Use of air displacement plethysmography for estimating body fat in a four-component model

Purpose: To compare measurements of body density (Db) obtained from air displacement plethysmography (AP) and hydrostatic weighing (HW) and to deter-mine the accuracy of substituting D-b via AP (D-b-AP) for D-b via HW (D-b-HW) in estimating body fatness (%Fat(4C)) and the composition and density of the fat-free mass (Dffm) from a four-component model (fat. mineral. water. and protein). Methods: D-b, was measured in 50 young adults using AP and HW. Total body water via deuterium dilution, bone mineral content via dual-energy x-ray absorptiometry, and D-b were used to estimate %Fat(4C) Results: D-b-AP and D-b-HW were highly correlated (r = 0.89 SEE = 0.008 g.mL(-1)), but D-b-AP (1.065 +/- 0.003 g.mL(-1)) was significantly higher (P < 0.05) than D-b-HW (1.058 +/- 0.003 g.mL(-1)), resulting in a mean difference of 2.8%fat. Differences between %Fat(4C-AP) (17.8 +/- 1.2%) and %Fat(4C.HW) (19.3 +/- 1.2%) were significant (P < 0.05), but the SD of the differences (2.3%) was low. When D-b-AP was used in a four-component model in place of D-b-HW, the calculated Dffm was significantly higher (1.109 <plus/minus> 0.002 vs 1.105 +/- 0.002 g.mL(-1)) based on a higher (P < 0.05) protein fraction (22.0 +/- 0.4% vs 20.6 +/- 0.4%) and tower (P < 0.05) water (71.1 +/- 0.4% vs 72.4 +/- 0.4%) and mineral fractions (7.0 +/- 0.1% vs 7.1 +/- 0.1%). Conclusions: AP yields a higher D-b than HW and may not be a valid method for measuring D-b or estimating %fat using densitometry. However, due to relatively small bias and low individual error, D-b-AP is an acceptable substitute for D-b-HW when estimating %fat with a four-component model in young adults.