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 underwate r weighing with correction of residual lung volume by nitrogen dilutio n and total body water (TBW) using isotope dilution of O-18 to estimat e body composition at 30 wk of gestation. D-B and TBW were used as ind ependent variables in the same equation. The hydration constant (HC) o f fat-free mass (FFM) was estimated as 0.762; based on this HC, new bo dy composition equations for both D-B and TBW were derived. These equa tions 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 equ ations and estimates of %F using both D-B and TBW. Ten of these forty women were evaluated postpartum. There was no significant difference i n %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 composit ion during pregnancy or conditions associated with increased body wate r.