Plasma glucose and immunoreactive insulin (IRI) were measured during four-hour oral glucose tolerance tests (GTT) and prednisolone glucose tolerance tests (PGTT) in thirty-six women suspected of having diabetes mellitus because they had recently delivered an infant weighing nine pounds or more. The results were compared to similar tests performed on ten nonobese women who had given birth to normal weight infants. All diabetes suspects had normal GTT, and the total IRI response (area circumscribed by the plasma insulin response curve) was similar to the control group. During PGTT, seventeen of the thirty-six suspects had a mild diabetic curve (44 per cent) despite the finding that total plasma IRI response remained similar to the control individuals and the nineteen suspects who had normal PGTT. Examination of the pattern of the hormonal response, however, revealed that in the subclinical diabetic group plasma insulin levels at thirty minutes during PGTT were subnormal relative to abnormally high plasma glucose concentrations at this time interval. This defect was not observed during GTT, when all individuals had normal carbohydrate tolerance. The majority of women who delivered large infants were also moderately obese whether or not diabetes was present. The results of these studies indicate that most women with subclinical diabetes mellitus who deliver large infants will escape detection in the immediate postpartum period if the evaluation is limited to a standard oral GTT. The impaired glucose tolerance elicited during PGTT would appear to be directly related to a defective insulin response during the initial thirty to sixty minutes following oral glucose administration and is spontaneously reversible when a diabetogenic stress such as that produced with oral glucocorticoids and presumably pregnancy is not present.

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