High Rates of Abnormal Glucose Metabolism Detected by 75 g Oral Glucose Tolerance Test in Major Psychiatric Patients with Normal HbA1c and Fasting Glucose Levels

Objectives: Comorbid diabetes is an important factor in the treatment of major psychiatric disorders. However, a substantial proportion of diabetic patients remain undetected by routine diabetic indices such as blood glucose and HbA1c. This study tried to estimate rates of such unidentified diabetic or prediabetic patients by using a 75 g oral glucose tolerance test (OGTT). Methods: Participants in the test were 25 patients with major depressive disorder (MDD), 28 patients with bipolar disorder (BP), 26 patients with schizophrenia, and 28 psychiatrically normal controls. They were all Japanese, and showed non-diabetic levels of blood glucose (<126 mg/dL) and HbA1c (<6.0%). Results: Relatively high rates of psychiatric patients showed diabetes mellites (DM)-type abnormality (32% of MDD, 21.4% of BP and 42.3% of schizophrenia v. 10.7% of controls). The difference in the rates between schizophrenia and control groups was statistically significant (p = 0.008). When abnormal glucose metabolism was defined as a prediabetic state (either normal high glycemia, impaired fast glycemia or impaired glucose tolerance) or DM type in OGTT, it was more frequently seen in the psychiatric patients than in controls (64% of MDD, 46.4% of BP and 46.2% of schizophrenia v. 35.7% of controls). Individuals with DM type showed higher HbA1c values compared with those with normal (p < 0.001) and prediabetic (p = 0.021) states. Conclusions: The results suggest that relatively high proportions of patients with a major psychiatric disorder remain undetected by routine indices for abnormal glucose metabolism, indicating the importance of OGTT even if the patients showed non-diabetic levels in blood glucose or HbA1c.