384), s well s to the subset of subjects with WT DXA of <40 kg (n = 1,160). Regression nlysis ws used to compre originl nd renlyzed results for totl body ft (TBF) mss, non-bone len body mss (LBM), nd WT DXA. Anlysis of covrince (ANCOVA) ws used to quntify the effects of ge, gender, nd WT DXA on the nlysis results. The effects of WT DXA nd ge on chnges in TBF nd %BF estimtes were evluted for boys nd girls seprtely. The chnges in TBF nd %BF were clculted by subtrcting the originl results from renlyzed results (V12.1 V11.2). For ll sttisticl comprisons, P vlue of <0.05 ws considered significnt. The reltionship between %BF nd BMI ws lso exmined for ech gender. BMI is the most commonly used nthropometric index (15,16) for levels of body ft. We wished to determine the effect of the new nlysis on the threshold levels of risk for overweight nd obesity ssocited with BMI. Since BMI chnges with ge s children mture, BMI percentiles nd z-scores bsed on ge re used to evlute child s BMI within norml popultion. We clculted BMI (using scle weight) nd gender-bsed BMI-for-ge z-scores for ll of the subjects, using the Ntionl Helth nd Nutrition Exmintion Survey dtset mde vilble by the US Centers for Disese Control nd Prevention (27). We used regression nlysis to compre BMI z-scores with %BF for both DXA nlysis versions. We lso compred the distribution of %BF vlues within ech gender group for both nlyses in order to explore the rmifictions of chnging %BF results on the clssifiction of obesity in peditric popultion. Results The DXA-derived estimtes for body weight for ll subjects were unltered by the new nlysis softwre (R 2 = 1.00, men difference = 0.12 g, SEE = g). WT DXA nd scle weight were in close greement (R 2 = 0.995). Regression of WT DXA ginst scle weight resulted in slope of 1.01 nd n intercept not significntly different from zero (P < 0.05). Thus, WT DXA ws used throughout to represent body weight, unless otherwise noted. The DXA-bsed body composition vlues for subjects with WT DXA >40 kg were lso unltered. This confirmed our finding in previous study of bone minerl (23), which showed tht only results for subjects with WT DXA <40 kg were ffected by the new softwre. Therefore, subsequent nlyses presented here, exploring effects of gender, ge, nd body weight, were performed using only the 1,160 subjects hving WT DXA below 40 kg. The bsic nthropometric chrcteristics of these subjects re presented in Tble 1. TBF nd LBM estimted by Hologic softwre versions 11.2 nd 12.1 differed significntly in these subjects (P < ). Differences in TBF vlues were greter in the smller children, nd the differences diminished with incresing TBF (Figure 1). Discrepncies between LBM estimtes followed similr but inverse pttern to TBF, with lrger differences t lower LBM vlues. For both TBF nd LBM, regression lines were significntly different from the line of identity (i.e., 95% confidence intervls for slope did not overlp unity nd intercepts were significntly different from zero). Tble 1 Descriptive chrcteristics of peditric popultion (n = 658) (n = 502) Men (±s.d.) Rnge Men (±s.d.) Rnge Age (yers) 7.8 ± ± Weight (kg) 26.4 ± ± Height (cm) ± ± BMI (kg/m 2 ) 17.1 ± ± BMI z-score 0.3 ± to ± to 4.5 Children with dul-energy X-ry bsorptiometry derived weight <40 kg. Scle weights. Ft (DXA V12.1) kg b Ft (DXA V12.1) kg FAT V12.1 = FAT V :1 line 1:1 line FAT V12.1 = FAT V Ft (DXA V11.2) kg Figure 1 Comprison of versions 11.2 nd 12.1 softwre on estimtes of body ft mss for girls (: R 2 = 0.991, P < ) nd boys (b: R 2 = 0.993, P < ) with WT DXA <40 kg. WT DXA, dul-energy X-ry bsorptiometry derived body weight. 458 VOLUME 16 NUMBER 2 FEBRUARY

3 rticles Methods nd Techniques Differences between the two softwre versions in TBF nd %BF for different WT DXA rnges re presented for boys nd girls in Tble 2. The differences were significnt (P 0.001) for ech of the weight rnges listed in the Tble, for both genders. The mgnitude of the differences between softwre versions stedily decresed with incresing body weight, with the results for the two softwre versions in greement by ~40 kg body weight. At the lowest body weight rnge, differences in TBF for girls nd boys were 0.81 ± 0.53 nd 0.66 ± 0.39 kg, respectively. For these sme groups, %BF incresed by 7.2 ± 4.5% for girls nd 6.5 ± 4.0% for boys. Including WT DXA s covrite, ANCOVA indicted tht the differences in TBF nd %BF differed significntly between boys nd girls (P < ). Gender still showed significnt effect when nlyzing the differences in TBF nd %BF using WT DXA nd initil body ft (V11.2 TBF nd %BF, respectively) s covrites (P < 0.007). The results of the renlysis bsed on ge groups re presented in Tble 3 for TBF nd Tble 4 for %BF. Similr effects s those obtined using body weight were observed s function of ge, since younger subjects tend to be smller. The differences in TBF nd %BF diminished with incresing ge, with no effect evident in the yers ge groups. For the 1 3 yers groups, the increse in TBF in girls, due to the renlysis, ws bout double tht observed for boys, which lso contributed to higher %BF increses for girls. Estimtes of %BF nd TBF with renlysis were significntly different between boys nd girls (ANCOVA with ge s covrite, P < ). After djusting for initil body ft levels (V11.2 %BF nd TBF) nd ge in the models, gender effects on the differences in body ft vlues were still significnt (P < 0.007). The effects of the renlysis on the reltionships between BMI z-scores nd %BF re shown in Figure 2 for girls nd Figure 3 for boys. An overll positive trend in %BF ws observed s function of n incresing BMI z-score (djusted R 2 = 0.55 nd 0.44 for girls nd boys, respectively). On verge, the newer softwre (version 12.1) produced systemtic increse in %BF of ~1.5% for ll BMI z-scores. Using ANCOVA to test the effect of the nlysis version on %BF, nd controlling for BMI z-score, the nlysis version hd significnt effect on %BF vlues for boys nd girls (P < ). The impct of renlysis on the %BF distributions within ech gender is illustrted in Tble 5. Using the %BF limits suggested for peditric popultions (13,28,29), ~59% of the girls nd 46% of the boys hd norml body ft levels ccording to Tble 2 Differences between two softwre versions in totl body ft nd percentge ft for boys nd girls ctegorized by body weight rnges WT DXA rnge (kg) n Δ Ft (kg) Δ% Ft P b P b ± ± ± ± ± ± ± ± ± ± 0.27 < ± ± 1.5 < ± ± ± ± ± ± 0.18 < ± ± 0.7 < ± ± ± ± ± ± ± ± ± ± ± ± Hologic softwre version 12.1 vs WT DXA, dul-energy X-ry bsorptiometry derived body weight. Anlysis versions significntly different (P 0.001) for ll weight bins. b P vlue for comprison between genders t the sme weight rnge. Tble 3 Effect of nlysis softwre on totl body ft (TBF) in boys nd girls by ge group Age rnge (yers) n TBF (kg) TBF (kg) V11.2 V12.1 Δ TBF (kg) V11.2 V12.1 Δ TBF (kg) P vlue ± ± 0.8 b 1.00 ± ± ± 0.5 b 0.45 ± ± ± 2.5 b 0.72 ± ± ± 2.4 b 0.59 ± ± ± 3.0 b 0.60 ± ± ± 2.9 b 0.41 ± 0.20 < ± ± 3.3 b 0.40 ± ± ± 3.0 b 0.27 ± 0.15 < ± ± 2.3 b 0.28 ± ± ± 2.3 b 0.18 ± 0.13 < ± ± 2.2 b 0.14 ± ± ± 2.3 b 0.08 ± ± ± l.5 c 0.10 ± ± ± 2.1 d 0.04 ± Children with dul-energy X-ry bsorptiometry derived body weight <40 kg. P vlue for difference in TBF between boys nd girls. b Anlysis versions significntly different (P < ). c Anlysis versions significntly different (P = 0.001). d No difference between nlysis versions (P = 0.113). obesity VOLUME 16 NUMBER 2 FEBRUARY

4 rticles Methods nd Techniques Tble 4 Effect of nlysis softwre on ft percentge (%BF) in boys nd girls by ge group Age rnge (yers) n %BF %BF V11.2 V12.1 Δ %BF V11.2 V12.1 Δ %BF ± ± 7.5 b 8.1 ± ± ± 3.4 b 3.5 ± ± ± 6.0 b 4.1 ± ± ± 6.1 b 3.4 ± P vlue ± ± 6.6 b 2.9 ± ± ± 6.8 b 1.8 ± 1.0 < ± ± 6.4 b 1.5 ± ± ± 6.9 b 1.0 ± 0.6 < ± ± 5.5 b 1.0 ± ± ± 5.2 b 0.6 ± 0.4 < ± ± 4.9 b 0.4 ± ± ± 5.2 b 0.2 ± ± ± 3.6 c 0.3 ± ± ± 5.0 d 0.1 ± Children with dul-energy X-ry bsorptiometry derived body weight <40 kg. P vlue for difference in %BF between boys nd girls. b Anlysis versions significntly different (P < ). c Anlysis versions significntly different (P = 0.004). d No difference between nlysis versions (P = 0.099). Percent ft 60% 50% 40% 30% 20% 10% Vers 12.1 Vers % BMI z-score Figure 2 Effect of version 12.1 softwre on ft percentge s function of BMI z-score in girls (WT DXA < 40 kg). WT DXA, dul-energy X-ry bsorptiometry derived body weight. Percent ft 50% 40% 30% 20% 10% Figure 3 Effect of version 12.1 softwre on ft percentge s function of BMI z-score in boys (WT DXA < 40 kg). WT DXA, dul-energy X-ry bsorptiometry derived body weight. 0% BMI z-score Vers 12.1 Vers 11.2 the originl softwre (version 11.2). Thirty-three percent of the girls were t risk for obesity, nd nine percent of the girls were clssified s obese. Forty percent of the boys were t risk nd fourteen percent were obese. After renlysis, 45% of the Tble 5 Popultion distribution mong obesity clssifictions bsed on %BF using two different DXA softwre nlysis versions Obesity clssifiction %BF V11.2 V12.1 %BF V11.2 V12.1 Norml < % (385) At risk % (217) Obese >36 b 8.5% (56) 45.0% (296) 40.4% (266) 14.6% (96) <21 c 45.6% (229) % (201) >30 d 14.3% (72) 35.7% (179) 45.2% (227) 19.1% (96) Vlues in prenthesis indicte number in ech group. %BF, percentge ft; DXA, dul-energy X-ry bsorptiometry. At risk of obesity threshold (girls). b Obesity threshold (girls). c At risk of obesity threshold (boys). d Obesity threshold (boys). girls nd 36% of the boys remined within the norml rnge. Forty percent of the girls were t risk nd fifteen percent were obese. Forty-five percent of the boys were t risk nd nineteen percent were obese. Seven percent of the girls hd moved into the obese ctegory, nd fourteen percent of the girls were no longer considered to be in the norml ctegory. Similrly, n dditionl 5% of the boys were ctegorized s obese, nd 10% moved out of the norml rnge. Discussion DXA is incresingly being used s criterion or reference for comprison with other body composition mesurement techniques (19 22), nd hs been considered by some to be gold stndrd for body composition studies (17). Our focus here hs been on the DXA-bsed estimtes for TBF becuse of worldwide concerns of n obesity epidemic in peditric popultions. We found tht the newest DXA softwre ffects body composition results for children with WT DXA of <40 kg, nd tht girls re ffected significntly more thn boys. would hve incresed TBF nd %BF vlues t younger ges, ccording to the newest nlysis lgorithms. lso would hve higher ft vlues, but to lesser degree. Thus, compred with DXA vlues previously reported in the literture especilly 460 VOLUME 16 NUMBER 2 FEBRUARY

5 rticles Methods nd Techniques in younger children discrepncy might be indvertently introduced when compring TBF or %BF simply on ccount of the selection of prticulr nlysis softwre. These differences my not necessrily represent true increse in TBF. In ddition to its incresing role s gold stndrd, DXA my potentilly be used to mesure %BF for the purpose of ssessing obesity in clinicl setting. Presently, BMI is the most commonly used obesity indictor (15,30), though it is most effective in epidemiologicl studies (15). Nevertheless, the use of BMI in clinicl ssessments is on the increse (31,32), even though its ccurcy for the individul hs been questioned (33). The use of BMI to estimte obesity my lso be misleding in children with diseses (34). Anlogous to the wy BMI percentile nd z-score thresholds re utilized, %BF levels cn lso be used to define normlity, overweight, nd obesity (13,28,29). To evlute the effect of the new nlysis on clssifictions of obesity, %BF levels indicting norml, t risk for obesity, nd obesity were defined. For boys we used n upper limit of 21% for norml %BF, nd vlues >30% to define obesity. The corresponding threshold vlues for girls were 27 nd 36%, respectively (13). These vlues were the mens of %BF rnges determined by Tylor et l. (13), which correspond to the BMI cutoffs clssifying children s overweight nd obese. These vlues lso spnned the discrete %BF vlues published by Willims et l. (35), who proposed tht %BF vlues of 25 nd 30% for boys nd girls, respectively, were pproprite for identifying excess levels of ft. As shown in Tble 5, the chnge in nlysis softwre lone cused substntil upwrd shift in the percentge of children tht would be clssified s t-risk or obese. This outcome would hve been similr if one were to choose the discrete limits proposed by Willims et l. (35). For exmple, if 25 nd 30% ft levels (for boys nd girls) were used s indictors of obesity, t lest 10% of the children, regrdless of gender, would be relbeled s being obese fter ppliction of the new softwre. This effect is evident in Figures 2 nd 3. Renlysis using version 12.1 shows systemtic increse in %BF cross ll BMI z-scores. Points t ny given BMI z-score cn represent children of ny given ge nd weight, thus the renlysis effect is consistent throughout the z-score rnge. BMI z-score thresholds currently in use would be ssocited with higher %BF vlues when using renlyzed DXA dt. Alterntively, to mintin consistency between %BF limits nd BMI, lower BMI would need to be used to define n obesity threshold. If %BF is to be used s reference or n obesity index, then DXA-bsed methods for its clcultion need to be consistent. A chnge in mesurement methodology one tht my include different nlysis lgorithm resulting in significnt chnges in the %BF estimtes, would not be desirble without independent evidence to verify the error with the originl vlues. Any technique, such s DXA, tht reports on bsolute mesures of body composition needs to mintin minimum level of consistency. Until tht consistency is chieved, such methods should not be considered s criterion or reference mesurements. In summry, the newest softwre version of Hologic s DXA instrument will hve significnt effect on body composition estimtes obtined for children weighing <40 kg. This represents most helthy children up to bout the ge of 14. Vlues for both TBF nd %BF re ffected, with estimted body ft levels incresing more substntilly in smller, younger children. This could lso become concern when exmining children with diseses, s these popultions re frequently smller thn their ge-mtched peers. The discrepncies in ft estimtes re significntly greter in girls thn in boys when using the newest softwre. The incresed %BF vlues would lso result in more children being clssified s t-risk or obese. The ccurcy of these effects is unknown. We re lso concerned tht when investigtors compre TBF or %BF for their studies (using the newest DXA softwre) with older studies published in the literture, n unfounded conclusion regrding incresing peditric obesity could be derived if the effects due to different softwre versions were not considered. We recommend tht body ft results from DXA should be crefully interpreted. In ll studies using DXA, investigtors should be required to clerly stte the softwre version tht generted their results, long with DXA instrument informtion. Comprison of DXA-derived body composition findings cross peditric studies cn be limited without this informtion. It is our opinion tht the vrible nture of the still- evolving DXA technology implies tht it cnnot yet be considered criterion method for body composition studies in peditric popultions. As we cnnot t this point determine which softwre version is more ccurte for the ssessment of body ft, further evlutions re needed to more thoroughly investigte the reltionship of this newest DXA nlysis version with other independent body composition mesurement techniques. Acknowledgments We re grteful to J. Prtt nd M. Lurent for performing the DXA mesurements nd ssisting with renlysis of whole body scns. We thnk Tom Kelly of Hologic for his technicl ssistnce. This work is publiction of the USDA/ARS Children s Nutrition Reserch Center, Deprtment of Peditrics, Bylor College of Medicine, nd Texs Children s Hospitl, Houston, TX. Funding hs been provided from the USDA/ARS under Coopertive Agreement No The contents of this publiction do not necessrily reflect the views or policies of the USDA, nor does mention of trde nmes, commercil products, or orgniztions imply endorsement by the US Government. Disclosure The uthors declred no conflict of interest The Obesity Society References 1. Thorpe KE. Fctors ccounting for the rise in helth-cre spending in the United Sttes: the role of rising disese prevlence nd tretment intensity. Public Helth 2006;120: Troino RP, Flegl KM, Kuczmrski RJ, Cmpbell SM, Johnson CL. Overweight prevlence nd trends for children nd dolescents. The Ntionl Helth nd Nutrition Exmintion Surveys, 1963 to Arch Peditr Adolesc Med 1995;149: Gortmker SL, Dietz WH, Sobol AM, Wehler CA. Incresing peditric obesity in the United Sttes. Am J Dis Child 1987;141: Mei Z, Scnlon KS, Grummer-Strwn LM et l. 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