Relationship Between Intravenous Glucose Tolerance and the Fasting Blood Glucose Level in Healthy and in Diabetic Subjects

Abstract

Rapid intravenous glucose injections (1.0 g/kg^0.7) were given to 33 healthy subjects and 70 diabetic subjects following an overnight fast. Many of the diabetic subjects were studied before and during therapy with Lente insulin, sulfonylureas or phenformin. Following the glucose injection the logarithm of the venous blood glucose level declined linearly with time. The fractional decay constant, k, of this line was calculated, and in the diabetic subjects was corrected for glycosuria. The decline in the amount of glucose within the volume of glucose distribution during the test was also calculated. In the untreated diabetic subjects with moderate and severe hyperglycemia, glucose tolerance was more impaired in men then in women. The higher the fasting blood glucose level, the greater was this sex difference. In the healthy subjects also glucose tolerance was slightly less in men than in women. The reason for these findings is not clear. In the moderately and severely hyperglycemic subjects both before and during therapy there was an inverse linear relationship between the value of k and the fasting blood glucose level. It is believed that these subjects did not release significant amounts of effective endogenous insulin in response to the glucose injection. In the healthy subjects the value of k was much greater, and in some of the mildly hyperglycemic subjects slightly greater, than that predicted by this relationship. These values were presumed to indicate that an effective insulinogenic response to the glucose injection had occurred. The decline in the amount of glucose within the volume of glucose distribution during the test provided a more direct measurement of glucose tolerance than did the value of k. Because of the inverse relationship between the value of k and the fasting blood glucose level, glucose decay during the test remained within the same range throughout the entire untreated diabetic group. Conventional therapy for diabetes with depot insulin, sulfonylureas or phenformin lowered the fasting blood glucose level while slightly increasing the value of k. The effect of these changes upon glucose decay was precisely equal and opposite. Thus, the low value for glucose decay in the diabetic subjects was not changed at all by conventional therapy. It was increased only when the effective circulating insulin level was acutely raised at the time of the glucose injection by the administration of soluble insulin.