Antihyperglycemic Effect of Oolong Tea in Type 2 Diabetes

Abstract

OBJECTIVE -To determine the efficacy of oolong tea for lowering plasma glucose in type 2 diabetic patients in Miaoli, Taiwan. RESEARCH DESIGN AND METHODS-A total of 20 free-living subjects who had type 2 diabetes and took hyperglycemic drugs as prescribed were enrolled in the present study. Subjects consumed oolong tea (1,500 ml) or water for 30 days each in a randomized crossover design. Tea was not consumed for 14 days prior to treatments.RESULTS -Relative to initial concentrations, oolong tea markedly lowered concentrations of plasma glucose (from 229 Ϯ 53.9 to 162.2 Ϯ 29.7 mg/dl, P Ͻ 0.001) and fructosamine (from 409.9 Ϯ 96.1 to 323.3 Ϯ 56.4 mol/l, P Ͻ 0.01), whereas the water control group had not changed (208.7 Ϯ 61.0 vs. 232.3 Ϯ 63.1 mg/dl for glucose and from 368.4 Ϯ 85.0 to 340.0 Ϯ 76.1 mol/l for fructosamine).CONCLUSIONS -Oolong tea may be an effective adjunct to oral hypoglycemic agents in the treatment of type 2 diabetes. Diabetes Care 26:1714 -1718, 2003R ecently, consumption of tea has become even more popular in Taiwan, Japan, and China. Consumption of tea leaves in Taiwan increased 2.2 times during last 20 years (1). Tea consumption may have an impact on plasma glucose concentrations. The concept was supported by in vivo studies of tea fed to diabetic rodents (2-4) and by in vitro studies where tea components were incubated with brush border membrane vesicles of rabbit small intestine (5).Caffeine is one of the most important components in tea. There are some studies examining caffeine, for example, in healthy volunteers (6,7), in type 1 diabetes (8 -10), and in type 2 diabetes (11). However, the efficacy of tea in lowering plasma glucose concentrations in humans was limited.There are three types of tea: green, oolong, and black. They are produced from a single plant species, but they are distinguished by the processing technique. Oolong tea is partially fermented during processing, whereas green tea is not fermented, and black tea is fully fermented (12).In general, the amount of the polymerized polyphenols is determined by the level of the fermentation. The degree of fermentation of oolong tea leaves from Taiwan is very low because of the preparation method. In contrast, oolong tea leaves from China contained an especially large amount of polymerized polyphenols, and fermentation was strong (13). For these reasons, oolong tea from China was used in this study.The purpose of this study was to assess the ability of oolong tea to lower plasma glucose concentrations in subjects with type 2 diabetes. RESEARCH DESIGN AND METHODS -The ethical committee of Providence University, an institutional review board, approved the study, and subjects gave informed consent. A total of 10 men and 10 women (average age 61.2 years, duration of diabetes 4.8 years) were recruited through their physicians at Chorng Kuang Hospital, Miaoli, Taiwan.The study used a crossover design; sex-matched subjects were randomly assigned to treatment order. Subjects were divided into two groups at random. The washout period was set ...

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