Data from a combined total of 1,133 subjects indicated that green tea consumption was associated with significantly lower blood sugar levels during fasting, and lower blood concentrations of hemoglobin A1c (HbA1c), a marker of long-term presence of excess glucose in the blood.
Subsequent analysis also showed that green tea consumption was associated with reduced fasting insulin levels, according to the paper in the American Journal of Clinical Nutrition.
“The current analyses showed slight inconsistencies in the effects of green tea on glycemic measures in the subgroup analyses,” wrote researchers from the Third Military Medical University in Chongqing, China.
“Green tea consumption significantly decreased fasting glucose concentrations only in studies that included subjects with risk of the metabolic syndrome. This suggested that the beneficial effect of green tea (ie, lowering fasting glucose concentrations) might be more pronounced in subjects with factors that increase their risk of the metabolic syndrome.”
The study adds to an ever-growing body of science supporting the potential benefits of green tea and its constituents, most notably EGCG (epigallocatechin gallate). To date green tea has been linked to a reduced risk of Alzheimer's and certain cancers, improved cardiovascular and oral health, as well as benefits in weight management.
Green tea contains between 30 and 40 per cent of water-extractable polyphenols, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 per cent. Oolong tea is semi-fermented tea and is somewhere between green and black tea. The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC).
As covered recently by NutraIngredients-USA, scientists from Oklahoma State University and the University of Oklahoma reported data from the first clinical investigation of the effects of green tea supplementation on endogenous antioxidant markers in obese participants with the metabolic syndrome (Nutrition Research, Vol. 33, pp. 180-187).
The Chinese scientists searched the literature for relevant studies using green tea and green tea extracts, and identified 17 randomized controlled trials. Nine trials used green tea extracts provided as capsules with doses ranging from about 200 to 800 mg of EGCG, and the other eight trials used beverages with 364-582 mg of catechins. The catechin dose for all 17 trials ranged from 208 to 1207 mg.
Crunching the numbers revealed that green tea consumption was associated with 0.09 mmol/L lower fasting glucose levels, and 0.3% lower HbA1c concentrations, compared with the non-green tea groups.
“The results of the meta-regression analysis did not show a significant dose-response effect between green tea and fasting glucose or fasting insulin concentrations,” wrote the researchers. “Thus, it is difficult to determine the optimal dose for a dietary program as part of a health policy aimed at improving diabetic health.”
“Additional long-term and high-quality RCTs specifically designed to evaluate the effects of green tea on glucose control and insulin sensitivity are needed to further evaluate and confirm these findings,” they concluded.
Source: American Journal of Clinical Nutrition
Published online ahead of print, doi: 10.3945/ajcn.112.052746
“Effect of green tea on glucose control and insulin sensitivity: a meta-analysis of 17 randomized controlled trials”
Authors: Kai Liu, Rui Zhou, Bin Wang, Ka Chen, Lin-Ying Shi, Jun-Dong Zhu, and Man-Tian Mi