Total Body Water Measurement in Normal and Diabetic Pregnancy: Evidence for Maternal and Amniotic Fluid Equilibrium

Abstract

The H₂[¹⁸O] tracer dilution method was applied to quantify total body water in 6 normal nonpregnant women, 8 normal pregnant women (gestation 29.3 ± 6.0 weeks, range 21–37 weeks), and 8 insulin-dependent diabetic pregnant subjects (gestation 29.4 ± 6.1 weeks, range 20–37 weeks). Plateau or equilibrium enrichment of ¹⁸O in the breath CO₂ was achieved at approximately 2h in nonpregnant normal subjects. In contrast, in pregnant subjects equilibrium plateau was reached later, at approximately 3 h. In order to examine whether the amniotic fluid/fetal compartment had also reached equilibrium with the maternal water compartment, amniotic fluid samples were obtained from 5 women undergoing elective cesarean section, approximately 5 h after the administration of tracer H₂[¹⁸O]. The ¹⁸O enrichment of amniotic fluid was similar to that of the mother. Total body water was similar in normal and diabetic pregnant subjects (40.4 ± 4.4 vs. 40.1 ± 5.8 kg) and represented 55.8 ± 5.4 and 58.7 ± 5.5% of body weight, respectively. These data demonstrate the usefulness of the H₂[¹⁸O] tracer method to measure total body water in pregnancy and emphasize the importance of an adequate equilibrium period 4–5 h in order to measure the dilution of administered tracer. The measurements by H₂[¹⁸O] tracer are similar to those previously reported with deuterium oxide. Insofar as total body water reflects lean body mass, the data suggest that body compositional changes in normal and diabetic pregnancy are qualitatively similar.

Glyceroneogenesis and the Source of Glycerol for Hepatic Triacylglycerol Synthesis in Humans

“…The subjects ate their last meals at 6:00 p.m. the evening before the day of the study. They were given [ 2 H]water orally (ϳ3 gm/kg of body water), assuming total body water to be 55% of body weight (14), in three divided doses at 11:00 p.m., 3:00 a.m., and 7:00 a.m. Blood samples in heparinized syringes were obtained at frequent intervals starting at 8:00 a.m. and lasting until 12:00 noon.…”

Comparison of multiple methods to measure maternal fat mass in late gestation

“…Subjects were instructed not to eat or drink anything after the dose. It has been suggested that full isotopic equilibration with the TBW pool may require up to 4 h or more in late gestation (32). However, in practical terms, a prolonged overnight fast in late gestation is quite difficult, and thus the postdose sample was collected 3 h after 2 H 2 O ingestion as per Lederman et al (17).…”

Body composition changes in pregnancy: measurement, predictors and outcomes

“…Two methods are utilized to determine TBW by dilution: plateau and back extrapolation. With the plateau method, a second physiological sample is obtained after an equilibration period; extended in pregnancy to 4 h for oral dosing and 3 h for subcutaneous dosing to ensure equilibrium, 44 as extracellular water compartments are expanded. With back extrapolation, physiological samples are collected at several intervals up to 14 days after dose administration, and TBW is calculated by back extrapolation to the time of dose.…”