COMPARISON OF THE METABOLIC EFFECTS OF MIXED MEAL AND STANDARD ORAL GLUCOSE-TOLERANCE TEST ON GLUCOSE, INSULIN AND C-PEPTIDE RESPONSE IN HEALTHY, IMPAIRED GLUCOSE-TOLERANCE, MILD AND SEVERE NON-INSULIN-DEPENDENT DIABETIC SUBJECTS

Dietary constituents other than glucose can influence insulin secretion in non-insulin-dependent diabetes mellitus and administration of a standard mixed meal has been proposed as a more physiological test in regard to human diet for evaluating the patient both at the time of diagnosis and during follow-up. This study was carried out to compare the effects of a standard meal and the oral glucose tolerance test on glucose, insulin and C-peptide plasma levels in four groups of subjects: healthy controls, subjects with impaired glucose tolerance, patients with mild non-insulin-dependent diabetes, and non-insulin-dependent diabetic patients with secondary failure to oral agents. Plasma glucose values were significantly higher after the oral glucose tolerance test than after the mixed meal in all four groups of subjects. Plasma insulin and C-peptide values were similar during the two tests in all groups of subjects except in non-insulin-dependent diabetics with secondary failure (flattened curves). Insulin and C-peptide responses per unit rise in blood glucose were significantly higher after the oral glucose tolerance test than after the mixed meal both in mild non-insulin-dependent diabetics (P < 0.05 and P < 0.05) and in non-insulin-dependent diabetics in secondary failure (P < 0.01 and P < 0.05). There was significant correlation between oral glucose tolerance test and mixed meal glucose incremental areas (r = 0.511, P < 0.001). The diagnostic relevance of the test was evaluated by comparing the 120 min plasma glucose levels following mixed meal and oral glucose tolerance test: there was a significant correlation between the values in the two tests (r = 0.956, P < 0.001); the differences among the four groups were statistically significant and there was a partial overlap between healthy controls and impaired glucose tolerance patients: 93 +/- 3 versus 111 +/- 7 mg/dl for controls; 107 +/- 3 versus 161 +/- 5 mg/dl for impaired glucose tolerance patients; 204 +/- 11 versus 284 +/- 12 mg/dl for mild non-insulin-dependent diabetics; 309 +/- 9 versus 440 +/- 17 mg/dl for NID diabetics in secondary failure. The sensitivity of the test was 73% and specificity 100%. The mixed meal is proposed for clinical practice as a more physiological test than the standard oral glucose tolerance test for further characterization and longitudinal evaluation of patients with impaired glucose tolerance or non-insulin-dependent diabetes mellitus.