Scientific Validation for Bod Pod

The objective of this study was to determine the accuracy, precision, bias, and reliability of percent fat (%fat) determined by air-displacement plethysmography (ADP) with the pediatric option against the four-compartment model in 31 children (4.1 ± 1.2 years, 103.3 ± 10.2 cm, 17.5 ± 3.4 kg). %Fat was determined by (BOD POD Body Composition System; COSMED USA, Concord, CA) with the pediatric option. Total body water (TBW) was determined by isotope dilution ((2)H(2)O; 0.2 g/kg) while bone mineral was determined by dual-energy X-ray absorptiometry (DXA) (Lunar iDXA v13.31; GE, Fairfield, CT and analyzed using enCore 2010 software). The four-compartment model by Lohman was used as the criterion measure of %fat. The regression for %fat by ADP vs. %fat by the four-compartment model did not deviate from the line of identity where: y = 0.849(x) + 4.291. ADP explained 75.2% of the variance in %fat by the four-compartment model while the standard error of the estimate (SEE) was 2.09 %fat. The Bland-Altman analysis showed %fat by ADP did not exhibit any bias across the range of fatness (r = 0.04; P = 0.81). The reliability of ADP was assessed by the coefficient of variation (CV), within-subject SD, and Cronbach’s α. The CV was 3.5%, within-subject SD was 0.9%, and Cronbach’s α was 0.95. In conclusion, ADP with the pediatric option is accurate, precise, reliable, and without bias in estimating %fat in children 2-6 years old.

OBJECTIVE:

To evaluate interdevice reliability in body density (Db) and percent body fat (%BF) using air-displacement plethysmography, the BOD POD (BP) body composition system.

DESIGN AND SETTING:

Duplicate body composition tests were performed in immediate succession on 50 adults (26 M, 24 F; 21-53 y) using two BP units located in the same body composition laboratory.

RESULTS:

Mean Db and %BF between BP1 and BP2 did not differ significantly for men (DeltaDb = 0.0003+/-0.0008 g/ml, P = 0.632; Delta%BF = 0.1+/-1.3, P = 0.665), while for women, there were small but significant differences in Db and %BF between BP1 and BP2 (DeltaDb = 0.0018+/-0.0003 g/ml, P = 0.001; Delta%BF = 0.8+/-1.1, P = 0.001). The regression between %BF by BP1 and BP2 did not deviate significantly from the line of identity for both men and women (R2 = 0.95, standard error of estimate (s.e.e.) = 1.23 %BF for men; R2 = 0.97, s.e.e = 1.13 %BF for women). Individual variations in %BF estimates between the two BP units were within acceptable ranges (95% limits of agreement = -2.5-2.7 %BF for men; -1.4-3.0 %BF for women), and there was no trend in individual differences as %BF varied (r = -0.19, P = 0.359 for men; r = 0.09, P = 0.677 for women). Other subject characteristics, including age, body mass, height, and body mass index, did not significantly contribute to the differences in %BF estimates by the two BP units.

CONCLUSIONS:

No clinically significant differences in Db and %BF estimates exist between the BP units, and the interdevice variability of the BP has minimal impact on %BF estimates. Further, test-to-test reliability between BP units appears to be as good as within one unit.

OBJECTIVE:

Patients with moderate and severe obesity, because of their physical size, often cannot be evaluated with conventional body composition measurement systems. The BOD POD air displacement plethysmography (ADP) system can accommodate a large body volume and may provide an opportunity for measuring body density (D(b)) in obese subjects. D(b) can be used in two- or three-compartment body composition models for estimating total body fat in patients with severe obesity. The purpose of this study was to compare D(b) measured by ADP to D(b) measured by underwater weighing (UWW) in subjects ranging from normal weight to severely obese.

RESEARCH METHODS AND PROCEDURES:

D(b) was measured with UWW and BOD POD in 123 subjects (89 men and 34 women; age, 46.5 +/- 16.9 years; BMI, 31.5 +/- 7.3 kg/m2); 15, 70, and 10 subjects were overweight (25 < or = BMI < 30 kg/m2), obese (30 < or = BMI < 40 kg/m2), and severely obese (BMI > or = 40 kg/m2), respectively.

RESULTS:

There was a strong correlation between D(b) (kilograms per liter) measured by UWW and ADP (r = 0.94, standard error of the estimate = 0.0073 kg/L, p < 0.001). Similarly, percent fat estimates from UWW and ADP using the two-compartment Siri equation were highly correlated (r = 0.94, standard error of the estimate = 3.58%, p < 0.001). Bland-Altman analysis showed no significant bias between D(b) measured by UWW and ADP. After controlling for D(b) measured by ADP, no additional between-subject variation in D(b) by UWW was accounted for by subject age, sex, or BMI.

DISCUSSION:

Body density, an important physical property used in human body composition models, can be accurately measured by ADP in overweight and obese subjects.

Laboratory-based body-composition techniques include hydrostatic weighing (HW), dual-energy X-ray absorptiometry (DXA), measurement of total body water (TBW) by isotope dilution, measurement of total body potassium, and multicompartment models. Although these reference methods are used routinely, each has inherent practical limitations. Whole-body air-displacement plethysmography is a new practical alternative to these more traditional body-composition methods. We reviewed the principal findings from studies published between December 1995 and August 2001 that compared the BOD POD method (Life Measurement, Inc, Concord, CA) with reference methods and summarized factors contributing to the different study findings. The average of the study means indicates that the BOD POD and HW agree within 1% body fat (BF) for adults and children, whereas the BOD POD and DXA agree within 1% BF for adults and 2% BF for children. Few studies have compared the BOD POD with multicompartment models; those that have suggest a similar average underestimation of approximate 2-3% BF by both the BOD POD and HW. Individual variations between 2-compartment models compared with DXA and 4 -compartment models are partly attributable to deviations from the assumed chemical composition of the body. Wide variations among study means, -4.0% to 1.9% BF for BOD POD – HW and -3.0% to 1.7% BF for BOD POD – DXA, are likely due in part to differences in laboratory equipment, study design, and subject characteristics and in some cases to failure to follow the manufacturer’s recommended protocol. Wide intersubject variations between methods are partly attributed to technical precision and biological error but to a large extent remain unexplained. On the basis of this review, future research goals are suggested.

A new air displacement plethysmograph, the BOD POD (BP), was evaluated in comparison to hydrostatic weighing (HW). Sixty-eight adult subjects (26 F, 42 M) varying widely in age (range 20-56 yr), ethnicity, and fatness participated in this study. Same-day test-retest reliability was assessed in a subsample of 16 subjects (9 F, 7 M) and validity was assessed in all subjects (N = 68). The test-retest coefficients of variation (CV) for %FAT measured by BP (%FATBP) and HW (%FATHW) were not significantly different (1.7% +/- 1.1% and 2.3% +/- 1.9% for BP and HW, respectively (mean +/- SD)), indicating excellent reliability for both methods. Validity of percent fat measured by the BP (%FATBP) was also excellent. The mean difference in %FAT (BP – HW) was -0.3 +/- 0.2 (SEM), with a 95% confidence interval of -0.6 to 0 %FAT. The regression equation (%FATHW = 1.86 + 0.94 %FATBP; r2 = 0.93, SEE = 1.81) was not significantly different from the line of identity (%FATHW = %FATBP), and did not differ by gender. These findings indicate that the BOD POD is a highly reliable and valid method for determining %FAT in adult humans in comparison to HW. This new method has several advantages over HW in that it is quick, relatively simple to operate and may be able to accommodate special populations such as the obese, elderly, and disabled.