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 18-69years. 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 β-cell function (HOMA-β) 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≥25kg/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-β 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.