ESTIMATING BODY-COMPOSITION IN LATE-GESTATION - A NEW HYDRATION CONSTANT FOR BODY DENSITY AND TOTAL-BODY WATER

Twenty women underwent body density (D-B) measurements using underwater weighing with correction of residual lung volume by nitrogen dilution and total body water (TBW) using isotope dilution of O-18 to estimate body composition at 30 wk of gestation. D-B and TBW were used as independent variables in the same equation. The hydration constant (HC) of fat-free mass (FFM) was estimated as 0.762; based on this HC, new body composition equations for both D-B and TBW were derived. These equations were prospectively tested in an additional 20 women at 30 wk of gestation. No significant differences were detected between estimates of percent body fat (%F) using either the newly derived D-B or TBW equations and estimates of %F using both D-B and TBW. Ten of these forty women were evaluated postpartum. There was no significant difference in %F estimated by either TBW or D-B compared with standard equations (hydration of FFM = 0.72) and %F using both D-B and TBW. These results highlight the importance of either measuring both D-B and TBW or using an appropriate hydration constant for FFM in estimating body composition during pregnancy or conditions associated with increased body water.