Fukino Y et al. (2008) Randomized controlled trial for an effect of green tea-extract powder supplementation on glucose abnormalities. Eur J Clin Nutr 62: 953–960

Green tea is believed to have several beneficial effects on health including antioxidant activity and an ability to reduce blood cholesterol. Findings from a new study suggest green tea also has beneficial effects on glycemic abnormalities in patients with impaired glucose tolerance.

Fukino and colleagues conducted a randomized, controlled, crossover study in Japan to investigate the effects of green tea extract on parameters of glucose metabolism in people with elevated blood glucose levels. A total of 60 individuals were randomly allocated to one of two groups—each participant consumed a packet of green tea extract that contained 544 mg polyphenols daily for 2 months before or after a 2-month nonintervention period. Biomarkers of glucose and lipid metabolism were measured in both groups.

A significant reduction in glycated hemoglobin levels was observed in both groups during consumption of the green tea extract. A nonsignificant trend for reduced diastolic blood pressure was also apparent. Other parameters of glucose metabolism were not significantly changed.

One important feature of this study is that individuals were permitted to continue their normal consumption of green tea in addition to taking the green tea extract. The authors speculate that this additional consumption might account of the lack of significant change in other indices of glucose metabolism. Importantly, however, this study demonstrates that an increase in polyphenol consumption that corresponds to approximately three cups of green tea can reduce glycated hemoglobin levels in patients with impaired glucose tolerance, who are at risk of diabetes.