Assessing the Shape of the Glucose Curve During an Oral Glucose Tolerance Test

Abstract

OBJECTIVE -The oral glucose tolerance test (OGTT) is used to define the status of glucose tolerance based on the plasma glucose level at 120 min. The purpose of the present study was to identify parameters that determine the shape of the plasma glucose course measured at 0, 30, 60, 90, and 120 min during an OGTT.RESEARCH DESIGN AND METHODS -OGTT data from 551 subjects (485 with normal glucose tolerance [NGT] and 66 with impaired glucose tolerance [IGT]) were analyzed. We distinguished between "monophasic," "biphasic," and unclassified glucose shapes. A "shape" index based on the extent and the direction of the plasma glucose change in the second hour allowed us to treat shape as a continuous variable.RESULTS -In the biphasic group, the NGT-to-IGT ratio was slightly higher (173/20 vs. 209/40, P ϭ 0.08) and the male-to-female ratio was lower (60/133 vs. 120/129, P ϭ 0.0003). Subjects with a biphasic shape had significantly lower age, BMI, waist-to-hip ratio (WHR), HbA 1c , plasma glucose, and area under the insulin curve (insulin AUC ) and a better estimated insulin sensitivity and secretion (using validated indexes) than monophasic subjects (all P Ͻ 0.05). By adjusting this shape index for glucose AUC (as continuous measure of glucose tolerance), correlations with age, BMI, WHR, HbA 1c , and insulin AUC were completely abolished. The adjusted shape index was still higher in female than in male subjects but lower in IGT than in NGT subjects (both P ϭ 0.0003). Finally, we tested common polymorphisms in insulin receptor substrate (IRS)-1, IRS-2, calpain-10, hepatic lipase, and peroxisome proliferator-activated receptor-␥ for association with the shape index.CONCLUSIONS -We conclude that the plasma glucose shape during an OGTT depends on glucose tolerance and sex. In addition, genetic factors seem to play a role. The shape index may be a useful metabolic screening parameter in epidemiological and genetic association studies. Diabetes Care 26:1026 -1033, 2003T he oral glucose tolerance test (OGTT) has traditionally been used to classify the status of glucose tolerance for diagnostic purposes: normal glucose tolerance (NGT) versus impaired glucose tolerance (IGT) versus diabetes (1). More recently, however, some authors have attempted to exploit the information contained in a 2-h OGTT to estimate insulin sensitivity (2-4) and ␤-cell function (5). While the derived indexes are less accurate than the respective gold-standard methods, they can be obtained more easily and used in large epidemiological or genetic association studies.These indexes take advantage of glucose and insulin concentrations at specific time points during the OGTT. To the best of our knowledge, with one exception, nobody has tried to answer the question of whether the shape of the glucose curve over time during the OGTT has any relevance. The one study addressing this issue that we found in the literature is a paper written in Japanese (with an abstract in English), where the authors classified the glucose curve during the OGTT as "biphasic," "domed...

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