BODY-FAT ESTIMATION IN LATE PREGNANCY AND EARLY POSTPARTUM - COMPARISON OF 2-COMPONENT, 3-COMPONENT, AND 4-COMPONENT MODELS

Accurate methods for determining body fat mass during reproduction are necessary to evaluate energy balance. However, determination of fat m ass is complicated during pregnancy by the accretion of water, which i nvalidates assumptions underlying standard two-compartment models. The extent to which the variability in body water during pregnancy invali dates use of pregnancy-corrected two-compartment models for determinat ion of fat mass in individual women is unknown, Moreover, it is unclea r whether body water returns to nonpregnant values by 2 wk postpartum, which is frequently used as the baseline in studies of postpartum wom en. The present study uses a four-component model as a criterion for e valuating two- and three-component models. Fifty-six healthy, normoten sive women between the ages of 19 and 35 y were studied at 36 +/- 1 wk gestation and 15 +/- 2 d postpartum. Total body water (TBW), total bo dy potassium (TBK), body density, and bone mineral content were measur ed by deuterium dilution, whole-body potassium counting, hydrodensitom etry, and dual-energy X-ray absorptiometry (postpartum only), respecti vely. At 2 wk postpartum; hydration and density of fat-free mass (FFM) had not returned to nonpregnant values, and differed between lactatin g and nonlactating women (P < 0.05). Accordingly, standard TBW and bod y density estimates of fat mass differed from four-component estimates at both time points (P < 0.005). Moreover, our data indicate that eve n when pregnancy-specific values for hydration or density of FFM are u sed in TBW and body density models, individual fat mass estimates may differ by > 3 kg from the Pour-component value. Fat mass by TBK may di ffer by > 10 kg from fat mass by the four-component model during pregn ancy. and by 6 kg postpartum. Use of standard two-compartment models t o estimate fat mass results in significant error both during pregnancy and at 2 wk postpartum. Pregnancy-corrected two-compartment models pr oduce reliable mean fat mass estimates during pregnancy, but individua l fat mass estimates may vary widely from four-component values.