Abdominal posture biases 3D optical imaging measurements while bioelectrical impedance analysis remains unaffected

Body composition assessment by bioelectrical impedance analysis (BIA) and three-dimensional optical (3DO) imaging is increasingly accessible due to low cost and ease of use, supporting integration into clinical practice and self-monitoring. Both methods estimate body composition based on body shape, yet the impact of acute changes in body shape on the estimation of body composition and, for 3DO, waist circumference (WC), has not been systematically evaluated.

Young adults completed three consecutive measurements on BIA and 3DO systems, in randomized order, under three abdominal positions: relaxed (normal), distended, and contracted. Mean fat mass percent (FM%) and 3DO-derived WC were compared across positions using repeated-measures analysis of variance with Tukey post hoc tests.

Twenty participants (age 21.3 ± 3.0 y; BMI 24.3 ± 4.3 kg/m²) completed all measures. BIA-estimated FM% did not differ across positions (23.3-23.5%; all p > 0.05). In contrast, 3DO-estimated FM% differed significantly across positions: distended 28.1 ± 7.0%, relaxed 26.7 ± 6.8%, contracted 25.8 ± 7.4% (all p < 0.05), with differences across the three positions as large as 8.2%. WC also differed by position: distended 79.6 ± 8.4 cm, relaxed 77.9 ± 8.1 cm, contracted 76.3 ± 7.7 cm (all p < 0.05).

Although body shape is fundamental to body composition estimation by BIA and 3DO, intentional changes in abdominal shape altered 3DO-derived FM% and WC but did not affect BIA-derived FM% in young adults. Standardized instructions that minimize abdominal distension or contraction are needed when using 3DO in clinics and at home to ensure accurate estimates and reliable longitudinal tracking.