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 (T2 DM). 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 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 ( HOMA_IR) index ( SMD −0.06; 95% CI −0.35 to 0.23) in participants at risk of T2 DM. 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 HOMA_IR in populations at risk of T2 DM. Larger, longer-term 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 T2 DM.