Coffee and green tea consumption is associated with insulin resistance in Japanese adults

Objective. Higher coffee and green tea consumption has been suggested to decrease risk of type 2 diabetes, but their roles in insulin resistance (IR) and insulin secretion remain unclear. This study examined the association between habitual consumption of these beverages and markers of glucose metabolism in a Japanese working population. Materials/Methods. Participants were 1440 Japanese employees (1151 men and 289 women) aged 18-69 years. Consumption of coffee and green tea was ascertained via a validated brief diet history questionnaire. Multilevel linear regression was used to estimate means (95% confidence intervals) of fasting insulin, fasting plasma glucose, homeostatic model assessment of IR (HOMA-IR), homeostatic model assessment of 3-cell function (HOMA-beta) and glycated hemoglobin (HbA1c) with adjustment for potential confounding variables. Results. Coffee consumption was significantly, inversely associated with HOMA-IR (P for trend = 0.03), and the association appeared to be confined to overweight subjects (BMI >= 25 kg/m(2)) (P for trend = 0.01, P for interaction = 0.08). Unexpectedly, green tea consumption was positively associated with HOMA-IR (P for trend = 0.02), though there was no dose-response relationship among daily consumers of green tea. Neither coffee nor green tea consumption was associated with HOMA-beta and HbA1c. Conclusions. Our findings indicate that coffee consumption may be associated with decreased IR, but not with insulin secretion. The positive association between green tea consumption and IR warrants further investigation. (C) 2014 Elsevier Inc. All rights reserved.