Air displacement plethysmography can detect moderate changes in body composition.

Objectives: To determine the sensitivity of air displacement plethysmography (APD) for evaluation of changes in body composition in normal subjects.

Design: Comparison of measurements with and without oil or water loads.

Subjects And Methods: Ten healthy volunteers were analyzed, without and with 1 l and 2 l of oil or water. The measured and true changes in fat mass and fat-free mass were compared by paired t-tests. A correlation study and a Bland & Altman procedure was performed on the 60 measurements of adiposity changes in 30 subjects carrying 0.5 l (n=8 x 2), 1 l (n=10 x 2) and 2 l (n=12 x 2) oil and water loads.

Results: Fat-free mass increased when the 10 subjects were carrying water. When they carried oil, fat mass increased, however, a approximately 0.5 kg increase of fat-free mass was also detected. Two liters loads led to distinct changes: +1.49+/-0.59 kg fat and +0.50+/-0.60 kg fat-free with oil and +0.37+/-0.57 kg fat and +1.70+/-0.56 kg fat-free with water (both P<0.001). Mixed loads (+1 l oil and 1 l water) led to detect +0.85+/-0.48 kg fat and +1.09+/-0.45 kg fat-free (both P<0.005 vs without load). For the 30 subjects analyzed thrice, measured changes in fat and fat-free mass were slightly underestimated (-15%, NS) but correlated with the true changes. Measured changes in adiposity were correlated with the true changes, with no bias as indicated by the Bland & Altman procedure.

Conclusion: APD detects approximately 2 kg changes in fat or fat-free mass in small populations.