Effects of green tea or green tea extract on insulin sensitivity and glycaemic control in populations at risk of type 2 diabetes mellitus: a systematic review and meta-analysis of randomised controlled trials

Background: Although the regular consumption of green tea or green tea extract has been considered to improve insulin sensitivity, the reported results are inconsistent. Therefore, we conducted a meta-analysis to evaluate the effect of green tea or green tea extract on insulin sensitivity and glycaemic control in populations at risk of type 2 diabetes mellitus (T2DM). Methods: Electronic databases, including PUBMED, The Cochrane Library, EMBASE, ISI Web of Knowledge, Chinese Biomedical Literature Database and Chinese Scientific Journals Fulltext Database, were systematically searched to identify randomised controlled trials (RCTs) up to December 2011, supplemented by the Clinicaltrials.gov websites and the reference lists of identified studies. Two reviewers independently selected trials, extracted data, and evaluated the methodological qualities and evidence levels. Results: Seven RCTs involving 510 participants were identified. There was no statistically significant difference between green tea or green tea extract group and placebo group with regard to fasting plasma glucose [standardised mean difference (SMD) 0.04; 95% confidence interval (CI) -0.15 to 0.24], fasting serum insulin (SMD -0.09; 95% CI -0.30 to 0.11), 2-h plasma glucose in the oral glucose tolerance test (OGTT-2 h) (SMD -0.14; 95% CI -0.63 to 0.34), haemoglobin A1c (SMD 0.10; 95% CI -0.13 to 0.33) and homeostasis model of insulin resistance (HOMAIR) index (SMD -0.06; 95% CI -0.35 to 0.23) in participants at risk of T2DM. Conclusions: The consumption of green tea did not decrease the levels of fasting plasma glucose, fasting serum insulin, OGTT -2 h glucose, haemoglobin A1c and HOMAIR in populations at risk of T2DM. Larger, longerterm and high-quality RCTs are needed to further definitely determine the effect of green tea or green tea extract on insulin sensitivity and glycaemic control in populations at risk of T2DM.