The Effectiveness of Green Tea or Green Tea Extract on Insulin Resistance and Glycemic Control in Type 2 Diabetes Mellitus: A Meta-Analysis

Background: Green tea or green tea extract (GT/GTE) has been demonstrated to reduce insulin resistance and improve glycaemic control. However, evidence for this health beneficial effect is inconsistent. This systematic review evaluated the effect of GT/GTE on insulin resistance and glycaemic control in people with pre-diabetes/type 2 diabetes mellitus (T2DM).

Methods: Ovid MEDLINE, Embase, AMED, Web of Science and the Cochrane library were searched up to April 2017 for Randomised Controlled Trials of participants with pre-diabetes or type 2 diabetes, where the intervention was GT/GTE. Meta-analysis was performed to assess the standardised mean difference (SMD) in biomarkers of insulin resistance and glycaemic control between GT/GTE and placebo groups.

Results: Six studies (n= 382) were pooled into random-effects meta-analysis. Overall, no differences were found between GT/GTE and the placebo for HbA1c (SMD -0.32; 95%CI -0.86, 0.23), HOMA-IR (SMD 0.10; 95%CI -0.17, 0.38), fasting insulin (SMD -0.25; 95%CI -0.64, 0.15), and fasting glucose (SMD -0.10; 95%CI -0.50, 0.3).

Conclusions: No evidence support the consumption of GT/GTE could reduce the levels of HbA1c, HOMA-IR, fasting insulin or fasting glucose in people with pre-diabetes/T2DM. However, the studies included were small and of varying quality.