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* Institute of Human Nutrition, Columbia University, New York, New York
Body Composition Unit of St Lukes-Roosevelt Hospital Center, Columbia University, New York, New York
Childrens Hospital of New York Presbyterian, Columbia University, New York, New York
| ABSTRACT |
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Methods. Height, weight, anthropometrics, body density by underwater weighing, total body water by deuterium dilution, and bone mineral content and %BF by DXA (Lunar DPX/DPX-L) were measured in 411 healthy subjects, aged 6 to 18 years. Values for %BF by 4-CM and DXA were compared using regression analysis.
Results. The mean ± standard deviation values for %BF by DXA (22.73% ± 11.23%) and by 4-CM (21.72% ± 9.42%) were different, but there was a strong relationship between the 2 methods (R2 = 0.85). DXA underestimated %BF in subjects with lower %BF and overestimated it in those with higher %BF. The relationship between the 2 methods was not affected by gender, age, ethnicity, pubertal stage, height, weight, or body mass index. The standard error of the estimate was 3.66%.
Conclusion. This analysis demonstrates a predictable relationship between DXA and 4-CM for %BF measurement. Because of its ease of use, consistent relationship with 4-CM, and availability, we propose that DXA has the capacity for clinical application including prediction of metabolic abnormalities associated with excess %BF in pediatrics.
Key Words: body composition percentage of body fat obesity pediatrics children adolescents 4-compartment model DXA
Abbreviations: DXA, dual-energy X-ray absorptiometry %BF, percentage of body fat 4-CM, 4-compartment model TBW, total body water Db, body density M, total body bone mineral content UWW, underwater weighing CV, coefficient of variation BMI, body mass index
In the midst of an epidemic of pediatric obesity13 and concerns about immediate and long-term complications,310 pediatricians are increasingly confronted with body composition information. This information is encountered both in the clinical evaluation of patients and in the pediatric literature. In the past 5 years, at least 27 articles in Pediatrics have included body composition variables from a variety of techniques, the majority from dual-energy X-ray absorptiometry (DXA). The quantity of body composition studies is expected to increase as pediatric investigators use these noninvasive techniques to define characteristics, such as percentage of body fat (%BF), that identify children and adolescents who are at health risk from obesity or abnormal body composition secondary to chronic disease or medication use.1114
A major issue in the interpretation of body composition analysis is that different methods may yield different results for the same variable in the same person. This is true in both children and adults. In fact, absolute truth is not achievable with any in vivo technique for body composition, because all are indirect and rely on numerous assumptions, never achieving the accuracy of direct actual chemical analysis.15 However, methods vary in their accuracy, defined as their ability to approximate the "true" value for a given body component. A criterion method is one that is accepted as the closest representation of true body composition and is used as a standard against which other methods are compared. The criterion method for body composition is the 4-compartment model (4-CM), combining measurements of total body water (TBW), body density (Db), and total body bone mineral (M) to estimate a fourth component%BF, fat, or fat-free mass. As pediatricians are well aware, children are not little adults, so the relationship of body composition techniques to the criterion method must be evaluated specifically in children and adolescents. The complex changes in body composition during childhood and adolescence make the interpretation of body composition in children particularly difficult.1620
In addition to accuracy, an important characteristic of body composition techniques used in children is ease of performance for subjects of all ages. The method should also be reproducible, readily available, and safe. The criterion 4-CM is tedious, time-consuming, and difficult to perform and requires fasting. Underwater weighing (UWW; holding breath repeatedly while completely submerged in a tank of water) was the method most likely to introduce error into the estimate of %BF by 4-CM even in adults.21 It has relatively low precision in young children22 and is difficult and uncomfortable for sick or young children to perform.23 In addition to UWW, 4-CM requires measurement of TBW as well as M by whole-body DXA scanning. Very few centers have all 3 methods available together. However, DXA scans are increasingly available and easily performed by children of all ages, making this method attractive for pediatric body composition measurement. Several papers have compared DXA with the criterion 4-CM for %BF in pediatric subject groups ranging in size from 25 to 141 with the greatest age range in a single study of 9 to 17, but the findings are not consistent.2327 We recently completed a cross-sectional body composition project during which 411 healthy subjects (aged 618 years) performed the criterion 4-CM. The goals of this article are to compare %BF by DXA with %BF by 4-CM in this large and heterogeneous group of children and adolescents, to evaluate factors that may influence the relationship between these 2 body composition methods, and to increase the awareness of pediatricians and pediatric investigators of these factors when analyzing body composition reports or data.
| METHODS |
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Studies were performed at least 1 hour after food intake, with all subjects wearing a hospital gown and foam slippers. Bathing suits were worn for UWW. Weight was measured to the nearest 0.1 kg on a balance-beam scale (Weight Tronix, New York, NY), and height to the nearest 0.1 cm using a wall-mounted stadiometer (Holtain, Crosswell, Wales).
DXA
Whole-body DXA scans were performed using Lunar models DPX with pediatric software version 3.8G and DPX-L with pediatric software 1.5G (GE Lunar Corporation, General Electric, Madison, WI).30 Each scan provided estimates of M in kilograms and %BF. Subjects who weighed <35 kg were scanned in the pediatric large mode, and those who weighed >35 kg were scanned in the adult medium mode. The coefficient of variation (CV) for repeated measures of %BF in adult subjects by whole-body DXA is 3.3% in our laboratory.31 The CV for repeated M measurements by whole-body DXA is 1.5% in adults30 and is 0.6% in our laboratory phantom.
An anthropomorphic spine phantom made up of calcium hydroxyapatite embedded in a 17.5 x 15 x 17.5-cm Lucite block was scanned with both DXA instruments for quality control each morning before subject evaluation. The phantom was also scanned immediately before and after all DXA system manufacturer maintenance visits. The measured phantom bone mineral density was stable throughout the study period at 1.166 to 1.196 g/cm2. Ethanol and water bottles (8-L volume), simulating fat and fat-free soft tissues, respectively, were scanned as soft-tissue quality control markers monthly. The range in measured R values during the study period was 1.255 to 1.258 (CV: 0.127%) and 1.367 to 1.371 (CV: 0.103%) for ethanol and water, respectively.
TBW
TBW in liters was measured by dilution of deuterium (2H2O) given orally. Saliva samples of 3 mL were collected at 0 and 120 minutes. The second sample was collected at 150 minutes if the subject weighed >91 kg.3234 The subject drank a dose (0.1 g/kg body weight) of 99.8 atom % excess 2H2O (Icon Corp, Summit, NJ) after the first saliva sample was collected. The subject then drank 30 mL of spring water, which was used to rinse the dosing cup. Subjects were reminded not to drink or eat anything until after the second saliva sample was collected. The dose concentration in the collected specimen was measured on a single-frequency infrared spectrophotometer after the specimen was lyophilized. The TBW volume was calculated by dividing the dose by the net 2H2O concentration in the specimen. The measured TBW was not corrected for nonaqueous exchange. In our laboratory, the CV for the TBW measurement by this method is 2.1% in adults.35
UWW
Db was determined using a 4-point platform scale system36 (Precision Biomedical System, Inc, University Park, PA). Residual lung volume was determined before UWW using the nitrogen washout technique.37 Subsequently, subjects entered the hydrodensitometry tank and were asked to exhale as much air as possible from their lungs during complete submersion. After between 5 and 10 trials were performed, an underwater weight was recorded as the average of the highest 3 values.38 The subjects wore bathing suits for all measurements. The between-day CV for measurement of Db by UWW corrected for residual lung volume in adults in our laboratory is 0.33%.38
4-CM Method
4-CM was used as the criterion method.39 The 4-CM equation is %BW = (2.747/Db 0.714 W + 1.146 M 2.0503)100, where Db is in kg/L, W is TBW (kg) as a fraction of weight (kg), and M is bone mineral content (kg) as a fraction of weight (kg).
Anthropometric Measurements
The following anthropometric measures were made as previously described40: chest, biceps, thorax, umbilicus, suprailiac, abdomen, thigh, subscapular, triceps, calf, and suprascapular skinfolds; upper arm, wrist, upper chest, chest, waist, iliac crest, thigh, and calf circumferences; and arm and thigh lengths.
Statistical Analysis
The mean values of %BF by DXA and 4-CM were compared by a paired 2-tailed t test. Regression analysis was used to assess agreement and bias between determinations of %BF. In the regression analysis, %BF by 4-CM was the dependent variable and %BF by DXA was the independent variable. The null hypothesis that the relationship was consistent with the line of identity was tested using the F distribution. Regression analysis was also used to determine whether the relationship between the 2 methods for %BF was affected by gender, pubertal stage, ethnicity, weight, height, body mass index (BMI), and anthropometric measurements. The method of backward elimination was used to identify a subset of anthropometric variables that had a significant effect on the relationship. The 95% limits of agreement, defined as the mean bias ± 2 standard deviations, were determined by the method of Bland and Altman.41
All statistical calculations were performed using the STATA version 7.0 statistical software package for personal computers (College Station, TX). The level of significance was .05 for all statistical tests of hypothesis.
| RESULTS |
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The relationships of %BF by DXA to 2 other 4-CMs (one specifically recommended for pediatric studies16,19 and the other for adults43) were very similar in slope, intercept, R2, and SEE (results not shown). The mean values for %BF for the study group by all methods (DXA and 4-CMs) were very close, ranging from 21.72% to 22.98%.
| DISCUSSION |
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Our findings are similar to those of other studies that compare Lunar DXA with 4-CM for measurement of %BF.24,44 Pediatric body composition studies using Hologic systems demonstrated a different relationship between the 2 methods.23,25,26 One report (n = 30) did not find a significant difference between %BF by Hologic DXA (model QDR 1000W) compared with 4-CM.23 However, 2 other reports found that Hologic DXA (models QDR 2000 and 2000W) systematically overpredicted %BF compared with the criterion.25,26 Unlike the current and previous small pediatric study using Lunar scanners, the overprediction of %BF was independent of subject %BF.25,26 Of interest, the SEE and limits of agreement were similar to our results. Although statistical modeling to create "translation" equations between DXA and the criterion are possible, the variability between methods persists.45 The 2 important issues for pediatricians are to recognize 1) that all DXA systems differ from the criterion 4-CM and 2) that this difference does not obviate the use of DXA for measurement of %BF in children and adolescents.
Relationship of DXA Systems to 4-CM for Measurement of %BF
The relationships between %BF by DXA and 4-CM differ by DXA manufacturer but have similar SEE and limits of agreement. The 2 major manufacturers are GE Lunar Corp and Hologic, Inc, and each has its own measurement algorithm. The different pattern of the relationship to %BF by 4-CM for each manufacturer but similar statistical characteristics suggests calibration differences associated with system-specific algorithms rather than a flaw in DXA technology. Factors that may differ between manufacturers algorithms include corrections for body thickness, body proportions, bone maturation, and bone edge characteristics and concerns about fat-free mass hydration.25,26,46,47
Like all indirect in vivo body composition methods, DXA technology relies on numerous assumptions of constancy that may not always be correct. For example, R values are theoretical constants related to photon attenuation for specific substances, but R values measured by DXA for homogeneous material may systematically change as thickness or depth varies.48 In an in vitro study that used a Lunar system to measure phantoms of varying depths, all fat values were close to chemical calculations, but percentage of fat of the phantoms was overestimated when phantom depth was greater and was underestimated when lower.49 These findings are similar to the relationship that we observed between our Lunar DXA system and 4-CM. The modest but significant effect of anthropometric measures on our model may represent an effect of body thickness or fatness. In a study of healthy adults, correction for anthropometric dimensions did not improve the relationship between DXA and the criterion 4-CM, emphasizing the importance of specific pediatric studies.50
Utility of DXA for the Measurement of %BF in Pediatrics
This study assessed the use of a Lunar DXA system (models DPX and DPX-L) for the measurement of %BF in the pediatric population by comparing it with the criterion 4-CM. Although the results differ, our findings suggest that there is a strong and predictable relationship. The 4-CM is not practical for large-scale projects or for young or sick children and is available in only a few centers. In addition, the criterion 4-CM is not perfect as illustrated by the minimum %BF of 0.11% in 1 of our subjects (Table 3). However, DXA is easily and quickly performed, safe, and increasingly available.
Demonstration of DXA precision in pediatrics would strengthen its role as a measure of %BF, particularly its value in longitudinal studies. DXA has been shown to be a precise measure of %BF in adults.31 One pediatric study showed adequate reproducibility of DXA in prepubertal girls who had 2 DXA scans 6 weeks apart.51 However, assessment of same-day intraindividual precision for %BF by DXA would be an important addition. Preliminary same-day intraindividual data in our laboratory indicate that DXA is precise for children and adolescents.
We propose that if each DXA systems specific characteristics are recognized, then they all have great potential as pediatric research and clinical tools. An example of a research use of DXA that may lead to clinical application is the prediction of the risk of comorbidities in obese children and adolescents.5
| CONCLUSION |
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Recognition that DXA differs from the criterion measure and that not all DXA systems are the same will lead to better interpretation of research and clinical results. Future areas of investigation include pediatric DXA precision studies and comparisons between DXA systems. Results from these will add to the findings of this report and will enhance the use of DXA for defining the relationship between body composition and health outcome. Because the prevention of adult disease is a central goal of pediatrics, practicing pediatricians should be knowledgeable about this body composition technique.
| ACKNOWLEDGMENTS |
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We thank all of the pediatric volunteers, the Body Composition Unit staff, and especially Barbara Fedun, RN, for maintaining everybodys enthusiasm during the course of this project.
| FOOTNOTES |
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Reprint requests to (M.H.) Body Composition Unit, Plant Basement, St Lukes-Roosevelt Hospital Center, 1111 Amsterdam Ave, New York, NY 10025. E-mail: mnh1{at}columbia.edu
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