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1st Department of Cardiology, Athens Medical School, Hippokration Hospital, (C.V., N.A., I.D., K.A., C.S.)
Department of Pharmaceutical Chemistry, School of Pharmacy, University of Athens (I.A.), Athens, Greece
Address correspondence to: Charalambos Vlachopoulos, MD, Kerassoundos 17, Athens 11528, GREECE. E-mail: cvlachop{at}otenet.gr
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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Methods: The acute effect of black and green tea on aortic stiffness and wave reflections was assessed in 29 healthy volunteers in a randomized, single-blind, sham-procedure controlled, cross-over design. In the black tea sub-study, 16 subjects received 6 gm of tea, caffeine (175 mg), or hot water in 3 different sessions. In the green tea sub-study, 13 subjects received 6 gm of tea, caffeine (125 mg), or hot water. Carotid-femoral pulse wave velocity and wave reflection indices were measured at baseline and for 3 hours after consumption.
Results: Black tea increased pulse wave velocity during the first 90 min (increase by 0.49 m/sec, P < 0.05), showing a rapid return towards baseline values thereafter (P = 0.07 for the whole study period); in contrast, green tea had no effect. Both black and green tea increased augmentation index (by 5.0% and by 6.6%, P < 0.01 and P < 0.001, respectively) throughout the study. These changes were less than the respective changes produced by caffeine. Both black and green tea had a significant pressor effect. No change in oxidant status was found with both types of tea.
Conclusions: Both black and green tea increases acutely wave reflections and only black tea increases aortic stiffness. Tea flavonoids may play a role in the attenuation of the effects of caffeine contained in tea.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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The most commonly consumed types of tea in the west world are black and green tea. Due to differences in the time and duration of fermentation, the amount of caffeine and the type of flavonoids are not the same in these two types of tea. Black tea has more caffeine than green tea, while catechins, i.e. the major type of flavonoids present in green tea, are largely oxidized to form dimers and polymers, such as theaflavins and thearubigins in black tea [13,14].
The pulse generated by the contraction of the left ventricle travels towards the periphery where a part of it is reflected. This reflected wave returns backwards and merges with the incident wave. When the reflected wave returns late traveling slow in elastic arteries it merges with the incident wave at diastole, thus facilitating coronary perfusion. In contrast, when the reflected wave returns early traveling fast in stiff arteries and is great in amplitude (due to peripheral vasoconstriction), it merges with the incident one before aortic closure, thus augmenting systolic pressure and, consequently, the load with which left ventricle must cope. Thus, a stiff aorta and enhanced wave reflections increase left ventricular load and myocardial oxygen demands and impair ventricular function. Concurrently, they compromise coronary blood flow and predispose to ischemia. Furthermore, by increasing pulse pressure, they increase pulsatile stretch of the arteries leading to mechanical fatigue of their elastic components [15,16]. This pathophysiological background determines the impact of arterial stiffness and wave reflections on disease progression and cardiovascular events. Indeed, aortic stiffness and wave reflections have a causative role in the pathogenesis of systolic hypertension. Furthermore, aortic stiffness and wave reflections and their pathophysiological manifestations, i.e. increased systolic pressure, increased pulse pressure (and especially central pulse pressure) and reduced diastolic pressure have been identified as independent markers or cardiovascular disease and predictors of cardiovascular risk [17–23].
Tea has been associated with improved short and long-term endothelial performance [24–26], however its effect on arterial stiffness and wave reflections has not been studied. The present randomized, sham procedure-controlled, cross-over (three arm) study was undertaken to evaluate the acute effects of both black and green tea on arterial stiffness and wave reflections in humans. Furthermore, we sought to test the hypothesis that any effect would be associated with changes in oxidant status.
MATERIALS AND METHODS
Study Population
The study population consisted of 29 healthy individuals studied on three separate occasions each, in two sub-studies; the black tea study (16 subjects, mean age 31.0 years, range 25 to 38, 10 men) and the green tea study (13 subjects, mean age 30.8 years, range 26 to 34, 7 men).
All subjects in both studies were nonobese (BMI < 27 kg/m2) and they did not have hypertension, diabetes, hyperlipidemia, or family history of premature vascular disease. Seven subjects in the black tea study and five in the green tea study were smokers. They were clinically well and taking no regular cardiovascular medications or antioxidant vitamin supplementation. Subjects abstained from caffeine and ethanol intake for at least 12 hours and from flavonoid-containing food for at least 24 hours before each session. Female participants were examined during the follicular phase of menstrual cycle and none was on oral contraceptives. The study protocol was approved by our Institutional Research Ethics Committee and all subjects gave written informed consent.
Study Design
Each sub-study was carried out using a randomized (sequence of exposure), single-blind (operator), sham-procedure-controlled, cross-over (tea-caffeine-placebo) design.
Black Tea Study.
The 16 subjects of this study arm were studied on three separate days on which they took either: a) 6 gm of black tea (Lipton Inc.) added in 450 ml of boiled water for 5 min, or b) 175 mg of caffeine (the amount contained in 6 gm of black tea, see below "measurement of caffeine content in tea") diluted into 450 ml of boiled water, or c) 450 ml of boiled water.
Green Tea Study.
The 13 subjects of this study arm were also studied on three separate days on which they took either: a) 6 gm of green tea (Lipton Inc.) added in 450 ml of boiled water for 5 min, or b) 125 mg of caffeine (the amount contained in 6 gm of green tea, see below "measurement of caffeine content in tea") diluted into 450 ml of boiled water, or c) 450 ml of boiled water.
Measurements were obtained in a quiet, temperature-controlled room at 23°C, while the subjects had fasted for at least 8 hours. After a 20-min rest period in the supine position baseline measurements for evaluation of arterial elastic properties were taken. Then, the subjects were randomized to the different arms of the study and measurements were repeated at 30, 60, 90, 120, 150, and 180 min after baseline measurements. Venous blood for measuring circulating malondialdehyde concentration (MDA) was drawn into Vacutainer tubes containing EDTA, at baseline and at 120 min when peak of plasma flavonoids is anticipated [27].
Measurement of Caffeine Contained in Tea
The caffeine content in 5 samples of black and green tea prepared in an identical manner as in the study sessions were analyzed for caffeine content in the General Chemical State Laboratory using high-performance liquid chromatography. The 450 ml of black tea beverage contained 172 ± 2 mg of caffeine and 450 ml of green tea beverage contained 128 ± 3 mg of caffeine.
Evaluation of Aortic Elastic Properties
The pulse travels at a higher velocity in a stiff aorta and vice versa. Carotid-femoral pulse wave velocity (PWV), an established index of aortic stiffness [15,18,28], was calculated from measurements of pulse transit time and the distance traveled between two recording sites (pulse wave velocity = distance [meters]/transit time [seconds]) using a validated non-invasive device (Complior®, Artech Medical, Pantin, France), which allows online pulse wave recording and automatic calculation of pulse wave velocity [29]. Two different pulse waves were obtained simultaneously at two sites (at the base of the neck for the common carotid and over the right femoral artery) with two transducers. The distance was defined as: (distance from the suprasternic notch to femoral artery)—(distance from carotid artery to the suprasternic notch).
Measurement of Wave Reflection Indices
Augmentation index (AIx) and augmented pressure of the central (aortic) pressure waveform were measured as indices of wave reflections [15,16,28,30,31]. Augmented pressure is the pressure added to the incident wave by the returning reflected one and represents the pressure boost that is caused by wave reflection and with which the left ventricle must cope. Augmentation Index (defined as augmented pressure divided by pulse pressure and expressed as a percentage) is a composite measure of the magnitude of wave reflection and arterial stiffness, which affects timing of wave reflection. Larger values of augmentation index indicate increased wave reflection from the periphery and/or earlier return of the reflected wave as a result of increased pulse wave velocity (due to increased arterial stiffness), and vice versa. Augmentation index was measured by using a validated, commercially available system (SphygmoCor®, AtCor Medical, Sydney, Australia), which employs the principle of applanation tonometry and appropriate acquisition and analysis software for non-invasive recording and analysis of the arterial pulse. The technique has been described in detail previously [15,16]. In brief, from radial artery recordings, the central (aortic) arterial pressure was derived with the use of a generalized transfer function which has been shown to give an accurate estimate of the central arterial pressure waveform and its characteristics [15,30–32]. Waveforms of radial pressure were calibrated according to sphygmomanometric systolic and diastolic pressure measured in the brachial artery since there is practically negligible pressure pulse amplification between the brachial and the radial artery [15].
Assessment of Plasma Oxidant Status
Immediately after acquisition of venous blood, plasma was separated by centrifugation (3000 x g at 4°C for 15 min) and stored at –80°C until analysis. Plasma oxidant status was evaluated with measurement of circulating malondialdehyde concentration (MDA), which is one of the most commonly used methods to evaluate lipid peroxidation (i.e. a marker of oxidative stress). MDA concentration was determined spectrophotometrically and expressed in µmol/l (Oxford Biomedical Research Colorimetric Assay for lipid peroxidation) as previously described [33]. Measurements of each group were performed in triplicate, and standard deviation was less than ±10%.
Statistical Analysis
Numerical data are expressed as the mean ± SEM. All variables were tested for homogeneity of variance and normal distribution, before any statistical analysis was applied. Baseline parameters between the three sessions were compared using one-way ANOVA. Changes in MDA concentration between tea or caffeine session and sham-procedure session were compared using the student t-test for paired measures. In order to evaluate the composite effect of the tea, or caffeine versus placebo over time on the variables of interest, an overall 7 x 2 analysis of variance (ANOVA) for repeated measures was performed (7 periods [baseline, 30, 60, 90, 120, 150 and 180 minutes] x 2 interventions [tea or caffeine versus placebo]). Sample size calculations were based on data from our unit according to which the standard deviation of the difference between two measurements of PWV and AIx in the same individual is 0.3 m/sec and 4% respectively. Therefore, we estimated that 13 subjects would provide 90% power at the 5% level of significance to detect a treatment-induced difference of 0.3 m/sec in PWV in a cross-over design. Similarly, 11 subjects would provide 90% power to detect an absolute difference of 5% in AIx. A P value <0.05 was considered statistically significant. Data analysis was performed using the SPSS statistical package for Windows (version 10.0, SPSS Inc., Chicago, Illinois).
RESULTS
Baseline characteristics of subjects in different sessions are shown in Tables 1 & 2.
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Effect on Heart Rate and Blood Pressure
Black Tea Study.
Although both black tea and caffeine (175 mg) decreased heart rate compared to placebo (by 3.3 bpm, nadir at 30 min with tea and by 5.7 bpm, nadir at 120 min with caffeine), only with caffeine the decrease was significant (P < 0.05).
Peripheral and aortic systolic pressures were significantly increased both with black tea and caffeine (Fig. 1). Peripheral and aortic diastolic pressures were significantly increased with black tea (by 8.3 mmHg and 8.4 mmHg respectively, peak at 60 min, P < 0.05 for both). Caffeine produced only a non-significant (P = 0.1) increase in peripheral and aortic diastolic pressures (by 5.3 and 5.2 mmHg respectively, peak at 60 min). Peripheral and aortic pulse pressure did not change either with black tea or caffeine.
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Peripheral systolic pressure was increased only with caffeine, while aortic systolic pressure was increased both with tea and caffeine (Fig. 1). Peripheral and aortic diastolic pressures were significantly increased both with green tea and caffeine, reaching their peak at 60 min (peripheral by 6.3 and 9.2 mmHg, P < 0.05 and <0.005 respectively; aortic by 6.2 and 8.8 mmHg, P < 0.05 and P < 0.005 respectively). Peripheral and aortic pulse pressure did not change significantly either with green tea or with caffeine (P = NS for all).
Effect on Aortic Pulse Wave Velocity and Wave Reflection Indices
Black Tea Study.
Caffeine produced a sustained increase in pulse wave velocity (Fig. 2). Black tea produced an initial increase in pulse wave velocity (by 0.49 m/sec, peak at 30 min, P < 0.05 for the initial 90 min), which was rapidly attenuated. When the total effect (180 min) was examined, only a trend to increase pulse wave velocity was observed (P = 0.07, Fig. 2). Augmentation index and augmented pressure were increased both with black tea and caffeine (Fig. 2, augmented pressure by 1.8 and 2.0 mmHg respectively, peak at 60 min for both, P < 0.01 and P < 0.001 respectively).
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Effect on Plasma Oxidant Status
Tea and caffeine had no effect on MDA plasma concentration either in the black or green tea studies.
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DISCUSSION |
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Mechanisms
The unfavorable effect of tea, especially at the early phase can be attributed to caffeine. Results of studies on the impact of caffeine consumption on cardiovascular risk are conflicting ranging from a strong positive association to even a J-shaped one [34,35]. However, as we [7–11] and others [12] have previously shown, caffeine results in an acute and chronic deterioration of arterial function with increased pulse wave velocity and wave reflections. Caffeine from tea is easily absorbed and plasma concentration of caffeine is comparable to that after consumption of pure caffeine in boiled water [36], which reaches a peak concentration at 30–75 min [37]. However, the effect of black and green tea on wave reflections and pulse wave velocity is less than expected according to the amount of caffeine contained in the two types of tea, especially at the late phase of the study. In addition, while blood pressure increased more with black tea than with caffeine, pulse wave velocity increased less. These findings indicate a counterbalancing effect of a tea ingredient, most likely flavonoids. Indeed, isoflavones—another type of flavonoids found in soy—have been shown to reduce systemic arterial compliance and pulse wave velocity [38]. Furthermore, as we have recently shown, dark chocolate rich in flavonoids reduces wave reflections [39]. These effects may be mediated through the favorable effect of tea flavonoids on endothelial function [24–26], which, in itself has a regulatory role on arterial stiffness [40]. A possible effect of flavonoids is further strengthened by the fact that, contrary to caffeine, tea flavonoids need more time to be absorbed, and their peak plasma concentration occurs at 90–120 min [27]; thus this time difference between caffeine and flavonoid peak plasma levels may account for the "delayed" effect of the latter on the indices of arterial function observed in our study.
Our results are in agreement with previous studies showing an acute increase in blood pressure with both black and green tea [24,26,36]. Interestingly, specifically for black tea, this effect was greater than the effect of caffeine. This finding is in accordance with previous studies that have examined the pressor effect of black and green tea [36] and indicates the presence of other substances with pressor effect as well. It can be speculated that the type of flavonoids (theaflavins and thearubigins in vs. catechins in green tea) or phenolic acids (black tea contains larger amounts compared to green tea [41]), may account for the increased pressor effect of black tea. However, the exact nature of these substances with pressor effects is unknown at present and this should be investigated in future studies.
An important finding of our study is that the attenuation of arterial function deterioration is not mediated by a beneficial effect on oxidant status since there was no effect of black or green tea on MDA. Although tea ingestion has been found to increase plasma antioxidant status in-vivo [42], our results are in line with other studies, which showed that ingestion of black or green tea does not inhibit in vivo lipid peroxidation [43], and that the beneficial effect of tea on endothelial function can not be attributed to a systemic antioxidant effect [44].
Study Limitations
Neither the amount, not the consumption pattern of tea studied in the present study are the average ones. However, we deliberately used this dosage in order to be in accordance with previous studies investigating the effect of tea on endothelial function [24,36]. Undoubtedly, different amounts and consumption patterns should be investigated in future studies.
Our study was conducted in young healthy volunteers with no risk factors for cardiovascular disease apart from smoking. Although our results cannot be extrapolated directly to other populations, they could be particularly important in "sensitive" populations such as the elderly or hypertensives.
We studied the acute effect of tea on arterial function and our findings cannot be translated to a chronic effect. However, they indicate an effect of considerable duration throughout the day, which is repeated with habitual consumption. Nevertheless, whether this is translated into a clinical impact remains to be determined.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONCONCLUSIONACKNOWLEDGMENTSREFERENCES |
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ACKNOWLEDGMENTS |
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