Use of a standard preparatory diet for the oral glucose tolerance test. Is it necessary?

A prospective investigation was performed to evaluate the necessity of a diet containing greater than or equal to 150 g/d of carbohydrate for three days before the oral glucose tolerance test (OGTT). Thirty-seven obstetric patients who had a value of greater than or equal to 135 mg/dL on the one-hour, 50-g oral glucose screening test underwent a subsequent OGTT. The patients were alternately assigned to one of two groups. Nineteen of 37 (51%) were asked to precede the OGTT with a three-day diet containing greater than 150 g/d of carbohydrate. Those patients were then asked not to alter their normal dietary pattern and to repeat the OGTT in one to two weeks (group I). Eighteen of 37 (49%) followed the testing scheme in reverse order (group II). There were no significant differences in mean fasting or in one-, two- or three-hour plasma glucose values at either testing period between groups I and II. There were no significant differences between groups I and II in the number of patients who had an abnormal OGTT at the initial or repeat testing period. Dietary preparation for the OGTT is not necessary, and patients can be scheduled for the test when convenient.