Lean and obese women with gestational diabetes (GDM) were given two different isocaloric meal challenge tests to assess glucose and insulin responses. Forty-three pregnant women received a 400-kcal isocaloric breakfast meal tolerance test (mini-MTT). Twenty of the subjects were also given a 2000-kcal isocaloric diet with three meals and three snacks during a 24-h period (maxi-MTT). This was the first study to utilize the physiologic challenge of mixed meals to compare insulin and glucose responses of both obese and lean normal pregnant women and women with GDM around the clock.

Normal obese pregnant women had higher integrated glucose and insulin values around the 24-h clock < 0.003 and < 0.03, respectively) than lean pregnant women. Lean and obese women with GDM also responded differently to the physiologic challenge of mixed meals. Some, but not all, obese diabetic subjects were markedly hyperinsulinemic in contrast to lean diabetic women who were relatively insulin deficient. One normoglycemic massively obese 18-yr-old pregnant woman was unexpectedly found to be severely hyperinsulinemic.

The two meal tolerance tests clearly defined a delay in the release of insulin in women with GDM (lean and obese) and markedly different quantitative insulin responses to identical meal challenges in obese diabetic subjects. Maternal hyperinsulinemia was positively correlated with prepregnancy body mass index (kg/m2) and heavier infants, but not with plasma glucose levels.

These studies provide evidence that GDM is a heterogeneous syndrome. Because of the variety of tests for GDM, the not unusual occurrence of carbohydrate intolerance in lean pregnant women without risk factors, and the worldwide differences in diagnostic criteria for this syndrome, we recommend screening for diabetes in all pregnant women. Finally, we have proposed that an extension of the National Diabetes Data Group classification be applied to pregnant women. In this scheme, gestational carbohydrate intolerance would be considered type III or GDM and be subdivided into nonobese and obese women. The increasing number of patients with carbohydrate intolerance secondary to renal dialysis, transplantation, cystic fibrosis, and other disorders might be called type IV or secondary diabetes.

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