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

Abstract

Green tea or green tea extract (GT/GTE) has been demonstrated to reduce insulin resistance and improve glycemic control. However, evidence for this health beneficial effect is inconsistent. This systematic review evaluated the effect of GT/GTE on insulin resistance and glycemic control in people with pre-diabetes/type 2 diabetes mellitus (T2DM). 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 T2DM, where the intervention was GT/GTE. Meta-analysis was performed to assess the standardised mean difference (SMD) in biomarkers of insulin resistance and glycemic control between GT/GTE and placebo groups. Six studies (n=382) were pooled into random-effects meta-analysis. Overall, no differences were found between GT/GTE and the placebo for glycosylated hemoglobin (HbA1c: SMD, -0.32; 95% confidence interval [CI], -0.86 to 0.23), homeostatic model assessment for insulin resistance (HOMA-IR: SMD, 0.10; 95% CI, -0.17 to 0.38), fasting insulin (SMD, -0.25; 95% CI, -0.64 to 0.15), and fasting glucose (SMD, -0.10; 95% CI, -0.50 to 0.30). 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.

Keywords: Diabetes mellitus, type 2; Glycemic control; Green tea; Insulin resistance; Meta-analysis.

Fig. 1. Flow-chart of study selection and exclusion in details.

Adapted from www.mdpi.com/link.

Fig. 2. Risk of bias summary: review authors' judgements about each risk of bias item for each included study.

Fig. 3. Meta-analysis results for each assessed outcome. (A) Comparison between decaffeinated green tea extract and placebo, outcome: glycosylated hemoglobin (HbA1c, %). (B) Comparison between decaffeinated green tea extract and green tea extract, outcome: fasting glucose. (C) Comparison between decaffeinated green tea extract and green tea extract, outcome: fasting insulin. (D) Comparison between decaffeinated green tea extract and green tea extract versus placebo, outcome: homeostatic model assessment for insulin resistance (HOMA-IR). SD, standard deviation; IV, independent variable; CI, confidence interval.