Jonathan M.
Hodgson
*a,
Richard J.
Woodman
b,
Ian B.
Puddey
a,
Theo
Mulder
c,
Dagmar
Fuchs
c and
Kevin D.
Croft
a
aSchool of Medicine and Pharmacology, University of Western Australia, Royal Perth Hospital Unit, GPO Box X2213, Perth, Western Australia 6847, Australia. E-mail: Jonathan.Hodgson@uwa.edu.au; Fax: +61 618 9224 0246; Tel: +61 618 9224 0267
bDiscipline of General Practice, Flinders University, Adelaide, South Australia, Australia
cUnilever Research and Development, Vlaardingen, The Netherlands
First published on 18th September 2012
There is increasing evidence that black tea polyphenols contribute to vascular health. We have recently shown that regular ingestion of polyphenol-rich black tea over 6 months results in lower systolic and diastolic blood pressure. However, the time course of these effects remains unclear. Therefore, our objective was to determine if short-term effects of tea on blood pressure could contribute to longer-term benefits of regular tea consumption on blood pressure. Men and women (n = 111) were recruited to a randomised placebo-controlled double-blind parallel designed trial. During a 4-week run-in, all participants consumed 3 cups per day of black tea. Participants then consumed 3 cups over 1 day of either powdered black tea solids containing 429 mg of polyphenols (tea), or a control product matched in flavour and caffeine content but containing no tea solids. The 24 h ambulatory blood pressure and heart rate was measured at the end of the 4-week run-in (baseline) and again during the 24 h intervention period. The 24 h day-time and night-time blood pressures were not significantly different between tea and control (P > 0.05). Baseline-adjusted net effects on mean 24 h ambulatory blood pressure for systolic and diastolic blood pressure were −0.2 mm Hg (95% CI, −1.5 to 1.0), P = 0.72, and 0.0 mm Hg (95% CI, −1.0 to 0.9), P = 0.95, respectively. Heart rate was significantly lower for tea compared to control during the night-time and early-morning periods (−2.0 (95% CI, −3.2, −0.8) bpm, and −1.9 (95% CI, −3.7, −0.2) bpm, respectively; P < 0.05 for both), but not during the day-time. These results suggest that the longer-term benefits of black tea on blood pressure are unlikely to be due to short-term changes.
Results of population studies indicate that a higher intake of black tea can protect against hypertension5,6 and cardiovascular disease.7,8 There is also evidence from randomised controlled trials that regular consumption of black tea can lower blood pressure,9,10 but the time-course of this effect is not clear. We have recently demonstrated that regular consumption of 3 cups per day of polyphenol-rich black tea over 6 months, compared to a control beverage matched for caffeine content, results in lower systolic and diastolic blood pressures by between 2 and 3 mm Hg.9 Grassi et al.10 found that regular consumption of black tea for 1 week resulted in lower office blood pressure by between 2 and 3 mm Hg, but not ambulatory blood pressure. We have also found that regular consumption of black tea for 1 week did not alter ambulatory blood pressure.11 In contrast, our studies exploring the acute effects of black tea on blood pressure indicate that when tea is consumed after an overnight fast it can cause a transient rise in blood pressure.11,12 However, the relevance of these findings to usual tea consumption in the general population may be limited. The acute intake of polyphenols and/or caffeine in these studies was high in comparison to intakes during usual tea consumption in the general population. Therefore, our objective of this analysis was to determine if short-term changes in blood pressure could contribute to the benefits of regular tea consumption on blood pressure.
Volunteers were randomized (1:1) using computer-generated random numbers. Randomization codes were sealed in envelopes, which were opened in consecutive order as each participant deemed eligible was entered into the study.
At baseline, characteristics of participants in the two groups were compared using the independent-samples t-test and the chi-squared test for categorical variables. Ambulatory blood pressure data were analysed using linear mixed models in STATA. The STATA “xtmixed” and “margins” commands were used to determine baseline-adjusted between-group differences. Subject was included as a random factor in each model. Fixed effects included baseline value, group (placebo or tea), hour (categories 1 to 24) and group × hour. The baseline-adjusted ambulatory blood pressure and heart rate values and between-group differences are presented as least squares means and 95% CI. P < 0.05 was used to indicate statistical significance. Separate models were used to assess 24 h, early-morning, daytime and night-time effects.
Placebo (n 43) | Tea (n 41) | P | |
---|---|---|---|
a P-value for between-group difference analysed using the independent samples t-test and the chi-squared test for categorical variables. b Values are presented as mean ± standard deviation. | |||
Men/women (n) | 16/27 | 15/26 | 0.95 |
Age (years) | 56.2 ± 10.9b | 56.1 ± 10.7 | 0.94 |
Body mass index (kg m−2) | 25.3 ± 3.5 | 24.8 ± 3.7 | 0.55 |
Total cholesterol (mmol L−1) | 5.1 ± 0.9 | 5.1 ± 0.9 | 92 |
LDL cholesterol (mmol L−1) | 3.2 ± 0.8 | 3.2 ± 0.8 | 0.95 |
HDL cholesterol (mmol L−1) | 1.40 ± 0.34 | 1.41 ± 0.31 | 0.95 |
Triglycerides (mmol L−1) | 1.1 ± 0.6 | 1.1 ± 0.4 | 0.94 |
Glucose (mmol L−1) | 5.1 ± 0.5 | 5.2 ± 0.9 | 0.28 |
Insulin (mU L−1) | 7.0 ± 4.9 | 8.3 ± 8.9 | 0.41 |
Placebo | Tea | Baseline-adjusted between-group difference | |||
---|---|---|---|---|---|
Baseline | Intervention | Baseline | Intervention | ||
a Analysed using mixed models in STATA using “xtmixed” and “margins” commands to determine baseline-adjusted between-group differences during intervention. Values are estimated marginal means (95% CI) for intervention versus placebo. | |||||
24 hour | |||||
Systolic blood pressure (mm Hg) | 121.1 (120.2, 122.0) | 122.0 (121.1, 122.9) | 121.3 (120.4, 122.2) | 121.8 (120.9, 122.7) | −0.2 (−1.5, 1.0) P = 0.72 |
Diastolic blood pressure (mm Hg) | 72.3 (71.6, 73.0) | 72.9 (72.2, 73.6) | 72.0 (71.3, 72.7) | 72.9 (72.1, 73.6) | 0.0 (−1.0, 0.9) P = 0.95 |
Heart rate (bpm) | 69.5 (68.7, 70.0) | 69.0 (68.2, 69.7) | 68.9 (68.2, 69.7) | 68.4 (67.6, 69.2) | −0.6 (−1.6, 0.5) P = 0.30 |
Day-time (6:00 am–9:59 pm) | |||||
Systolic blood pressure (mm Hg) | 126.0 (125.0, 127.0) | 127.4 (126.4, 128.3) | 126.0 (125.0, 127.0) | 126.5 (125.5, 127.5) | −0.9 (−2.2, 0.5) P = 0.22 |
Diastolic blood pressure (mm Hg) | 76.5 (75.8, 75.2) | 77.4 (76.6, 78.1) | 76.0 (75.2, 76.7) | 77.0 (76.3, 77.8) | −0.4 (−1.4, 0.7) P = 0.50 |
Heart rate (bpm) | 73.9 (72.8, 75.0) | 72.4 (71.3, 73.5) | 73.6 (72.5, 74.7) | 73.4 (72.3, 74.6) | 1.0 (−0.6, 2.7) P = 0.20 |
Night-time (10:00 pm–5:59 am) | |||||
Systolic blood pressure (mm Hg) | 111.3 (110.4, 112.3) | 111.3 (110.4,112.3) | 111.6 (110.6, 112.6) | 111.9 (110.9, 112.9) | 0.6 (−0.8, 2.0) P = 0.42 |
Diastolic blood pressure (mm Hg) | 63.9 (63.1, 64.6) | 63.9 (63.1, 64.7) | 63.7 (62.9, 64.5) | 64.2 (63.4, 65.1) | 0.3 (−0.8, 1.5) P = 0.57 |
Heart rate (bpm) | 60.8 (60.0, 61.6) | 61.9 (61.0, 62.7) | 60.4 (59.6, 61.3) | 59.8 (59.0, 60.8) | −2.0 (−3.2, −0.8) P < 0.001 |
Early-morning (6:00 am–8:59 am) | |||||
Systolic blood pressure (mm Hg) | 122.7 (121.5, 123.8) | 123.4 (122.2, 124.5) | 122.5 (121.4, 123.7) | 125.0 (123.7, 126.2) | 1.6 (−0.1, 3.3) P = 0.06 |
Diastolic blood pressure (mm Hg) | 63.8 (63.1, 64.6) | 63.9 (63.1, 64.7) | 63.7 (62.9, 64.5) | 64.2 (63.4, 65.1) | 0.3 (−0.8, 1.5) P = 0.57 |
Heart rate (bpm) | 71.9 (70.8, 73.0) | 71.6 (70.4, 72.8) | 71.8 (70.7, 73.0) | 69.7 (68.4, 71.0) | −1.9 (−3.7, −0.2) P = 0.03 |
Results of population studies indicate that a higher intake of tea may protect against hypertension.5,6 There is also evidence from randomised controlled trials that regular consumption of black tea9,10 and green tea15,16 can result in lower blood pressure. We have recently demonstrated that regular consumption of 3 cups per day of black tea over 6 months resulted in lower systolic and diastolic blood pressures of between 2 and 3 mm Hg.9 We have addressed here any short-term contribution to longer-term effects on blood pressure in this study, and demonstrated that longer-term benefits of tea on blood pressure are unlikely to be due to immediate effects on ambulatory blood pressure. Our data are consistent with a recent short-term controlled trial that assessed the effects of green tea on blood pressure.17 Rudelle et al.17 demonstrated that green tea consumption over 3 days did not alter mean 24 h blood pressure measured while participants resided in an indirect calorimeter chamber.
Several previous studies have investigated acute effects of tea and its components on blood pressure.11,12,18–20 These studies have primarily been interested in the acute effects of a single dose of tea and/or its components, and often in the morning after an overnight fast. In this setting, tea and caffeine have been shown to cause a transient increase in blood pressure.11,12,18 These changes in blood pressure may have a bearing on acute improvements in cognitive performance.19,20 However, their relevance to the impact of regular tea consumption on ambulatory blood pressure and ultimately to the risk of hypertension and cardiovascular disease is uncertain.
We have shown here that a powdered black tea rich in polyphenols, in comparison to a control beverage matched for caffeine, as it would usually be consumed in the general population did not influence ambulatory blood pressure over a single day. We also found that blood pressure was not significantly altered in the early-morning period, a time comparable to previous acute studies, or during day-time or night-time periods. Our study assessed the effects of black tea against a background of regular tea drinking. It remains possible that effects of black tea could be different if participants were not consuming tea, coffee or caffeine during the run-in period. These results indicate that the previously observed rapid onset and short-lived pressor response to tea11,12 has minimal impact on ambulatory blood pressures in the short-term. Therefore, although non-caffeine components in black tea when consumed at high levels in the fasting state have the potential to raise blood pressure acutely, their effects on ambulatory blood pressure during usual tea consumption are likely to be minimal.
We have also shown that heart rate was significantly lower with black tea compared to control during the night-time and early-morning periods. These effects can be interpreted as an effect of black tea polyphenols to reduce heart rate, but could equally be interpreted as an effect of withdrawal of non-caffeine black tea solids to raise heart rate. The mechanisms responsible are not clear. Caffeine can acutely influence heart rate.21 However, the observed effects were independent of caffeine, because the two beverages were matched for caffeine content. Results of previous human trials suggest that regular ingestion of a high polyphenol diet leads to small and mostly non-significant effects on heart rate.9,22,23 An additional possibility is that other components of black tea such as L-theanine could influence heart rate.24,25 Further, because heart rate differences were found only during the night-time and early-morning periods, it is possible that the withdrawal of polyphenols, L-theanine, or other component of black tea could influence sleep quality, perhaps via attenuation of sympathetic nervous activation.25–27
Elevated heart rate may be an independent risk factor for cardiovascular disease.28 However, the magnitude of the effects on night-time and early-morning heart rate, −2 bpm, was small. The clinical importance of small differences in heart rate is not clear. Therefore, an important question relating to the clinical relevance of these findings is whether effects are maintained longer-term. Analysis of the longer-term data9 showed that an effect of tea to reduce heart rate at night was still present at 3 months but was not present at 6 months. Thus the clinical relevance of these short-term effects is therefore uncertain.
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