Controlled oral glucose tolerance test: evaluation of insulin resistance with an insulin infusion algorithm that forces the OGTT glycaemic curve within the normal range. A feasibility study

This is a technical study to show the feasibility of a computer-controlled oral glucose tolerance test (OGTT) using a specific algorithm, consisting of an OGTT carried out while insulin is infused as required to keep glycaemia within the normal range (National Diabetes Data Group 1979 criteria). This technique allows (a) the amount of insulin (insulin area) required to maintain a normal glycaemic curve to be assessed, a parameter indicating the degree of insulin resistance; and (b) the unique parameter consisting of the insulin secretory response (C-peptide) to a normal glycaemic curve under the inhibitory feedback exerted by the insulin levels required to maintain normal glycaemia to be obtained. Preliminary results confirmed the feasibility of this approach by showing that during the test while the glycaemic area was kept normal the insulinaemic area (endogenous + infused insulin) increased markedly in obese (n = 8) and obese diabetic (n = 5) subjects compared with normal subjects (n = 6), with values of 145.10 +/- 26.71, 204.75 +/- 20.77 and 68.25 +/- 5.93 nmol l-1 min-1 respectively (P < 0.01 in both instances). In contrast, endogenous insulin secretion (C-peptide levels) remained almost unchanged. Compared with data in normal subjects, free fatty acid (FFA) values were basally elevated in the obese and obese diabetic patients, and underwent a smaller decrease during the test. The FFA areas were greater than normal in both groups of patients, suggesting that FFAs were not fully suppressible despite the highest possible insulin levels (higher insulin levels would produce hypoglycaemia). The computer-controlled OGTT might be useful for the metabolic study of patients in the clinical setting.