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 ) .
Coffee Consumption and Risk for Type 2 Diabetes Mellitus
Context A recently published study in a Dutch population identified an association between higher coffee consumption and lower risk for type 2 diabetes mellitus  but did not distinguish between intake of regular and decaffeinated coffee . Contribution This large epidemiologic study in a U.S. sample confirmed the Dutch study findings . It also found a statistically significant protective association between total caffeine intake and type 2 diabetes mellitus and a modest inverse association with decaffeinated coffee consumption . Implication s Consumption of caffeinated coffee is associated with a reduced risk for type 2 diabetes mellitus . Further studies should explore the long-term effects of caffeine on glucose metabolism . The Editors The prevalence of type 2 diabetes mellitus has increased dramatically in the past decades , and the disease now affects approximately 8 % of U.S. adults ( 1 ) . Diet and lifestyle factors are primary determinants of risk for type 2 diabetes ( 2 ) . In addition to overweight and obesity , other independent behavioral and lifestyle factors include physical inactivity , smoking , and a diet higher in glycemic load and trans fatty acids and lower in fiber and polyunsaturated fatty acids . Coffee consumption may be related to diabetes because short-term metabolic studies have suggested that caffeine adversely affects insulin sensitivity and glucose metabolism ( 3 ) . Coffee consumption is widespread ; more than 50 % of Americans drink coffee , and average per capita intake is about 2 cups per day ( 4 ) . Coffee is the primary source of caffeine intake , providing approximately 210 mg/d per person in the United States . In humans , acute administration of caffeine decreases insulin sensitivity and impairs glucose tolerance ( 3 , 5 - 8 ) . On the other h and , caffeine stimulates thermogenesis and increases energy expenditure ( 9 - 11 ) , which may facilitate weight reduction and maintenance . Because of these complex physiologic effects of caffeine and because tolerance to the humoral and hemodynamic effects of caffeine typically develops with long-term use ( 12 ) , it is difficult to extrapolate findings from short-term metabolic studies to long-term use of coffee and other caffeinated beverages . A recent epidemiologic study has found a statistically significant inverse association between coffee consumption and risk for type 2 diabetes  in a sample of Dutch participants ( 13 ) . After adjustment for potential confounders , the relative risk for type 2 diabetes among participants consuming at least 7 cups of coffee per day as compared with those consuming 2 cups or less per day was 0.50 ( 95 % CI , 0.35 to 0.72 ; P < 0.001 for trend ) . The study , however , could not distinguish regular coffee from decaffeinated coffee and did not evaluate the association with total caffeine intake . In our study , we examined long-term intake of coffee and other caffeinated beverages and decaffeinated coffee in relation to incidence of type 2 diabetes in 2 large prospect i ve cohorts of men and women . In particular , we examined whether the associations were modified by smoking and body mass index . Methods Study Sample The Health Professionals Follow-up Study The Health Professionals Follow-up Study ( HPFS ) was established in 1986 when 51 529 male health professionals , including dentists , optometrists , veterinarians , osteopathic physicians , podiatrists , and pharmacists , who were 40 to 75 years of age returned a mailed question naire providing detailed information about their medical history , lifestyle , and other risk factors ( 14 ) . Information on the cohort is up date d every 2 years to identify newly diagnosed diseases . For our analysis , we excluded men with a previous diagnosis of type 2 diabetes , coronary heart disease , or cancer at baseline . Participants who did not complete more than 70 of 131 food items in the 1986 dietary question naire or had extreme scores for total daily intake of energy ( < 800 kcal or > 4200 kcal ) were excluded . After exclusions , the analysis included 41 934 eligible men who were followed from 1986 to 1998 . The Nurses ' Health Study The Nurses ' Health Study ( NHS ) was established in 1976 when 121 700 female nurses 30 to 55 years of age living in 11 large states completed a mailed question naire providing detailed information about their medical history , lifestyles , and other risk factors ( 15 ) . The information is up date d every 2 years to identify newly diagnosed diseases . In 1980 , a semiquantitative food frequency question naire was added . For our analysis , we excluded women with a previous diagnosis of type 2 diabetes , coronary heart disease , or cancer at baseline . Participants who did not complete more than 10 items on the 1980 dietary question naire or had extreme scores for total daily intake of energy ( < 500 kcal or > 3500 kcal ) were also excluded , leaving 84 276 eligible women who were followed from 1980 to 1998 . Assessment of Coffee and Caffeine Intake Vali date d dietary question naires were sent to the HPFS participants in 1986 , 1990 , and 1994 and to the NHS participants in 1980 , 1984 , 1986 , 1990 , and 1994 . In all the question naires , participants were asked how often on average during the previous year they had consumed coffee and tea . Decaffeinated coffee and different types of caffeinated soft drinks were first assessed in 1986 in the HPFS and in 1984 in the NHS . We assessed the total intake of caffeine  by summing the caffeine content for a specific amount of each food during the previous year ( 1 cup for coffee or tea , one 12-ounce bottle or can for carbonated beverages , and 1 ounce for chocolate ) multiplied by a weight proportional to the frequency of its use . The participants could choose from 9 frequency responses ( never , 1 to 3 per month , 1 per week , 2 to 4 per week , 5 to 6 per week , 1 per day , 2 to 3 per day , 4 to 5 per day , and 6 or more per day ) . Using the U.S. Department of Agriculture food composition sources , we estimated that the caffeine content was 137 mg per cup of coffee , 47 mg per cup of tea , 46 mg per bottle or can of cola beverage , and 7 mg per serving of chocolate c and y. In our validation studies , high correlations were recorded for coffee and other  caffeinated beverage intake assessed by the food frequency question naire and several 1-week diet records ( coffee , r = 0.78 ; tea , r = 0.93 ; and caffeinated sodas , r = 0.85 ) ( 16 ) . For these analyses , coffee consumption was categorized into 5 groups : never , less than 1 cup per day , 1 to 3 cups per day , 4 to 5 cups per day , and 6 cups or more per day . Caffeine intake was categorized into quintiles . Assessment of Diabetes Cases Participants who reported having diabetes were sent a supplementary mailed question naire that asked about symptoms , diagnostic tests , and treatment . We considered any 1 of the following criteria to represent a new case of diabetes : classic symptoms and elevated glucose levels ( fasting plasma glucose concentration 7.8 mmol/L [ 140 mg/dL ] or r and omly measured concentration 11.1 mmol/L [ 200 mg/dL ] ) ; at least 2 elevated plasma glucose concentrations on different occasions in the absence of symptoms ( plasma glucose level 11.1 mmol/L [ 200 mg/dL ] 2 or more hours after oral glucose tolerance testing ) ; or treatment with oral hypoglycemic agents or insulin . Our criteria for diabetes classification are consistent with those proposed by the National Diabetes Data Group ( 17 ) because most cases were diagnosed before 1997 . The validity of this diagnostic procedure has been verified in a sub sample of this study sample ( 18 ) . The diagnosis of type 2 diabetes was confirmed by medical records in 98 % of the participants . In addition , another sub study assessing the prevalence of undiagnosed diabetes suggested a very low rate of false-negative results ( 19 ) . Statistical Analysis We calculated the follow-up period from the return of the  baseline question naires ( in 1986 for men and in 1980 for women ) to the diagnosis of type 2 diabetes , death , or the end of follow-up ( January 1998 for men and June 1998 for women ) , whichever occurred first . Incidence rates for diabetes were calculated by dividing incident cases by person-years of follow-up in each category of coffee intake .  Relative risks were calculated as the rate of occurrence of type 2 diabetes in each quintile of caffeine intake divided by the corresponding rate in the lowest quintile , and 95 % CIs were calculated . To inspect potential confounding variables for the analyses , we examined baseline characteristics ( directly st and ardized to the age distribution of each cohort ) according to coffee consumption levels . We used proportional hazards models ( 20 ) to adjust for several risk factors , including age in 5 categories ( < 50 , 50 to 54 , 55 to 59 , 60 to 64 , or 65 years of age ) ; body mass index ( weight divided by height [ kg/m2 ] ) in 8 categories ( < 23 , 23 to 24 , 25 to 28 , 29 to 30 , 31 to 32 , 33 to 34 , 35 to 39 , or 40 kg/m2 ) ; physical activity ( hours of moderate or vigorous exercise in NHS or quintiles of metabolic equivalents in HPFS ) ; family history of diabetes ( yes or no ) ; hormone use ( never , current , or past [ in NHS only ] ) ; smoking status ( never , past , or current smokers [ 1 to 14 , 15 to 24 , or 25 cigarettes per day ] ) ; alcohol consumption ( 0 , 0.1 to 4.9 , 5 to 9.9 , 10 to 14.9 , or 15 g/d ) ; total calorie intake ( continuous ) ; and quintiles of trans fat , glycemic load , and cereal fiber intake . We also adjusted for magnesium intake because our previous analyses have shown an inverse association between dietary magnesium and risk for diabetes ( 21 ) . Information on covariates was up date d periodically during follow-up . We performed a test for trend by treating median values of each category of caffeine and coffee intake as continuous variables . To represent long-term intake of coffee and caffeine and to reduce measurement error , we conducted analyses using cumulative up date d caffeine and coffee intake from all question naires ( 22 ) . For example , in the NHS ,  diabetes incidence between 1980 and 1984 was related to the coffee intake from the 1980
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 ) .