Plasma Insulin and Glucose Responses of Healthy Subjects to Varying Glucose Loads During Three-hour Oral Glucose Tolerance Tests

Abstract

The plasma glucose and plasma immunoreactive insulin concentrations of twelve healthy, nonobese, adult subjects were compared following administration of oral glucose loads of 50, 75 and 100 gm., and 1.75 gm./kg. body weight. The time of occurrence and the magnitude of maximal plasma glucose and insulin concentrations were unaffected by the 25 gm. increases in glucose load. The 50 gm. glucose load gave the smallest plasma glucose response during the three-hour tests and this response was statistically distinguishable from the plasma glucose responses due to the 100 gm. and 1.75 gm./kg. loads (p < 0.05). The plasma glucose responses due to the other loads did not differ from each other. Insulin responses, estimated by either summing allthe plasma insulin values or by measuring the areas under the curves, increased approximately 35 per cent with the first 25 gm. glucose load increment (50 to 75 gm.) and 27 per cent with the second 25 gm. load increment (75 to 100 gm.) during the three-hour study. The differences in insulinresponses between the 50 gm. load and the 100 gm. or 1.75 gm./kg. loads were highly significant (p < 0.01) while the difference between the insulin responses following the 75 gm. and 1.75 gm./kg. loads had lower significance (p < 0.05). Differences in insulin responses of the other loads were not significant. Increases in glucose loads above 120 gm. did not further increase the insulin response. This provides evidence that maximal insulin responses provoked by an oral glycemic stimulus alone in subjects are achieved with 100 gm. A wide range was observed in the plasma insulin concentrations among these healthy individuals for any given load, at any one time, and was probably due to biological variation. The fact that incremental changes in glucose load particularly influenced plasma glucose and insulin concentrations relatively late in the glucose tolerance tests, and that mean insulin peakconcentrations did not differ, indicates that increasing glucose load results in a more prolongedinsulinogenic stimulus rather than the acute release of greater quantities of insulin relatively abruptly. These studies provide evidence that the response associated with increasing glucose load is to some extent a function of the quantity of glucose ingested, and that the significant increases in insulin response commonly occur in association with concurrent alterations in plasma glucose concentrations. The results also indicate that the 50 gm. glucose load results in a different plasma glucose response than does the 100 gm. and 1.75 gm./kg. loads and consequently requires different criteria for interpretation of glucose tolerance testing.