Changes in fat-free mass and fat mass in postpartum women: a comparison of body composition models.

Objective: (1) To compare 2-, 3- and 4-component models of body composition based on total body water (TBW), underwater weighing (UWW), skinfold thicknesses (SF), total body potassium (TBK), dual-energy X-ray absorptiometry (DXA) and total body electrical conductivity (TOBEC); (2) to compare postpartum changes in body composition estimated by the 2-, 3- and 4-component models and (3) to test for an effect of pregnancy or lactation on the hydration, density and potassium content of fat free mass (FFM) in postpartum women.

Methods: Longitudinal measurements of body composition at 3, 6 and 12 months postpartum. Methods: Thirty-five healthy postpartum women, aged 30.2 +/- 3.5 y. Methods: Body composition was estimated by 2-component models based on TBW, UWW, SF, TBK, DXA or TOBEC; 3-component models based on TBW and UWW (Fuller 3, Siri 3); and a 4-component model (Fuller 4) based on TBW, UWW and bone mineral content.

Results: Systematic differences were seen among the various body composition models, with the following ranking from lowest to highest estimate of fat mass (FM): TOBEC, TBW, Fuller 3, Siri 3, Fuller 4, UWW, SF, TBK, and DXA. Estimated changes in FFM and FM were not significantly different among methods, except for the 3-6 months FFM and FM changes estimated from TBW, which differed from SF, DXA, and TOBEC. Pregnancy-induced changes in the hydration, density and potassium content of FFM were not evident by 3 months postpartum (0.73 +/- 0.02, 1.099 +/- 0.015 kg/l and 2.31 +/- 0.10 g/kg, respectively).

Conclusions: In spite of systematic differences among body composition models for the measurement of FFM and FM, changes in FFM and FM did not differ significantly among the models. Since there was no apparent effect of pregnancy or lactation on the postpartum composition of FFM, 2-component models of body composition are acceptable for use in postpartum women beyond the puerperium.