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 population s at risk of T2DM .
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 population s at risk of type 2 diabetes mellitus ( T2DM ) .
The Relationship between Green Tea and Total Caffeine Intake and Risk for Self-Reported Type 2 Diabetes among Japanese Adults
Context Caffeine intake from coffee has been associated with a lower incidence of diabetes . Research ers have not studied the relationship of green tea , a popular beverage in Japan , where the incidence of diabetes is increasing rapidly . Contributions The authors estimated the intake of caffeine-containing beverages in a community-based survey in Japan and measured the 5-year incidence of diabetes . Greater intake of green tea or coffee was associated with a reduced incidence of diabetes . The effect of green tea was largely observed in women , was dose-related , and reflected caffeine intake . Implication s Higher intake of caffeine , whether from coffee or green tea , is associated with a lower incidence of diabetes . The Editors The prevalence of type 2 diabetes has increased worldwide , particularly in Asian countries where it was previously low ( 1 ) . In Japan , population -based studies have shown a 2-fold increase in the prevalence of diabetes during the past 2 decades , from 5 % to 10 % to 10 % to 15 % ( 2 ) . Several cohort studies done in Europe and in the United States reported an association between coffee consumption , a major source of caffeine , and reduced risk for diabetes ( 3 - 7 ) . Although these studies did not show any association between consumption of black tea and the risk for diabetes , they did not examine the effect of green or oolong teas , major sources of caffeine in Asian countries . Consumption of green tea is common in Japan ; 80 % of the population drinks green tea , and the average consumption per capita is 2 cups per day ( 8) . We wanted to determine whether there is a relationship between consumption of green tea and the risk for type 2 diabetes and , if so , whether caffeine fully accounts for this relationship . To examine these questions , we analyzed data from a large cohort study of 19487 middle-aged men and women in 25 communities across Japan . We also examined the effect of age , sex , body mass index ( BMI ) , family history , smoking status , alcohol use , magnesium intake , and physical activity on the association between this mode of caffeine consumption and risk for diabetes . Methods The Japan Collaborative Cohort Study for Evaluation of Cancer Risk ( JACC Study ) started between 1988 and 1990 . A total of 110792 individuals ( 46465 men and 64327 women ) who were 40 to 79 years of age and living in 45 communities across Japan participated in municipal health screening examinations and completed self-administered question naires regarding lifestyle and medical history of cardiovascular disease and cancer ( 9 ) . Informed consent was obtained before the completion of the question naire . Participants from 25 of the 45 communities completed 5-year follow-up surveys . Among 35690 participants ( 15177 men and 20513 women ) who were 40 to 65 years of age at baseline without a history of type 2 diabetes , stroke , coronary heart disease , or cancer , 17413 individuals ( 49 % ; 6727 men and 10686 women ) completed the 5-year follow-up question naire and provided valid responses on tea or coffee consumption and history of type 2 diabetes . The data from these 17413 individuals were used for the analyses . The mean age of the nonrespondents was 1 year younger for men ( 52.3 vs. 53.3 years of age ) and did not differ for women ( 53.1 vs. 53.0 years of age ) compared with the respondents . The mean BMI and the prevalence of a BMI of 25.0 kg/m2 or greater did not differ between the respondents and nonrespondents . Mean BMI was 22.7 kg/m2 versus 22.8 kg/m2 for men and 22.8 kg/m2 versus 22.9 kg/m2 for women , respectively ; the prevalence of overweight was 18 % versus 19 % for men and 21 % versus 22 % for women , respectively . The ethical committees at Nagoya University and the University of Tsukuba approved the study . Assessment of Consumption of Tea and Coffee and Caffeine Intake At baseline , consumption of tea and coffee  was assessed by using a self-administered dietary question naire . Participants were asked to state their average consumption of green tea , black tea , oolong tea , and coffee during the previous year . They could select any of 4 frequency responses : less than once a week , about 1 to 2 times a week , about 3 to 4 times a week , and almost every day . Participants who selected the response of almost every day were also asked to state their average consumption of these beverages in number of cups per day . We classified the categories of consumption as less than 1 cup per week , 1 to 6 cups per week , 1 to 2 cups per day , 3 to 5 cups per day , and 6 or more cups per day . The highest 2 or 3 consumption categories were combined for coffee , black tea , and oolong tea because of the small number of participants in these categories . The consumption of decaffeinated coffee or tea  was not recorded because these products were not commercially available in Japan in the early 1990s . The total intake of caffeine was calculated by adding the caffeine content from each specific beverage ( 1 cup for coffee or tea ) and multiplying it by the participant 's weight proportional to the frequency of caffeine use . We estimated the size of the cup for each beverage from a validation study ( 10 ) and the caffeine content per 100 mL of each beverage from the Japan Food Tables ( 11 ) . The estimated caffeine content was 153 mg per cup ( 170 mL ) of coffee , 30 mg per cup ( 200 mL ) of green tea , 51 mg per cup ( 170 mL ) of black tea , and 38 mg per cup ( 190 mL ) of oolong tea . The mean caffeine intake was 229 mg/d for men and 215 mg/d for women .  Relative proportions of caffeine intake by beverage were 46 % from green tea , 44 % to 47 % from coffee , 3 % from black tea , and 4 % to 5 % from oolong tea . For reproducibility , the Spearman correlation coefficients between the 2 question naires , administered 1 year apart for 85 participants ( 8 men and 77 women ) , were 0.79 for green tea , 0.87 for coffee , 0.77 for black tea , and 0.56 for oolong tea ( 10 ) . The validity of the data was confirmed for the 85 participants by comparing the data from the question naire with those from four 3-day dietary records collected approximately 3 to 4 months apart ( 10 ) . The mean frequency of consumption of green tea  was 25.4 cups per week according to the question naire and 30.1 cups per week according to four 1-week dietary records ( Spearman correlation coefficient , 0.47 ) . The respective mean frequencies were 8.0 cups and 7.1 cups per week with a correlation coefficient of 0.79 for coffee , 1.4 cups and 1.6 cups per week with a correlation coefficient of 0.70 for black tea , and 1.8 cups and 1.2 cups per week with a correlation coefficient of 0.55 for oolong tea . When we restricted the data to the 77 women , the results were essentially the same . Assessment of Diabetes Cases Participants who reported having diabetes newly diagnosed by physicians on the 5-year follow-up question naire were considered to have incident diabetes . To examine the validity of self-reporting of diabetes , we compared self-report data with laboratory findings and treatment status in a sample of 1230 men and 1837 women . We considered elevated glucose concentrations ( fasting serum glucose concentration 7.8 mmol/L [ 140 mg/dL ] or a r and omly measured concentration of 11.1 mmol/L [ 200 mg/dL ] ) or treatment with oral hypoglycemic agents or insulin to indicate new cases of diabetes . Recent criteria from the American Diabetes Association ( 12 ) were not used because the cases in our study were diagnosed before 1995 . The sensitivity of self-reporting was 70 % for men and 75 % for women ; the specificity was 95 % for men and 98 % for women . Statistical Analysis To examine potential confounding variables reported from previous studies ( 3 - 7 ) , we presented baseline characteristics according to the frequency of consumption for each beverage . Tests for trends were conducted by using the median values of confounding variables in each category of beverage ; the linear regression model was used for continuous variables , and the logistic regression model was used for categorical variables . The odds ratios for incident type 2 diabetes were calculated in each category of beverage consumption and in each quartile of caffeine intake ; less than 1 cup per week or the lowest quartile was used as the reference category . We estimated age , sex , and BMI -adjusted odds ratios and multivariable odds ratios using the logistic regression model , adjusting for age ( in years ) , sex , sex-specific quintiles of BMI ( weight in kilograms divided by the square of height in meters ) , parental history of diabetes ( yes or no ) , smoking status ( never , former , or current [ 1 to 19 , 20 to 29 , or 30 cigarettes/d ] ) , alcohol intake ( never , former , or current [ 1 to 22 , 23 to 45 , 46 to 68 , or 69 g/d ] ) , sex-specific quintiles of magnesium intake , hours of walking ( < 0.5 , 0.5 , 0.6 to 0.9 , and 1.0 h/d ) , and hours of participation in sports ( < 1 , 1 to 2 , 3 to 4 , and 5 h/wk ) . Sex-specific quintiles of BMI and magnesium intake were used because of different distributions between the sexes . We adjusted for magnesium intake because previous cohort studies indicated an inverse association between magnesium intake and risk for diabetes ( 13 , 14 ) . We conducted a test for trend by treating median values of each category of beverage or caffeine intake as continuous variables . We examined the association between caffeine intake and the risk for diabetes stratified by age group ( 40 to 54 years and 55 to 65 years ) , sex , family history of diabetes ( yes or no ) , current smoking status ( yes or no ) ,  current alcohol intake ( yes or no ) ,  magnesium  intake ( below and above the sex-specific median ) , BMI ( < 25.0 kg/m2 and 25.0 kg/m2 ) , hours of walking ( < 0.5 and 0.5 h/d ) , and hours of participation in sports ( < 5 and 5 h/wk ) . The interactions with these stratified variables were tested by using cross-product terms of caffeine intake and the stratified variables . All analyses were conducted by using the SAS statistical package , version 8.2 ( SAS Institute Inc. , Cary , North Carolina ) . P values for statistical tests were 2-tailed , and 95 % CIs were estimated . Role of
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 population s at risk of T2DM .
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 population s at risk of type 2 diabetes mellitus ( T2DM ) .