Excess glycaemic excursions after an oral glucose tolerance test compared with a mixed meal challenge and self-measured home glucose profiles: is the OGTT a valid predictor of postprandial hyperglycaemia and vice versa?

ABSTRACT

INTRODUCTION: Postprandial hyperglycaemia is often assumed in individuals with high glucose excursions during an oral glucose tolerance test (OGTT), but the relationship between glucose levels during the OGTT and after a mixed meal is yet unclear. We addressed whether (i) glucose concentrations after an oral glucose load are similar to those after a test meal or under daily life conditions and (ii) impaired glucose tolerance (IGT) predicts postprandial hyperglycaemia. PATIENTS AND METHODS: A total of 60 subjects with normal (NGT), IGT or diabetic (DM) glucose tolerance were examined with an OGTT, a mixed meal challenge (3433 kJ) and a self-determined 10-point home glucose profile. RESULTS: There was a significant correlation between the 120-min OGTT glucose levels and the glycaemic excursions after the test meal and during everyday conditions. However, glucose excursions during the OGTT exceeded those after the test meal and during everyday conditions by approximately 20 and approximately 30% respectively. Likewise, insulin and C-peptide levels rose to higher levels after oral glucose compared with mixed meal ingestion. The mean self-determined diurnal glucose levels were already 10% higher in subjects with IGT compared with NGT subjects (p < 0.0001). CONCLUSIONS: Glucose levels reached after an oral glucose challenge and during real life are correlated to some extent, but the absolute levels of glycaemia greatly differ between both conditions. Therefore, 'postchallenge' glucose levels measured during an OGTT might be used as a predictor of 'postprandial hyperglycaemia', but caution should be taken when both terms are used synonymously. Furthermore, subjects with IGT during an OGTT already exhibit increased postprandial glucose levels under real-life conditions. This suggests that IGT should already be considered an overt disease condition rather than merely a high-risk situation.