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Our primary objective was to evaluate changes in energy expenditure and body composition in women with normal glucose tolerance (NGT) and gestational diabetes mellitus (GDM). A secondary objective was to examine the relationship between maternal leptin and nutrient metabolism. Fifteen obese women, eight with NGT and seven with GDM, were evaluated before conception (P), at 12-14 wk (E), and at 34-36 wk (L). Energy expenditure and glucose and fat metabolism were measured using indirect calorimetry. Basal hepatic glucose production was measured using [6,6-[sup.2][H.sub.2]]glucose and insulin sensitivity by euglycemic clamp. There was a significant increase (6.6 kg, P = 0.0001) in fat mass from P to L. There was a 30% (P = 0.0001) increase in basal [O.sub.2] consumption (V[O.sub.2], ml/min). There were no significant changes in carbohydrate oxidation during fasting or storage from P to L. There was, however, a significant (P = 0.0001) 150% increase in basal fat oxidation (mg/min) from P to L. Under hyperinsulinemic conditions, there were similar 25% increases in V[O.sub.2] (P = 0.0001) from P to L in both groups. Because of the significant increases in insulin resistance from P to L, there was a significant (P = 0.0001) decrease in carbohydrate oxidation and storage. There was a net change from lipogenesis to lipolysis, i.e., fat oxidation (30-40 mg/min, P = 0.0001) from P to L. Serum leptin concentrations had a significant positive correlation with fat oxidation at E (r = 0.76, P = 0.005) and L (r = 0.72, P = 0.009). Pregnancy in obese women is associated with significant increases in fat mass and basal metabolic rate and an increased reliance on lipids both in the basal state and during the clamp. These modifications are similar in women with NGT and GDM. The increased reliance on fat metabolism is accompanied by a concomitant decrease in carbohydrate metabolism during hyperinsulinemia. The increase in fat oxidation may be related to increased maternal serum leptin. gestational diabetes; pregnancy; leptin

Use of an underwater weighing technique that uses isotope dilution of (18)O for residual lung volume correction by total body water (TBW) and by nitrogen dilution helps measure body density (D(B)) and determine the body composition of twenty women at 30 wk of gestation. The new hydration constant of fat-free mass is 0.762. The percent body fat estimates derived from the new D(B) values or TBW equations and from D(B) and TBW measurements are similar.

A study to characterize carbohydrate metabolism abnormalities associated with the development of gestational diabetes mellitus (GDM) is presented. The study protocol involved the evaluation of six control and 10 women with GDM using an intravenous glucose tolerance test and hyperinsulinemic-euglycemic clamp with labeled glucose prior to conception and during early and late gestation. The results showed that decreased peripheral insulin sensitivity is the main metabolic abnormality which is associated with GDM development.