The relationship between plasma glucose levels during an oral glucose tolerance test (OGTT) in the morn-ing and in the afternoon is interesting and has been reported previously. The purpose of this study was to examine the potential differences in insulin action and secretion between morning and afternoon OGTTs. Method: Twenty-two out-patients (mean age, 52.4±2.4 years) comprising 6 patients with normal glucose tolerance, 10 patients with impaired glu-cose tolerance and 6 patients with diabetes were studied. All subjects received separately OGTT in the morning and af-
ternoon (M-OGTT and A-OGTT, respectively) and a frequently sampled intravenous glucose tolerance test (FSIGT). The surrogates of insulin resistance (IROGTT) and insulin sensitivity (SIOGTT) were compared with the insulin sensitivity derived from FSIGT (SIFSIGT). Results: OGTT-derived insulin secretion (ISECOGTT) was compared with the acute insulin response to glucose (AIRg) derived from FSIGT. The IROGTT and SIOGTT during M-OGTT unanimously indicated higher insulin re-sistance and were signifi cantly better correlated with SIFSIGT (mean r = 0.62 vs. 0.47, p < 0.01) than those during A-OGTT. Moreover, compared with A-OGTT values, ISECOGTT values were higher during M-OGTT and also better correlated with
AIRg (mean r = 0.41 vs. 0.20, p = 0.021). Conclusion: Higher insulin resistance and insulin secretion were noted during M-OGTT. Insulin action and secretion during M-OGTT were better correlated with SIFISGT and AIRg, respectively. These signifi cant differences in glucose metabolism suggest that OGTT in the afternoon may be less reliable in evaluating insu-lin action and secretion or diagnosing diabetes.