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Comparison of Bod Pod® and DXA in Female Collegiate Athletes

BALLARD, TASHA P.; FAFARA, LAURA; VUKOVICH, MATTHEW D.

Medicine & Science in Sports & Exercise: April 2004 - Volume 36 - Issue 4 - p 731-735
doi: 10.1249/01.MSS.0000121943.02489.2B
APPLIED SCIENCES: Physical Fitness and Performance
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BALLARD, T. P., L. FAFARA, and M. D. VUKOVICH. Comparison of Bod Pod® and DXA in Female Collegiate Athletes. Med. Sci. Sports Exerc., Vol. 36, No. 4, pp. 731–735, 2004.

Purpose This study was designed to evaluate the reliability and validity of air displacement plethysmography (ADP) compared with dual energy x-ray absorptiometry (DXA) Hologic QDR 4500A (Waltham, MA) in female collegiate athletes.

Methods Forty-seven females representing various Division II collegiate sports and 24 controls participated in the current study. All women underwent both methods of testing within a 30-min period.

Results Comparison of means indicated that the ADP and DXA are not different when measuring body fat (BF%) in the athletes (ADP = 22.5 ± 5.5%, DXA = 22.0 ± 4.7%P = 1.0). Furthermore, this study determined that ADP is a reliable measure of body fat (BF%; r = 0.96, P < 0.001; 0.97, P < 0.001) in collegiate female athletes and nonathletes, respectively.

Conclusion The results from this study indicate that ADP is a valid measure of body composition in female athletes and nonathletes when compared with DXA.

South Dakota State University, Department of HPER, Brookings, SD

Address for correspondence: Matthew Vukovich, Ph.D., Dept. of HPER, Box 2820, South Dakota State University, Brookings, SD 57007; E-mail: matthew_vukovich@sdstate.edu.

Submitted for publication May 2003.

Accepted for publication December 2003.

Excess adiposity can alter athletic performance (4,22). Athletes often undergo body composition testing to determine ideal competitive weights for their specific sport. Therefore, it is important that body composition testing be valid, reliable, and readily accessible for a variety of athletes to assist in guiding training both in and out of the competitive season.

Dual energy x-ray absorptiometry (DXA) is a three-component model which considers total body mineral mass, mineral free mass, and fat mass. DXA is a rapid method that delivers highly reliable results (19), and its open table is minimally stressful for most individuals. However, the expense of the machine makes it slightly impractical for many laboratories and service centers.

Recently, air displacement plethysmography (ADP) has been validated against hydrostatic weighing (HW) and is considered to be a both quick and more comfortable method of body composition assessment (17). Whereas HW uses water displacement to measure body volume, ADP uses air displacement to determine body volume and ultimately body density. Although ADP has been considered to provide reliable and valid information in adults (7,17,24), documentation of population subgroups is warranted.

As ADP becomes more common among collegiate athletics, it is important to deem the use of this method valid among this population. Collins et al. (3) reported in collegiate male football players, ADP significantly underpredicted body fat when compared with HW and DXA. In a recent report of female collegiate athletes representing various sports, Vescovi et al. (23) reported an 8.5% higher body fat from ADP when compared with HW. In contrast, Utter et al. (21) reported that ADP provided similar estimates of body composition when ADP was compared with HW in normally hydrated and acutely dehydrated wrestlers. To date, we are unaware of any research that has evaluated the use of ADP in reference to DXA in female college athletes. Therefore, the purpose of this study was to determine the reliability and validity of ADP for determining body density (Db), fat free mass (FFM), and body fat percentage (BF%) in female collegiate athletes when compared with DXA.

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METHODS

Subjects.

Forty-seven Division II female athletes representing various sports and 24 female nonathletes participated in the present study. Sports represented included basketball, soccer, volleyball, swimming, fastpitch softball, and track. All subjects were non-Hispanic Euro-American women, apparently healthy, between the ages of 18 and 21. The nature, purpose, risks, and benefits were explained to each participant before obtaining written informed consent. The South Dakota State University Human Research Committee approved all experimental protocols.

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Experimental protocols and procedures.

All subjects reported to the lab after a minimum 4-h fast. In addition, all subjects were instructed to refrain from exercise before testing. After using the restroom, subjects were weighed to the nearest 0.1 kg on a calibrated electronic scale, and height was determined via a stadiometer to the nearest centimeter. All participants underwent two methods of body composition in the following order: DXA then ADP. All measurements were performed the same day, within a 30-min period. Dual energy x-ray absorptiometry (DXA) was performed and analyzed by one technician using a Hologic QDR 4500A software version 12.01 (Waltham, MA). This method requires approximately a 5-min period where the subject lies supine while the whole body is scanned.

Body volume was measured using air displacement plethysmography (Bod Pod® Life Measurement Instruments, Concord, CA). Methods for the Bod Pod® have been previously described (17). Briefly, after a two-point calibration of the ADP, the subject entered the chamber and instructed to sit still with hands on thighs and breathe normally while body volume was assessed. Thoracic gas volume was estimated according to the methods described by Dempster and Aitkens (5). The use of predicted thoracic volumes is supported by McCrory et al. (18), who reported that the use of predicted lung volumes in conjunction with ADP was acceptable for group mean comparisons. All subjects were tested in nylon swimming suits and nylon swimming caps with all jewelry removed.

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Reliability testing.

Repeatability measures were acquired for 12 controls and 10 athletes in the ADP. After the first pair of measurements, each woman repeated the entire process including the measurement of body weight and the two-point calibration. All repeated measures were performed immediately after the initial measurements and were completed by the same technician.

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Statistical analysis.

JMPIN version 4.0 was used for all statistical analysis. Student’s t-tests were used for group mean comparisons. Regression analysis was performed to determine reliability of ADP in estimating body density, percent body fat, and fat-free mass (FFM) from trial 1 to trial 2. Bland-Altman residuals were plotted for percent body fat in athletes and controls to distinguish trends in methods and values (2). Lohman’s criteria were used to evaluate prediction errors (16). Significance was set a priori at P < 0.05. Results are reported as mean ± standard deviations.

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RESULTS

Test-retest data for body density are reported in Figures 1. Trial 1 measures were highly correlated to trial 2 for BF%, FFM, and density in both controls and athletes (Table 1).

FIGURE 1

FIGURE 1

TABLE 1

TABLE 1

Subject characteristics are defined in Table 2. In the athletic population there were no differences of BF% (ADP = 22.5 ± 5.5%, DXA = 22.0 ± 4.7%) or FFM (ADP = 15.1 ± 5.1 kg, DXA = 15.1 ± 4.5 kg) in ADP when compared with DXA. There were no differences between DXA and ADP for BF% (ADP = 28.5 ± 6.7%, DXA = 28.2 ± 5.2%) or FFM (ADP = 45.9 ± 5.7 kg, DXA = 44.9 ± 5.1 kg) in the control group. Bland-Altman residual plots were used to display the difference in error between the methods for density and BF% (DXA vs ADP). These are depicted in Figure 2. All plots display a “shot-gun” appearance with no apparent trend.

TABLE 2

TABLE 2

FIGURE 2

FIGURE 2

Correlational relationship of ADP and DXA for BF% are defined in Figure 3. SEE, derived from regression analysis, indicated that ADP measurements of body density (SEE = 0.007, 0.006), BF% (SEE = 2.856, 3.157), and FFM (SEE = 1.20, 2.09) in athletes and controls were all acceptable predictors according to Lohman’s criteria.

FIGURE 3

FIGURE 3

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DISCUSSION

The results of this study indicate that ADP is both reliable and valid in female collegiate athletes and nonathletes when compared with DXA. In our populations of women, there were no significant differences in BF% or FFM between the two methods when DXA was used as the reference measure for ADP.

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Reliability.

Regression analysis suggests test-retest reliability of ADP for body composition measurement is well correlated in both female athletes and nonathletes. In addition, there are no significant mean differences between trial 1 and trial 2 in either group, collectively regarding it as a reliable tool for body composition assessment. To our knowledge, this is the first study that addresses ADP reliability in female athletes. The results are in agreement with previous studies testing the reliability of ADP in collegiate football players (3) and in adults of various ethnicities (17,20).

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Validity.

The current study suggests that ADP is a valid measure of BF% and FFM when referenced by DXA. This is the first study, to our knowledge that compared ADP with DXA in female collegiate athletes. Other work in female athletes that investigated the use of ADP for body composition analysis used hydrostatic weighing as a reference measure. One such report by Vescovi et al. (23) reported a significant overestimation in BF% in female collegiate athletes when ADP was compared with HW. In another study by Vescovi et al. (24) that included women ages 18–49, they reported that although the comparison between ADP and HW revealed no differences in those of “average” fat, ADP overestimated BF% in lean individuals. Research in other female populations have found upward trends in Bland-Altman analysis when comparing ADP with hydrostatic weighing (1,15), and ADP has been documented to overpredict BF% in lean and obese females as well as African-American men (9,25). Whether the results are influenced by sex or body fatness per se has yet to be completely elucidated. When comparing ADP with DXA, however, as in our population, the current data suggest that there are no significant differences in BF% between methods in female athletes and nonathletes. This is in contrast to reports of underestimation of BF% by ADP when compared with DXA in middle-aged men and women (20,26). Although, in one of the studies, the middle-aged men were overweight by definition, whereas the subjects in the present study were lean or normal weight (20). However, a study using a 4-C model that included DXA and isotope dilution documented that ADP under-predicted BF% in women of all levels of fatness (8).

The differences in results between our research and that of others may, in part, be due to certain methodological limitations. A limitation of the current study is that thoracic lung volumes were obtained using prediction equations instead of from measured volumes. However, McCrory et al. (18) suggests that predicted thoracic lung volumes are valid when performing ADP as compared with measured thoracic lung volumes. In addition, Bod Pod® has recently developed an S/T model that does not require lung volume measurements. We decided to use prediction equations rather than measuring thoracic lung volumes because most service facilities are unable to budget the money to support the necessary equipment nor the time to perform the lung volume measurements.

Of further consideration is the appropriateness of DXA verses hydrostatic weighing as a criterion measure in female athletes. Bone, mineral, protein, and hydration status are all biological factors, which may alter the correctness of this technique in female athletic populations under the constant assumptions. Therefore three-compartment methods such as DXA, which considers bone mineral density (BMD) presents challenge to two-compartment HW as a criterion measure by accounting for more individual variability in lean body tissue in effort to reduce error in body fat measurement (10). The use of DXA as a reference measure of body composition is supported by a report that indicated differences in BF% in DXA versus HW may be due to differences in bone mineral content (BMC) and lean body mass. This report by Kohrt et al. (14) suggests the potential for DXA as a superior reference measure for body composition compared with HW. Further support by Prior et al., who compared DXA with a 4-c model, suggests DXA is an accurate method of body composition assessment in a variety of ages, races, body types, and women of various ethnicities (11–13,19,27). In our study population of female athletes, this is a relevant consideration due to the wide variety of body geometry and composition of the females, as well as the nature of the sports represented within the athletic grouping. For instance, a sport such as track that loads the bone by nature of the activity would potentially present a distinct bone characteristics as compared with swimming, a weightless sport. In support, Duncan et al. (6) has documented that female track athletes had greater site-specific BMD than female swimmers.

These differences in bone characteristics present the need for a more complex model beyond the (2-C) that considers sport-related discrepancies such as bone. As previously stated, in our population of female college athletes, there were no significant differences in BF% when DXA was used as a comparison method for ADP. This is an important consideration in that DXA is becoming increasingly more popular as a criterion measure because of its inclusion of BMC and its relative comfort compared with HW.

In conclusion, ADP seems to be a reliable and valid method of body composition assessment in female athletes and nonathletes. Because of the high reliability of ADP, it would be an acceptable measure to track changes in body composition as often requested by coaches at different points in and out of season. Furthermore, ADP appears to be a quick, comfortable, and valid measure of body composition in female college-aged athletes and nonathletes.

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Keywords:

AIR-DISPLACEMENT; BODY COMPOSITION; DUAL ENERGY X-RAY ABSORBTIOMETRY; BODY FAT; FAT-FREE MASS

©2004The American College of Sports Medicine