Plasma glucose levels were higher 120 min after ingestion of the meal with green tea than after the ingestion of the meal with water. No significant differences were found in serum insulin levels, or the area under the curve for glucose or insulin. Subjects reported significantly higher satiety, having a less strong desire to eat their favorite food and finding it less pleasant to eat another mouthful of the same food after drinking green tea compared to water.
Conclusions
Green tea showed no glucose or insulin-lowering effect. However, increased satiety and fullness were reported by the participants after the consumption of green tea.
Trial registration number
NCT01086189
Background
Tea is the second most commonly consumed beverage worldwide after water. Green tea is produced from the plant Camellia sinensis. The compounds thought to contribute to the health-promoting effects ascribed to green tea are polyphenolic compounds called catechins [1]. There are four major catechins in green tea: epicatechin (EC), epicatechin gallate (ECG), epigallocatechin (EGC) and epigallocatechin gallate (EGCG), of which EGCG is the most abundant. The focus of many previous studies on green tea has been on the anti-oxidative properties of catechins, and their potential role in preventing cancer and cardiovascular disease [2]. Green tea may also have a beneficial effect on glucose tolerance and the risk of developing diabetes. In a large cohort study on green tea, frequent consumption was found to be inversely associated with the risk of type 2 diabetes among Japanese women [3]. A cross-sectional study in Japan revealed an inverse correlation between daily consumption of green tea at a high concentration and fasting glucose levels in male subjects [4]. Intervention studies with green tea extract (GTE) in healthy rodents [5] and humans [6] have demonstrated increased insulin sensitivity after an oral glucose tolerance test (OGTT) based on lower insulin levels and unchanged glucose levels. Furthermore, EGCG was the catechin found to have most insulin-enhancing activity in an animal in vitro study [7]. Several randomized controlled studies investigating the effect of green tea on glucose metabolism have been performed in humans, although with varying results. A crossover trial demonstrated that two months' supplementation with GTE significantly lowered HbA1c in individuals with glucose abnormalities [8]. In contrast, no significant effect on HbA1c was seen after a 3-month trial with GTE supplementation in patients with type 2 diabetes [9]. A crossover study performed on healthy human participants showed that green tea lowered glucose levels after OGTT [10]. In several studies, neither GTE nor EGCG was found to have any effect on fasting glucose, insulin sensitivity or glucose levels after OGTT [11-13].
Changes in lifestyle, such as increased energy intake and decreased physical activity, are causing overweight and obesity, leading to an epidemic increase in type 2 diabetes. Low-glycemic index (GI) diets are associated with lower risk of type 2 diabetes and heart disease [14] and can be useful in the management of glucose levels in patients with diabetes [15]. To our best knowledge, this is the first study to examine the effect of green tea on both the glucose metabolism and satiety, after the ingestion of a regular meal. The primary objective of this study was to determine whether ingestion of a regular meal and green tea lower postprandial plasma glucose levels, glycemic index, and insulin levels. The secondary objective, was to establish whether consumption of a regular meal including green tea increase the satiety. This study was therefore conducted to examine the postprandial effects of green tea on glucose levels, glycemic index, insulin levels and satiety in healthy individuals after the consumption of a meal including green tea.
Methods
Fifteen healthy subjects volunteered to participate in the study. One subject was excluded on the first occasion due to an inability to ingest the food within the required time. Data were thus collected for seven male and seven female participants [(mean ± SD): age 27 ± 3 years (range 22-35 years); body mass index 22.3 ± 3.4 kg/m2 (range 17.0-30.8 kg/m2)]. One subject was a smoker, one was a snuff user and one subject used inhalator agents for asthma. All subjects were recruited from the student population in southern Sweden, and provided their written informed consent. The study was approved by the Ethics Committee of Lund University, and performed according to the Helsinki Declaration. The trial is registered in the US National Library of Medicine with the trial registration number NCT01086189. Subjects received a financial reward for their participation.
The meal consisted of 100 g white bread (Skogaholms Originalrost, Bageri Skogaholm AB, Eskilstuna, Sweden) containing 50 g carbohydrates, 8 g protein, 3 g fat, and 2.5 g dietary fiber. In order to resemble a normal meal, 25 g smoked turkey was added, containing 4.5 g protein, 0.75 g fat, and 0.25 g carbohydrates (Cascina serena, H. Kemper GmbH & Co. KG, Nortrup, Germany). The total amount of carbohydrates in the meal was 50 g, as recommended by Brouns et al., in their description of GI methodology [16]. The same meal was served with either green tea (green tea meal) or hot water (reference meal). The tea, Japanese Sencha Makato (AFTEK Te & Kryddor AB, Arbrå, Sweden) was prepared by brewing 9.00 g of loose-leaf green tea in 300 ml water (initial temperature 80-85°C) for 3 min. The serving temperature of the beverages was 60-65°C. The amount of caffeine in the brewed tea was 26.5 mg/100 ml, and the amounts of catechins were: EC 8.5 mg/100 ml, ECG 29.9 mg/100 ml, EGC <1.0 mg/100 ml, and EGCG 10.8 mg/100 ml.
The design of the study was a crossover randomized control trial without blinding. The study was conducted between 25 January and 11 February 2010. The subjects attended the clinical research department (Skåne University Hospital, Malmö, Sweden) on two different occasions following a minimum 10-h overnight fast. Smoking, snuff taking and medication were prohibited in the morning prior to and during the test. After obtaining a fasting blood sample by finger-prick, venous blood was collected from an indwelling plastic catheter for insulin analysis. The subjects were randomly assigned to either the green tea group or the hot water group, at intervals of at least one week. Each meal was to be consumed within 10 min, after which further blood samples (as described above) were taken at 15, 30, 45, 60, 90, and 120 min after the start of the meal. A validated visual analog score (VAS) was used to assess the participants' subjective satiety on both occasions, according to the method of Hauber et al. based on a scoring system from -10 (extreme hunger) to + 10 (extreme satiety) [17]. A more extensive questionnaire was also used for self-reported ratings on different feelings of satiety. The questionnaires were presented in small booklets showing only one question at a time. The questions asked were: "How hungry are you?" (hereafter denoted "hunger"), "How pleasant would you find eating another mouthful of this food?" ("pleasant"), "How strong is your desire to eat your favorite food right now?" ("desire"), "How full do you feel right now?" ("fullness"), "How sick do you feel right now?" ("sickness"), and "How strongly do you feel that you have had enough to consume?" ("enough"). The subjects were asked to rate the different sensations on a 15 cm VAS anchored by the phrases "Not at all" and "Extremely" [18]. Hunger, desire, sickness and fullness were estimated before the meal (0 min) and 15, 30, 45, 60, 90, and 120 min after the start of the meal. Pleasant was estimated 15, and 30, 45, 60, 90, and 120 min after the start of the meal. The subjects were asked how strongly they felt they had had enough to consume at 15, and 30, 45, 60, 90, and 120 min after the start of the meal. At the same time, the acceptability of the meal was rated on a bipolar hedonic scale, where 1 represents "dislike extremely", 5 represents a neutral response ("neither"), and 9 represents "like extremely".
Capillary plasma glucose samples were collected from all subjects (n = 14), although insulin measurements could not be performed on one subject due to problems associated with vein cannulation. Glucose concentrations were measured with the HemoCue Glucose system (HemoCue AB, Ängelholm, Sweden), which converts blood glucose to plasma-equivalent glucose concentrations by multiplying by a constant factor of 1.11 [19]. The precision of the HemoCue Glucose system is better than ± 0.3 SD between 0 and 22.2 mmol/l. All venous blood samples were centrifuged at 3000 × g for 10 min at 4°C. Aliquots of serum were immediately stored at -25°C for later analysis. Insulin concentrations were measured using an immunoassay with an alkaline phosphatase conjugate (Access Ultrasensitive Insulin, Beckman-Coulter AB, Bromma, Sweden). The sensitivity of the insulin immunoassay was 0.03 mUnit/l (mU/l), and the intra-assay coefficient of variation was less than 10% in the interval 0.03 to 300 mU/l.
Statistical analysis
The incremental area under the curve (AUC) was calculated for glucose, insulin and satiety for each subject and meal (using GraphPad Prism ver 3.0; GraphPad Software, San Diego, CA, USA). All areas below the baseline were excluded from the calculations. The GI was calculated by expressing each participant's glucose incremental AUC following the test meal as a percentage of the same participant's AUC following the reference meal. Descriptive statistics were run on all measures, and the results are given as means ± SEMs. All statistical calculations were performed using SPSS for Windows software (version 14.0, 2005). Differences in blood glucose, insulin levels, GI, and the questions regarding satiety were evaluated with the Wilcoxon's signed rank sum test. Significance was set at P ≤ 0.05. This study, employing fourteen healthy subjects, had an 80% power to detect a 20% change in GI at a level of P < 0.05 [17].
Results
Postprandial glucose and insulin response
Ingestion of the green tea meal resulted in a significantly (P = 0.019) higher blood glucose response at 120 min than did the reference meal. The postprandial change in glucose level from baseline was also significantly higher after the green tea meal than the reference meal at 120 min (Figure 1). No significant differences were seen in the areas under the plasma glucose curves (Table 1). No significant differences in serum insulin levels or insulin AUCs were observed between the green tea meal and the reference meal during the 120 min postprandial observation period (Figure 2 and Table 1). The mean GI of the green tea meal was 134.2 ± 16.3 (range 66-265) and there was no significant difference compared to the reference meal (GI: 100, P = 0.096).
 
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