Journal of Human Hypertension (2015), 1–7 © 2015 Macmillan Publishers Limited All rights reserved 0950-9240/15 www.nature.com/jhh
ORIGINAL ARTICLE
Correlation between tea consumption and prevalence of hypertension among Singaporean Chinese residents aged ⩾ 40 years W Li1,2, J Yang3, XS Zhu4, S-C Li5 and PC Ho6 By a cross-sectional epidemiology study, we attempted to correlate the consumption of tea and/or health supplements, living habits and socio-demographic factors to the prevalence of hypertension among Singaporean Chinese residents. Singaporean Chinese residents aged ⩾ 40 years were randomly selected and interviewed face-to-face by clinical research assistants. Hypertension was defined as measured systolic blood pressure at least 140 mm Hg and/or diastolic blood pressure at least 90 mmHg or self-reported history/treatment for hypertension. The prevalence of hypertension among the whole investigated population (N = 1184, 58.27% females) was 49.73% and the prevalence increased to 66.47% in the sub-population aged ⩾ 60 years. High risk of hypertension was associated with age ⩾ 60 years (odds ratio (OR): 4.15–4.19, P o 0.01), obesity (body mass index 425 kg m− 2, OR: 2.10–2.11, P o 0.01), family history of hypertension (OR: 2.69–2.76, Po 0.01), diabetes history (OR: 2.29–2.33, Po 0.01), hyperlipidemia history (OR: 1.79–1.80, P o0.01), male (OR: 1.56–1.59, P o0.01) and coffee intake (OR: 1.44–1.46, P o0.05). In contrast, drinking green tea at least 150 ml per week was associated with lower hypertension risk (OR: 0.63, 95% confidence interval (CI): 0.43–0.91, P o 0.05). Drinking combination of green tea and British tea was associated with higher reduction in the risk of hypertension (OR: 0.58, 95% CI: 0.39–0.85, P o0.05). This cross-sectional study suggests that consumption of tea, especially green tea and British tea, was associated with lowering the risk of hypertension. On the other hand, consumption of coffee could be a risk factor of hypertension. These findings may provide useful information for health promotion to reduce risk of hypertension and warrant further study to confirm and elucidate such association. Journal of Human Hypertension advance online publication, 28 May 2015; doi:10.1038/jhh.2015.45
INTRODUCTION Hypertension, or also known as high or raised blood pressure, is a global public health issue.1 Hypertension has been recognized as a vital risk factor for the incidence of cerebrovascular disease,2 coronary heart disease,3 heart failure4 and chronic renal failure.5 According to Global Health Observatory group of World Health Organization (WHO), due to the global population growth and ageing, the overall prevalence of hypertension in adults aged ⩾ 25 years was around 40% in 2008 and the number of hypertensive patients has increased globally from 600 million in 1980 to 1 billion in 2008.6 Male and age 440 years were found to be highly associated with more severe hypertension.7 In adults ⩾ 40 years, signs of hypertensive retinopathy were frequently found and predictive of future incident of stroke, congestive heart failure and cardiovascular mortality.8 Glaucoma and ocular hypertension were also more commonly found in individuals aged 440 years.9 From the Statistics of the Ministry of Health, Singapore, the prevalence of hypertension in adults aged 18–69 years was 23.5% (2010),10 but the prevalence increased markedly from age 40 years onwards. More than half of the elderly (53.4%) in Singapore aged 60–69 years were found to have hypertension.10 A survey of senior Singaporeans by Malhotra et al.11 in 2010 also found nearly 1
three-fourths (73.9%) of Singaporeans aged ⩾ 60 years to be hypertensive. As the prevalence and risk of hypertension increased greatly from age 40 years, we targeted the population with age ⩾ 40 years in our current study. Chinese folks have 4–5 thousand years of tea-drinking history. To Chinese, the three most widely consumed tea are black tea (or the British tea) (fully fermented), oolong tea (semi-fermented) and green tea (non-fermented).12 The tea extracts have been identified to have pharmacological effects highly associated with reducing blood pressure,13,14 obesity,15,16 hyperglycemia17 and so on. There are clinical studies indicating that tea consumption could lower blood pressure.18,19 However, no similar studies on the correlation between tea drinking and its potential beneficial effect on lowering blood pressure among Singaporeans have been conducted. Health supplements are used extensively in the management of hypertension,20–23 most of which are nutritional supplements, such as vitamins, amino acids and coenzyme Q10. Clinical studies on complementary consumption of herbs for the treatment of hypertension have also been reported.24,25 Nevertheless, it is yet not clear whether Singaporean Chinese can benefit from their habits of consumption of tea and/or health supplements to control hypertension. Hence, we proposed to undertake an epidemiology study to investigate the prevalence of
Singapore College of Traditional Chinese Medicine, Singapore, Singapore; 2Department of Pharmaceutics, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Piscataway, NJ, USA; 3National University Hospital of Singapore, Singapore, Singapore; 4Centre for Complementary Medicine Research, University of Western Sydney, Sydney, New South Wales, Australia; 5School of Biomedical Sciences and Pharmacy, University of Newcastle, Newcastle, New South Wales, Australia and 6Department of Pharmacy, National University of Singapore, Singapore, Singapore. Correspondence: Dr W Li, Department of Pharmaceutics, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Room 013, 160 Frelinghuysen Road, Piscataway, NJ 08854-8020, USA. E-mail: [email protected]. Received 11 December 2014; revised 20 March 2015; accepted 16 April 2015
Tea consumption and Singaporean Chinese hypertension W Li et al
2 hypertension in Singapore and also to correlate hypertension to consumption of tea and/or health supplements, living habits and socio-demographic factors. This would be the first study focusing on correlation between tea consumption and hypertension among Singaporean Chinese residents. The findings would provide useful information for effective prevention and treatment of hypertension.
Table 1. Association of socio-demographic factors with the hypertension prevalence
MATERIALS AND METHODS Ethical consideration The study protocol followed ethical standards of Declaration of Helsinki 1975 as revised in 2008 and was approved by institutional review board of Singapore College of Traditional Chinese Medicine.
Sample size calculation Following the method the sample size was
as reported by Charan calculated according to
and Biswas,26 the equation:
z 21 - α Pð1 - PÞ
sample size ¼ 2 d2 . We set type 1 error α = 0.05, hence Z1-α/2 = 1.96; and precision (d) as 0.05. According to Statistics of the Ministry of Health, Singapore,10 the prevalence of hypertension in adults aged 18–69 years was 23.5%, so expected portion of population was set as 0.235. Hence the minimum sample size ¼ 1:962 ´ 0:235 ´ ð1 - 0:235Þ =0:052 ¼ 276.
Subjects' recruitment A simple random sampling was carried out with proportional fixation by communities and neighborhoods. Chinese residents (citizen or Permanent Resident) of five major districts of Singapore (Northern East, Northern West, Southern East, Southern West and Centre) aged ⩾ 40 years were randomly selected from community-based visits and invited to participate in the study. Ethnic groups other than Chinese, aged o40 years, those who were unable or refused to complete the interview, those with cognitive dysfunction, pregnant women and lactating women were excluded. The participants were randomly selected and interviewed faceto-face by 56 trained clinical research assistants from Singapore College of Traditional Chinese Medicine from June 2013 to December 2013. Each participant was explained the details of the study and asked to sign an informed consent form before the survey was conducted. Each subject was interviewed by two research assistants. During and after the survey, one of the investigators (Wenji Li) would randomly follow-up the participants to validate the quality of acquired data.
Socio-demographics information and blood pressure acquisition During the interview, all the subjects were enquired about their sociodemographic information, such as age, race, gender, social identity number, telephone number, weight, height, address and occupation. Their medical history such as hypertension history, family hypertension history, diabetes history and hyperlipidemia history were also recorded. Blood pressure for each subject was taken after 5-min rest. Three seated blood pressure measurements were recorded with an automatic sphygmomanometer (HEM-7203, OMRON, Singapore, Singapore). Each reading and the average of the three readings were recorded. The average of the three readings was the blood pressure value used in the analysis. Hypertension was defined as tested systolic blood pressure ⩾ 140 mm Hg and/or diastolic blood pressure ⩾ 90 mm Hg or self-reported history/ treatment for hypertension.
Journal of Human Hypertension (2015), 1 – 7
Hypertension
P
Age (average age), years o50 50–59 60–69 70–79 ⩾ 80
56.76 ± 8.52 72 (64.29%) 323 (63.96%) 126 (40.91%) 35 (23.03%) 9 (19.15%)
62.92 ± 10.36 40 (35.71%) 182 (36.04%) 182 (59.09%) 117 (76.97%) 38 (80.85%)
0.000 0.000
Gender Male Female BMI (kg m −2) Underweighta Normal weightb Overweightc Obesityd
208 (44.35%) 357 (54.50%) 22.91 ± 3.49 39 (62.90%) 410 (55.18%) 98 (37.69%) 18 (30.51%)
261 (55.65%) 298 (45.50%) 24.25 ± 3.89 23 (37.10%) 333 (44.82%) 162 (62.31%) 41 (69.49%)
0.001
Family history of hypertension No 381 (58.08%) Yes 184 (39.32%)
275 (41.92%) 284 (60.68%)
0.000
History of hyperlipidemia No Yes
475 (55.69%) 90 (33.21%)
378 (44.31%) 181 (66.79%)
0.000
History of diabetes mellitus No Yes
521 (54.61%) 44 (25.88%)
433 (45.39%) 126 (74.12%)
0.000
Occupation Retired Working
196 (41.26%) 369 (56.86%)
279 (58.74%) 280 (43.14%)
0.000
0.004 0.000
Abbreviation: BMI, body mass index. aUnderweight (BMI o18.5 kg m − 2). b Normal (18.5 ⩽ BMI o25 kg m − 2). cOverweight (25 ⩽ BMI o30 kg m − 2). d Obesity (BMI ⩾ 30 kg m − 2). P-value represents the difference in characteristics based on a chi-square test (for dichotomous variables) or analysis of variance (for continuous variables) as appropriate, similarly hereinafter.
450 ml). The frequency record mark multiplied by the usual consumed volume mark provided the total volume index. A subject was classified as tea nondrinker if the total volume mark equaled zero. Total low and total high intakes were defined as total volume mark o300 ml per week and ⩾ 300 ml per week, respectively. Low intake of a single type of tea (green, oolong or British) and high intake of a single type of tea were defined as the total volume mark o150 ml per week and ⩾ 150 ml per week, respectively.
Other data acquisition Subjects were asked to recall the frequency with which they have consumed the following items or performance of exercise in the past 12 months. ●
Tea intake inquiry Subjects were asked to recall the average frequency of tea intake in the past 12 months. The questions were designed according to habitual intake of common types of tea among locals by using local references and terms: black tea (or British tea), oolong tea, or green tea. Black tea or British tea is usually consumed with sugar and milk, whereas oolong tea and green tea are almost always drunk without sugar or milk. The frequency of consumption of each type of tea was classified into five categories: never or rarely (marked 0), ⩾ once per week (marked 1), ⩾ 7 times per week (marked 7), ⩾ 14 times per week (marked 14), and ⩾ 21 times per week (marked 21). The subjects were also asked to provide the usual consumed volume set as ⩾ 150 ml (one small cup, marked 150 ml), ⩾ 300 ml (two small cups, marked 300 ml) and ⩾ 450 ml (three small cups, marked
Non-hypertension
Variables
● ● ● ●
Consumption of nutrition supplements (such as coenzyme Q10, fish oil, garlic, vitamin C) or TCM herbs or alcohol and exercise performance were determined according to four categories (0: None, 1: ⩾ once per month, 2: ⩾ once per week, and 3: ⩾ once per day). Cigarette smoking was classified into four categories (0: never, 1: 1–2 per day, 2: 3–4 per day, and 3: ⩾ 5 per day). Consumption of coffee was recorded as following: 0: none, 1: 1 cup per week, 2: 2–3 cups per week, and 3: ⩾ 3 cups per week. Consumption of vegetables and fruits were encoded as: 0: none, 1: once per week, 2: 2–3 times per week, and 3: ⩾ 3 times per week. The categorical variables (0, 1, 2 and 3) were created for consumption of salt ( o6g per day, ⩾ 6g per day, 7–10g per day and ⩾ 10g per day). Followed by the WHO Body Mass Index (BMI) Classification Standard,27 the classification of body weight was as follows: underweight (BMIo 18.5), normal (18.5 ⩽ BMIo25), overweight (25 ⩽ BMIo30), and obesity (BMI ⩾ 30). © 2015 Macmillan Publishers Limited
Tea consumption and Singaporean Chinese hypertension W Li et al
3 Table 2. Association of the living habits with the prevalence of hypertension Variables Nutrition supplement Not taking Taking
Non-hypertension
Hypertension
Table 3. Correlation between the prevalence of hypertension and categorical tea intakea or binominal tea intakeb P
311 (48.52%) 254 (52.59%)
330 (51.48%) 229 (47.41%)
0.177
Functional supplement Not taking 404 (50.25%) Taking 161 (50.31%)
400 (49.75%) 159 (49.69%)
0.985
Cigarette Not smoking Smoking
494 (49.50%) 65 (51.59%)
0.659
504 (50.50%) 61 (48.41%)
Non-hypertension
Hypertension
P
354 (47.39%) 102 (53.40%) 109 (58.60%)
393 (52.61%) 89 (46.60%) 77 (41.40%)
0.015
Intake of oolong teaa None Low (⩽150) High (4150)
372 (49.47%) 81 (47.65%) 112 (55.45%)
380 (50.53%) 89 (52.35%) 90 (44.55%)
0.243
Intake of British teaa None Low (⩽150) High (4150)
349 (47.81%) 99 (55.93%) 117 (53.92%)
381 (52.19%) 78 (44.07%) 100 (46.08%)
0.074
Intake of tea as a wholea None 189 (46.10%) Low (⩽300) 143 (50.71%) High (4300) 233 (53.94%)
221 (53.90%) 139 (49.29%) 199 (46.06%)
0.074
Variables Intake of green tea None Low (⩽150) High (4150)
a
Alcohol Not drinking Drinking
459 (49.68%) 106 (53.00%)
465 (50.32%) 94 (47.00%)
0.394
Exercise No Yes
156 (47.13%) 409 (51.58%)
175 (52.87%) 384 (48.42%)
0.174
Coffee Not drinking Drinking
152 (58.46%) 413 (47.80%)
108 (41.54%) 451 (52.20%)
0.003
Intake of green teab No Yes
354 (47.39%) 211 (55.97%)
393 (52.61%) 166 (44.03%)
0.007
Salt intake Low high
165 (50.61%) 400 (50.13%)
161 (49.39%) 398 (49.87%)
0.882
Intake of oolong teab No Yes
372 (49.47%) 193 (51.88%)
380 (50.53%) 179 (48.12%)
0.446
Vegetable and fruit Not taking Taking
10 (43.48%) 555 (50.41%)
13 (56.52%) 546 (49.59%)
0.511
Intake of British teab No Yes
349 (47.81%) 216 (54.82%)
381 (52.19%) 178 (45.18%)
0.025
Intake of tea as a wholeb No 189 (46.10%) Yes 376 (52.66%)
221 (53.90%) 338 (47.34%)
0.034
Statistical analysis Univariate analysis was employed to evaluate between the two groups with hypertension and without hypertension, respectively, by using a chi-square test for dichotomous variables or by using analysis of variance for continuous variables. All variables which were significant in the previous test were enrolled in logistic regression analysis to examine the relation between tea intake and other correlated factors and prevalence of hypertension. Three models were utilized to analyze the influence of tea intake. Model 1 included the tea intake (yes or no) and other significant univariables. Model 2 included the consumption level of different types of tea (none, low or high level) and other significant univariables. Model 3 included single or synergetic effects of green tea and British tea (none, green tea only, British tea only or a combination of both) and other significant univariables. All the statistical tests were two-sided, and Po0.05 was regarded as significant. All data were double entered by Epidata (version 3.1; The EpiData Association, Odense, Denmark) and analyzed by STATA respectively (version 11; published by Stata Press, College Station, TX, USA).
RESULTS During the study period, a total of 1184 participants were recruited. Among them, 60 subjects were omitted because of missing data and 1124 participants were analyzed for the present study (Table 1). As shown in Table 1, in the univariate analysis of the socio-demographic factors and hypertension, the subjects with hypertension indicated a higher average age than the nonhypertensive subjects (Po 0.01). Data also indicated higher ratio of hypertension in the older age groups (Po 0.01) when the age group was stratified (P o0.01). Among all the 1124 Singaporean Chinese subjects, there were more male hypertensive subjects (55.65% vs 45.50%, Po 0.01). High body weight was highly associated with hypertension. In the subjects ranging from underweight to obesity, the ratio of hypertension increased from © 2015 Macmillan Publishers Limited
a
Model 1: categorical tea intake evaluation models. bModel 2: binominal tea intake evaluation models.
37.10% to 69.49% (P o 0.01). Prevalence of hypertension also closely associated with the family history of hypertension (P o 0.01), history of hyperlipidemia (Po 0.01) and history of diabetes mellitus (Po 0.01). Hypertension was more common in retirees than those still working (58.74% vs 43.14%, P o0.01). For the univariate analysis of the effects of supplements and living habits on the prevalence of hypertension, we only found that drinking of coffee had higher correlation with hypertension (52.20% vs 41.54%, P o 0.01; Table 2). To understand the effect of tea intake on hypertension, the categorical intake and binominal intake were analyzed together (Table 3). Higher category intake of green tea was more likely to be associated with reducing the hypertension prevalence (high intake, low intake and non-intake: 41.40, 46.60 and 52.61%, respectively, P o 0.05). Comparing with non-drinkers, drinkers of green tea, British tea or tea as a whole had lower hypertension ratio (P o0.05). Tea intake habits among subjects with various backgrounds were analyzed and only results with significant differences are listed in Table 4. Higher portion of subjects in the 50–59 age group were more frequent tea drinkers than any other age groups (43%, P o 0.01). In addition, larger percentage of this group was high-volume green tea drinkers (19%, P o 0.01) than others. Most subjects aged ⩾ 80 years did not drink tea (51%, P o 0.01). Most hypertensive subjects did not drink green tea (70%, Po 0.05). More retired people were non-tea-drinker than those still working (41% vs 33%, P o 0.05). Alcohol drinkers tended to consume larger volume of tea than the alcohol abstainers (50% vs 36%, Po 0.01). Journal of Human Hypertension (2015), 1 – 7
Journal of Human Hypertension (2015), 1 – 7
142 (15) 49 (25)
637 (69) 110 (55)
233 (70) 514 (65)
Alcohol No Yes
Exercise No Yes
34 (13) 157 (18)
58 (18) 133 (17)
(18) (19) (15) (13) (13)
51 (20) 135 (16)
40 (12) 146 (18)
145 (16) 41 (21)
139 (17) 47 (15)
62 (13) 124 (19)
109 (19) 77 (14)
15 98 47 20 6
0.082
0.033
0.001
0.048
0.001
0.015
0.001
P
(73) (61) (71) (72) (68)
183 (70) 569 (66)
242 (73) 510 (64)
651 (70) 101 (51)
549 (68) 203 (63)
334 (70) 418 (64)
372 (66) 380 (68)
82 309 220 109 32
None
High
(15) (18) (13) (13) (13)
32 (12) 138 (16)
41 (12) 129 (16)
125 (14) 45 (23)
121 (15) 49 (15)
59 (12) 111 (17)
81 (14) 89 (16)
17 89 39 19 6
(12) (21) (16) (16) (19)
45 (17) 157 (18)
48 (15) 154 (19)
148 (16) 54 (27)
134 (17) 68 (21)
82 (17) 120 (18)
112 (20) 90 (16)
13 107 49 24 9
Number (% of subgroup)
Low
Oolong tea
0.291
0.017
0.000
0.175
0.060
0.243
0.052
P
(56) (64) (69) (64) (74)
179 (69) 551 (64)
230 (69) 500 (63)
617 (67) 113 (57)
526 (65) 204 (64)
313 (66) 417 (64)
349 (62) 381 (68)
63 322 212 98 35
None
High
(26) (17) (13) (13) (6)
27 (10) 150 (17)
56 (17) 121 (15)
133 (14) 44 (22)
120 (15) 57 (18)
74 (16) 103 (16)
99 (18) 78 (14)
29 85 40 20 3
(18) (19) (18) (22) (19)
54 (21) 163 (19)
45 (14) 172 (22)
174 (19) 43 (22)
158 (20) 59 (18)
88 (19) 129 (20)
117 (21) 100 (18)
20 98 56 34 9
Number (% of subgroup)
Low
British tea
All records are presented as 'actual number (percentage of the related subgroup)'. 68 (82) means 68 cases, which accounts for 82% of its subgroup.
175 (67) 572 (66)
123 (15) 68 (21)
Functional supplement No 542 (67) Yes 205 (64)
Coffee No Yes
67 (14) 124 (19)
346 (73) 401 (62)
Occupation Retired Working
102 (18) 89 (16)
(19) (17) (10) (4)
354 (63) 393 (70)
94 51 15 2
Hypertension No Yes
(82) (62) (68) (77) (83)
High
Number (% of subgroup)
Low
68 313 210 117 39
None
Green tea
Correlation between tea intake and various study factors
Age (years) o50 50–59 60–69 70–79 ⩾ 80
Variables
Table 4.
0.025
0.007
0.009
0.476
0.824
0.074
0.038
P
(32) (32) (40) (42) (51)
100 (38) 310 (36)
144 (44) 266 (34)
362 (39) 48 (24)
295 (37) 115 (36)
197 (41) 213 (33)
189 (33) 221 (40)
36 164 122 64 24
None
High
(35) (24) (25) (26) (13)
55 (21) 227 (26)
79 (24) 203 (26)
229 (25) 53 (27)
199 (25) 83 (26)
114 (24) 168 (26)
143 (25) 139 (25)
39 122 76 39 6
(33) (43) (36) (32) (36)
105 (40) 327 (38)
108 (33) 324 (41)
333 (36) 99 (50)
310 (39) 122 (38)
164 (35) 268 (41)
233 (41) 199 (36)
37 219 110 49 17
Number (% of subgroup)
Low
Tea total
0.248
0.005
0.000
0.916
0.010
0.074
0.007
P
Tea consumption and Singaporean Chinese hypertension W Li et al
4
© 2015 Macmillan Publishers Limited
Tea consumption and Singaporean Chinese hypertension W Li et al
5 Table 5.
Logistic regression analysis of the outcomes in the three models (models 1–3) OR
s.e.
P-value
95% CI
Age460a Family history of Hypertension BMI 425e History of diabetes History of hyperlipidemia Male Coffee intake Tea intakeb
4.19b,c, 4.15d 2.69b, 2.76c, 2.72d 2.11b, 2.10c,d 2.29b, 2.33c, 2.32d 1.8b, 1.79c,d 1.59b, 1.56c,d 1.45b, 1.44c, 1.46d 0.74b
0.6b,c, 0.59d 0.39b, 0.40c,d 0.32 0.48b, 0.49c,d 0.3 0.22 b,c 0.23 , 0.24d 0.1b
0 0 0 0 0 0 0.02 0.03b
3.17–5.54b, 3.16–5.55c, 3.14–5.49d 2.02–3.58b, 2.07–3.68c, 2.04–3.62d 1.56–2.84b, 1.55–2.83c, 1.56–2.83d 1.52–3.44b, 1.55–3.52c, 1.54–3.51d 1.31–2.49b, 1.30–2.48c,d 1.21–2.08b, 1.18–2.04c, 1.19–2.05d 1.06–1.99b,1.05–1.98c, 1.07–2.01d 0.56–0.97b
Green tea intake (ml per week)c,f 0 0–150 4150
1.00 (referent)c 0.91c 0.63c
0.17c 0.12c
0.60c 0.01c
0.63–1.31c 0.43–0.91c
British tea intake (ml per week)c,g 0 0–150 >150
1.00 (referent)c 0.76c 0.74c
0.15c 0.13c
0.16c 0.09c
0.51–1.11c 0.52–1.05c
Tea typed,h None Green tea only British tea only Both green and British tea
1.00 (referent)d 0.76d 0.76d 0.58d
0.14d 0.14d 0.11d
0.14d 0.13d 0.01d
0.52–1.09d 0.53–1.09d 0.39–0.85d
Subgroups
Abbreviations: BMI, body mass index; CI, confidence interval; OR, odds ratio. aPatients with age ⩽ 60 were reference group. bModel 1: total tea intake evaluation models. cModel 2: green tea and British tea intake evaluation models. dModel 3: one type tea intake (either green tea or black tea) or combination intake of both green tea and black tea evaluation models. ePatients with BMI ⩽ 25 were reference group. fPatients without green tea consumption were the reference group. gPatients without British tea consumption were the reference group. hPatients without tea consumption were reference group.
Exercisers consumed higher volume of tea than the non-exercisers (41% vs 33%, P o 0.01). The results of the multivariate analyses after control for the confounding effects of other risk factors are shown in Table 5. All significant variables in univariate test were presented. Among all the three multivariable regression models, older age (age 460 years, odds ratio (OR): 4.15–4.19, Po 0.01), obesity (BMI425 kg m − 2, OR: 2.10–2.11, P o0.01), family history of hypertension (OR: 2.69–2.76, P o0.01), diabetes history (OR: 2.29–2.33, P o0.01), hyperlipidemia history (OR: 1.79–1.80, P o0.01), male (OR: 1.56–1.59, P o0.01) and coffee intake (OR: 1.44–1.46, Po 0.05) were all associated with higher risk in the prevalence of hypertension. Model 1 examined the correlation of hypertension and intake of tea and showed that tea consumption (of any kinds) was associated with a reduction in the risk of hypertension (model 1, OR: 0.74, 95% confidence interval (CI): 0.56–0.97, P o 0.05). Model 2 investigated the influence of the tea intake amount and indicated that green tea consumption (can blend with other kind of tea) at high volume (4150 ml per week) had good potential of lowering blood pressure (model 2, OR: 0.63, 95% CI: 0.43–0.91, P o 0.05). Model 3 analyzed the effect of tea combination and demonstrated that combination of drinking green tea and British tea was associated with a greater reduction in the risk of hypertension (model 3, OR: 0.58, 95% CI: 0.39–0.85, P o0.05). DISCUSSION In our study, the prevalence of hypertension for the subjects aged ⩾ 40 years and ⩾ 60 years was 49.73% and 66.47%, respectively (calculated from Table 1) and were congruent with the Singapore Ministry of Health 2010 statistics10 and the survey by Malhotra et al.11 The prevalence among subjects aged 70–79 years and ⩾ 80 years were 76.97% and 80.85%, respectively, further showing the seriousness of the situation among the elderly in Singapore. © 2015 Macmillan Publishers Limited
Our study also produced some interesting observations about the potential effect of tea consumption and hypertension. The major constituents of tea are flavonoids and polyphenols, including catechin, epicatechin, epigalloca-techin, epicatechin gallate, epigallocate-chin gallate, quercetin, kaempferol and so on.28 The objective in the production of green tea is to avoid the oxidation of those polyphenols. In contrast, black tea is produced by promoting enzymatic oxidation of tea polyphenols. During this process, catechins are oxidized to complex theaflavins. The partially fermented oolong tea represents an intermediate between black tea and green tea.29 In this large population study of Singaporean Chinese adults aged ⩾ 40 years, we found that more frequent tea consumption, especially consumption of the combination of green tea and British tea, was associated with a lower risk of hypertension (Table 5, model 3, OR: 0.58, 95% CI: 0.39–0.85, P o 0.05). Drinking green tea at least one cup every week (4150 ml per week) for 12 months was associated with lowering the risk of hypertension (Table 5, model 2, OR: 0.63, 95% CI: 0.43–0.91, P o0.05). Hence, if this association can be further confirmed by large-scale, longer-term cohort study or proven as causal relationship by randomized controlled clinical trial, it may be advisable to encourage subjects with low-tea consumption, such as elderly subjects (age 480 years), non-exercisers and retirees in our study, to increase tea consumption. In addition, our results indicated coffee intake was associated with a slightly increased risk of hypertension (OR: 1.44–1.46, P o0.05). This finding has been shown by other researchers as well. Hu et al.30 also found coffee drinking had the potential to increase the risk of compromising the effects of antihypertensive drug treatment, and likewise, Zhang et al.31 indicated that slightly elevated hypertension risk was associated with light-to-moderate consumption of coffee. Hence, patients with high risk of hypertension should limit the intake of coffee. As caffeine, the major active ingredient in coffee, is also one important component in tea, there may also be worries about the risk of tea in raising blood pressure. According to the report by Hodgson Journal of Human Hypertension (2015), 1 – 7
Tea consumption and Singaporean Chinese hypertension W Li et al
6
et al.,32 ingestion of green tea or black tea did cause greater acute increase in blood pressure than taking caffeine alone. Nevertheless, any acute effects of tea on blood pressure was found not to translate into significant alterations in ambulatory blood pressure during regular tea consumption.32 This is probably due to the effect of flavonoids or polyphenols.33–35 However, longterm cohort study and/or randomized control trials are needed to confirm this hypothesis. Finally, our study did not find any correlation of hypertension prevalence with consumption of supplements, cigarette, alcohol, salt, vegetable and fruit and exercising. One most plausible explanation would be due to the sample size or sampling limitation. Certainly, there are other limitations in our current study. Being a cross-sectional study, it was not possible to establish a causal relationship between the factors evaluated and the risk of hypertension. Another intrinsic limitation due to our study design was that the temporal relationship between tea drinking and appearance of hypertension could not be established. Hence, our observations would need to be verified by longer-term cohort study and better still confirmed by a randomized control trial. In conclusion, this cross-sectional study suggests an association between the consumption of tea, especially green tea and British tea, and lowering of the risk of having hypertension, while the consumption of coffee may have the opposite effect. These findings may provide insights to develop public health-promotion strategies to reduce the risk of hypertension and warrant further studies to confirm the observed association.
3
4
5
6 7
8 9
10
11
12 13
14
What is known about this topic ● Hypertension is a global public health issue and associated closely with the incidence of cardiovascular and cerebrovascular diseases. The prevalence and risk of hypertension increased greatly from age 40 years. ● Clinical studies indicating that tea consumption could lower blood pressure in countries other than Singapore. What this study adds ● This is the first study examining the correlation between tea consumption and hypertension among Singaporean Chinese residents. ● We found an association between consumption of tea with reduced prevalence of hypertension and a stronger association between high tea consumption and prevalence of hypertension. ● We found no association between supplement intake and other known risk factors with the prevalence of hypertension in Singaporean Chinese residents aged ⩾ 40 years.
15
16
17
18
19
20 21
CONFLICT OF INTEREST
22
The authors declare no conflict of interest. 23
ACKNOWLEDGEMENTS The research was sponsored by a seed grant from the Singapore College of Traditional Chinese Medicine (STCM 2013-1). The authors are grateful for the support and help from Chung Hwa Medical Institution, Singapore. The support and research assistance from the students of the STCM are highly appreciated.
24
REFERENCES
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1 Organization WH. A global brief on Hypertension--Silent killer, global public health crisis, World Health Organization: 20 Avenue Appia, 1211 Geneva 27, Switzerland, 2013. 2 Lip GY, Frison L, Halperin JL, Lane DA. Comparative validation of a novel risk score for predicting bleeding risk in anticoagulated patients with atrial fibrillation: the HAS-BLED (hypertension, abnormal renal/liver function, stroke, bleeding history
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or predisposition, labile INR, elderly, drugs/alcohol concomitantly) score. J Am Coll Cardiol 2011; 57: 173–180. Aburto NJ, Hanson S, Gutierrez H, Hooper L, Elliott P, Cappuccio FP. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ 2013; 346: f1378. Okin PM, Kjeldsen SE, Julius S, Hille DA, Dahlof B, Devereux RB. Effect of changing heart rate during treatment of hypertension on incidence of heart failure. Am J Cardiol 2012; 109: 699–704. Bakris GL, Sarafidis PA, Weir MR, Dahlof B, Pitt B, Jamerson K et al. Renal outcomes with different fixed-dose combination therapies in patients with hypertension at high risk for cardiovascular events (ACCOMPLISH): a prespecified secondary analysis of a randomised controlled trial. Lancet 2010; 375: 1173–1181. World Health Organization. Global status report on noncommunicable diseases 2010. WHO: Geneva, Switzerland, 2011. Piccini RX, Facchini LA, Tomasi E, Siqueira FV, Silveira DS, Thume E et al. Promotion, prevention and arterial hypertension care in Brazil. Rev Saude Publica 2012; 46: 543–550. Wong TY, Mitchell P. The eye in hypertension. Lancet 2007; 369: 425–435. Han JA, Frishman WH, Wu Sun S, Palmiero PM, Petrillo R. Cardiovascular and respiratory considerations with pharmacotherapy of glaucoma and ocular hypertension. Cardiol Rev 2008; 16(2): 95–108. Epidemiology & Disease Control Division MoH, Singapore. National Health Survey 2010. Epidemiology & Disease Control Division, vol. 2010. Ministry of Health, Singapore: Singapore, Singapore, 2010. Malhotra R, Chan A, Malhotra C, Ostbye T. Prevalence, awareness, treatment and control of hypertension in the elderly population of Singapore. Hypertens Res 2010; 33: 1223–1231. Zhen Y-S. Tea: Bioactivity and Therapeutic Potential, vol. 17. CRC Press: 11 New Fetter Lane, London EC4P 4EE, 2002. Bogdanski P, Suliburska J, Szulinska M, Stepien M, Pupek-Musialik D, Jablecka A. Green tea extract reduces blood pressure, inflammatory biomarkers, and oxidative stress and improves parameters associated with insulin resistance in obese, hypertensive patients. Nutr Res 2012; 32: 421–427. Ihm SH, Jang SW, Kim OR, Chang K, Oak MH, Lee JO et al. Decaffeinated green tea extract improves hypertension and insulin resistance in a rat model of metabolic syndrome. Atherosclerosis 2012; 224: 377–383. Yang HY, Yang SC, Chao JC, Chen JR. Beneficial effects of catechin-rich green tea and inulin on the body composition of overweight adults. Br J Nutr 2012; 107: 749–754. Zhou CJ, Huang S, Liu JQ, Qiu SQ, Xie FY, Song HP et al. Sweet tea leaves extract improves leptin resistance in diet-induced obese rats. J Ethnopharmacol 2013; 145: 386–392. Sundaram R, Naresh R, Shanthi P, Sachdanandam P. Modulatory effect of green tea extract on hepatic key enzymes of glucose metabolism in streptozotocin and high fat diet induced diabetic rats. Phytomedicine 2013; 20: 577–584. McKay DL, Chen CY, Saltzman E, Blumberg JB. Hibiscus sabdariffa L. tea (tisane) lowers blood pressure in prehypertensive and mildly hypertensive adults. J Nutr 2010; 140: 298–303. Mozaffari-Khosravi H, Ahadi Z, Barzegar K. The effect of green tea and sour tea on blood pressure of patients with type 2 diabetes: a randomized clinical trial. J Diet Suppl 2013; 10: 105–115. Forman JP, Scott JB, Ng K, Drake BF, Suarez EG, Hayden DL et al. Effect of vitamin D supplementation on blood pressure in blacks. Hypertension 2013; 61: 779–785. Li Z, Ye R, Zhang L, Li H, Liu J, Ren A. Folic acid supplementation during early pregnancy and the risk of gestational hypertension and preeclampsia. Hypertension 2013; 61: 873–879. Trimarco V, Cimmino CS, Santoro M, Pagnano G, Manzi MV, Piglia A et al. Nutraceuticals for blood pressure control in patients with high-normal or grade 1 hypertension. High Blood Press Cardiovasc Prev 2012; 19: 117–122. Neri I, Monari F, Sgarbi L, Berardi A, Masellis G, Facchinetti F. L-arginine supplementation in women with chronic hypertension: impact on blood pressure and maternal and neonatal complications. J Matern Fetal Neonatal Med 2010; 23: 1456–1460. Yang TY, Wei JC, Lee MY, Chen CM, Ueng KC. A randomized, double-blind, placebo-controlled study to evaluate the efficacy and tolerability of Fufang Danshen (Salvia miltiorrhiza) as add-on antihypertensive therapy in Taiwanese patients with uncontrolled hypertension. Phytother Res 2012; 26: 291–298. Pan C, Huo Y, An X, Singh G, Chen M, Yang Z et al. Panax notoginseng and its components decreased hypertension via stimulation of endothelial-dependent vessel dilatation. Vascul Pharmacol 2012; 56: 150–158. Charan J, Biswas T. How to calculate sample size for different study designs in medical research? Indian J Psychol Med 2013; 35: 121–126. World Health Organization. BMI Classification, 2004 http://apps.who.int/bmi/ index.jsp?introPage = intro_3.html. In.
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7 28 Khan N, Mukhtar H. Tea and health: studies in humans. Curr Pharm Des 2013; 19: 6141–6147. 29 Stangl V, Lorenz M, Stangl K. The role of tea and tea flavonoids in cardiovascular health. Mol Nutr Food Res 2006; 50: 218–228. 30 Hu G, Jousilahti P, Nissinen A, Bidel S, Antikainen R, Tuomilehto J. Coffee consumption and the incidence of antihypertensive drug treatment in Finnish men and women. Am J Clin Nutr 2007; 86: 457–464. 31 Zhang Z, Hu G, Caballero B, Appel L, Chen L. Habitual coffee consumption and risk of hypertension: a systematic review and meta-analysis of prospective observational studies. Am J Clin Nutr 2011; 93: 1212–1219.
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32 Hodgson JM, Puddey IB, Burke V, Beilin LJ, Jordan N. Effects on blood pressure of drinking green and black tea. J Hypertens 1999; 17: 457–463. 33 Bhardwaj P, Khanna D. Green tea catechins: defensive role in cardiovascular disorders. Chin J Nat Med 2013; 11: 345–353. 34 Huang WY, Davidge ST, Wu J. Bioactive natural constituents from food sourcespotential use in hypertension prevention and treatment. Crit Rev Food Sci Nutr 2013; 53: 615–630. 35 Li F, Ohnishi-Kameyama M, Takahashi Y, Yamaki K. Tea polyphenols as novel and potent inhibitory substances against renin activity. J Agric Food Chem 2013; 61: 9697–9704.
Journal of Human Hypertension (2015), 1 – 7
ORIGINAL ARTICLE
Correlation between tea consumption and prevalence of hypertension among Singaporean Chinese residents aged ⩾ 40 years W Li1,2, J Yang3, XS Zhu4, S-C Li5 and PC Ho6 By a cross-sectional epidemiology study, we attempted to correlate the consumption of tea and/or health supplements, living habits and socio-demographic factors to the prevalence of hypertension among Singaporean Chinese residents. Singaporean Chinese residents aged ⩾ 40 years were randomly selected and interviewed face-to-face by clinical research assistants. Hypertension was defined as measured systolic blood pressure at least 140 mm Hg and/or diastolic blood pressure at least 90 mmHg or self-reported history/treatment for hypertension. The prevalence of hypertension among the whole investigated population (N = 1184, 58.27% females) was 49.73% and the prevalence increased to 66.47% in the sub-population aged ⩾ 60 years. High risk of hypertension was associated with age ⩾ 60 years (odds ratio (OR): 4.15–4.19, P o 0.01), obesity (body mass index 425 kg m− 2, OR: 2.10–2.11, P o 0.01), family history of hypertension (OR: 2.69–2.76, Po 0.01), diabetes history (OR: 2.29–2.33, Po 0.01), hyperlipidemia history (OR: 1.79–1.80, P o0.01), male (OR: 1.56–1.59, P o0.01) and coffee intake (OR: 1.44–1.46, P o0.05). In contrast, drinking green tea at least 150 ml per week was associated with lower hypertension risk (OR: 0.63, 95% confidence interval (CI): 0.43–0.91, P o 0.05). Drinking combination of green tea and British tea was associated with higher reduction in the risk of hypertension (OR: 0.58, 95% CI: 0.39–0.85, P o0.05). This cross-sectional study suggests that consumption of tea, especially green tea and British tea, was associated with lowering the risk of hypertension. On the other hand, consumption of coffee could be a risk factor of hypertension. These findings may provide useful information for health promotion to reduce risk of hypertension and warrant further study to confirm and elucidate such association. Journal of Human Hypertension advance online publication, 28 May 2015; doi:10.1038/jhh.2015.45
INTRODUCTION Hypertension, or also known as high or raised blood pressure, is a global public health issue.1 Hypertension has been recognized as a vital risk factor for the incidence of cerebrovascular disease,2 coronary heart disease,3 heart failure4 and chronic renal failure.5 According to Global Health Observatory group of World Health Organization (WHO), due to the global population growth and ageing, the overall prevalence of hypertension in adults aged ⩾ 25 years was around 40% in 2008 and the number of hypertensive patients has increased globally from 600 million in 1980 to 1 billion in 2008.6 Male and age 440 years were found to be highly associated with more severe hypertension.7 In adults ⩾ 40 years, signs of hypertensive retinopathy were frequently found and predictive of future incident of stroke, congestive heart failure and cardiovascular mortality.8 Glaucoma and ocular hypertension were also more commonly found in individuals aged 440 years.9 From the Statistics of the Ministry of Health, Singapore, the prevalence of hypertension in adults aged 18–69 years was 23.5% (2010),10 but the prevalence increased markedly from age 40 years onwards. More than half of the elderly (53.4%) in Singapore aged 60–69 years were found to have hypertension.10 A survey of senior Singaporeans by Malhotra et al.11 in 2010 also found nearly 1
three-fourths (73.9%) of Singaporeans aged ⩾ 60 years to be hypertensive. As the prevalence and risk of hypertension increased greatly from age 40 years, we targeted the population with age ⩾ 40 years in our current study. Chinese folks have 4–5 thousand years of tea-drinking history. To Chinese, the three most widely consumed tea are black tea (or the British tea) (fully fermented), oolong tea (semi-fermented) and green tea (non-fermented).12 The tea extracts have been identified to have pharmacological effects highly associated with reducing blood pressure,13,14 obesity,15,16 hyperglycemia17 and so on. There are clinical studies indicating that tea consumption could lower blood pressure.18,19 However, no similar studies on the correlation between tea drinking and its potential beneficial effect on lowering blood pressure among Singaporeans have been conducted. Health supplements are used extensively in the management of hypertension,20–23 most of which are nutritional supplements, such as vitamins, amino acids and coenzyme Q10. Clinical studies on complementary consumption of herbs for the treatment of hypertension have also been reported.24,25 Nevertheless, it is yet not clear whether Singaporean Chinese can benefit from their habits of consumption of tea and/or health supplements to control hypertension. Hence, we proposed to undertake an epidemiology study to investigate the prevalence of
Singapore College of Traditional Chinese Medicine, Singapore, Singapore; 2Department of Pharmaceutics, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Piscataway, NJ, USA; 3National University Hospital of Singapore, Singapore, Singapore; 4Centre for Complementary Medicine Research, University of Western Sydney, Sydney, New South Wales, Australia; 5School of Biomedical Sciences and Pharmacy, University of Newcastle, Newcastle, New South Wales, Australia and 6Department of Pharmacy, National University of Singapore, Singapore, Singapore. Correspondence: Dr W Li, Department of Pharmaceutics, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Room 013, 160 Frelinghuysen Road, Piscataway, NJ 08854-8020, USA. E-mail: [email protected]. Received 11 December 2014; revised 20 March 2015; accepted 16 April 2015
Tea consumption and Singaporean Chinese hypertension W Li et al
2 hypertension in Singapore and also to correlate hypertension to consumption of tea and/or health supplements, living habits and socio-demographic factors. This would be the first study focusing on correlation between tea consumption and hypertension among Singaporean Chinese residents. The findings would provide useful information for effective prevention and treatment of hypertension.
Table 1. Association of socio-demographic factors with the hypertension prevalence
MATERIALS AND METHODS Ethical consideration The study protocol followed ethical standards of Declaration of Helsinki 1975 as revised in 2008 and was approved by institutional review board of Singapore College of Traditional Chinese Medicine.
Sample size calculation Following the method the sample size was
as reported by Charan calculated according to
and Biswas,26 the equation:
z 21 - α Pð1 - PÞ
sample size ¼ 2 d2 . We set type 1 error α = 0.05, hence Z1-α/2 = 1.96; and precision (d) as 0.05. According to Statistics of the Ministry of Health, Singapore,10 the prevalence of hypertension in adults aged 18–69 years was 23.5%, so expected portion of population was set as 0.235. Hence the minimum sample size ¼ 1:962 ´ 0:235 ´ ð1 - 0:235Þ =0:052 ¼ 276.
Subjects' recruitment A simple random sampling was carried out with proportional fixation by communities and neighborhoods. Chinese residents (citizen or Permanent Resident) of five major districts of Singapore (Northern East, Northern West, Southern East, Southern West and Centre) aged ⩾ 40 years were randomly selected from community-based visits and invited to participate in the study. Ethnic groups other than Chinese, aged o40 years, those who were unable or refused to complete the interview, those with cognitive dysfunction, pregnant women and lactating women were excluded. The participants were randomly selected and interviewed faceto-face by 56 trained clinical research assistants from Singapore College of Traditional Chinese Medicine from June 2013 to December 2013. Each participant was explained the details of the study and asked to sign an informed consent form before the survey was conducted. Each subject was interviewed by two research assistants. During and after the survey, one of the investigators (Wenji Li) would randomly follow-up the participants to validate the quality of acquired data.
Socio-demographics information and blood pressure acquisition During the interview, all the subjects were enquired about their sociodemographic information, such as age, race, gender, social identity number, telephone number, weight, height, address and occupation. Their medical history such as hypertension history, family hypertension history, diabetes history and hyperlipidemia history were also recorded. Blood pressure for each subject was taken after 5-min rest. Three seated blood pressure measurements were recorded with an automatic sphygmomanometer (HEM-7203, OMRON, Singapore, Singapore). Each reading and the average of the three readings were recorded. The average of the three readings was the blood pressure value used in the analysis. Hypertension was defined as tested systolic blood pressure ⩾ 140 mm Hg and/or diastolic blood pressure ⩾ 90 mm Hg or self-reported history/ treatment for hypertension.
Journal of Human Hypertension (2015), 1 – 7
Hypertension
P
Age (average age), years o50 50–59 60–69 70–79 ⩾ 80
56.76 ± 8.52 72 (64.29%) 323 (63.96%) 126 (40.91%) 35 (23.03%) 9 (19.15%)
62.92 ± 10.36 40 (35.71%) 182 (36.04%) 182 (59.09%) 117 (76.97%) 38 (80.85%)
0.000 0.000
Gender Male Female BMI (kg m −2) Underweighta Normal weightb Overweightc Obesityd
208 (44.35%) 357 (54.50%) 22.91 ± 3.49 39 (62.90%) 410 (55.18%) 98 (37.69%) 18 (30.51%)
261 (55.65%) 298 (45.50%) 24.25 ± 3.89 23 (37.10%) 333 (44.82%) 162 (62.31%) 41 (69.49%)
0.001
Family history of hypertension No 381 (58.08%) Yes 184 (39.32%)
275 (41.92%) 284 (60.68%)
0.000
History of hyperlipidemia No Yes
475 (55.69%) 90 (33.21%)
378 (44.31%) 181 (66.79%)
0.000
History of diabetes mellitus No Yes
521 (54.61%) 44 (25.88%)
433 (45.39%) 126 (74.12%)
0.000
Occupation Retired Working
196 (41.26%) 369 (56.86%)
279 (58.74%) 280 (43.14%)
0.000
0.004 0.000
Abbreviation: BMI, body mass index. aUnderweight (BMI o18.5 kg m − 2). b Normal (18.5 ⩽ BMI o25 kg m − 2). cOverweight (25 ⩽ BMI o30 kg m − 2). d Obesity (BMI ⩾ 30 kg m − 2). P-value represents the difference in characteristics based on a chi-square test (for dichotomous variables) or analysis of variance (for continuous variables) as appropriate, similarly hereinafter.
450 ml). The frequency record mark multiplied by the usual consumed volume mark provided the total volume index. A subject was classified as tea nondrinker if the total volume mark equaled zero. Total low and total high intakes were defined as total volume mark o300 ml per week and ⩾ 300 ml per week, respectively. Low intake of a single type of tea (green, oolong or British) and high intake of a single type of tea were defined as the total volume mark o150 ml per week and ⩾ 150 ml per week, respectively.
Other data acquisition Subjects were asked to recall the frequency with which they have consumed the following items or performance of exercise in the past 12 months. ●
Tea intake inquiry Subjects were asked to recall the average frequency of tea intake in the past 12 months. The questions were designed according to habitual intake of common types of tea among locals by using local references and terms: black tea (or British tea), oolong tea, or green tea. Black tea or British tea is usually consumed with sugar and milk, whereas oolong tea and green tea are almost always drunk without sugar or milk. The frequency of consumption of each type of tea was classified into five categories: never or rarely (marked 0), ⩾ once per week (marked 1), ⩾ 7 times per week (marked 7), ⩾ 14 times per week (marked 14), and ⩾ 21 times per week (marked 21). The subjects were also asked to provide the usual consumed volume set as ⩾ 150 ml (one small cup, marked 150 ml), ⩾ 300 ml (two small cups, marked 300 ml) and ⩾ 450 ml (three small cups, marked
Non-hypertension
Variables
● ● ● ●
Consumption of nutrition supplements (such as coenzyme Q10, fish oil, garlic, vitamin C) or TCM herbs or alcohol and exercise performance were determined according to four categories (0: None, 1: ⩾ once per month, 2: ⩾ once per week, and 3: ⩾ once per day). Cigarette smoking was classified into four categories (0: never, 1: 1–2 per day, 2: 3–4 per day, and 3: ⩾ 5 per day). Consumption of coffee was recorded as following: 0: none, 1: 1 cup per week, 2: 2–3 cups per week, and 3: ⩾ 3 cups per week. Consumption of vegetables and fruits were encoded as: 0: none, 1: once per week, 2: 2–3 times per week, and 3: ⩾ 3 times per week. The categorical variables (0, 1, 2 and 3) were created for consumption of salt ( o6g per day, ⩾ 6g per day, 7–10g per day and ⩾ 10g per day). Followed by the WHO Body Mass Index (BMI) Classification Standard,27 the classification of body weight was as follows: underweight (BMIo 18.5), normal (18.5 ⩽ BMIo25), overweight (25 ⩽ BMIo30), and obesity (BMI ⩾ 30). © 2015 Macmillan Publishers Limited
Tea consumption and Singaporean Chinese hypertension W Li et al
3 Table 2. Association of the living habits with the prevalence of hypertension Variables Nutrition supplement Not taking Taking
Non-hypertension
Hypertension
Table 3. Correlation between the prevalence of hypertension and categorical tea intakea or binominal tea intakeb P
311 (48.52%) 254 (52.59%)
330 (51.48%) 229 (47.41%)
0.177
Functional supplement Not taking 404 (50.25%) Taking 161 (50.31%)
400 (49.75%) 159 (49.69%)
0.985
Cigarette Not smoking Smoking
494 (49.50%) 65 (51.59%)
0.659
504 (50.50%) 61 (48.41%)
Non-hypertension
Hypertension
P
354 (47.39%) 102 (53.40%) 109 (58.60%)
393 (52.61%) 89 (46.60%) 77 (41.40%)
0.015
Intake of oolong teaa None Low (⩽150) High (4150)
372 (49.47%) 81 (47.65%) 112 (55.45%)
380 (50.53%) 89 (52.35%) 90 (44.55%)
0.243
Intake of British teaa None Low (⩽150) High (4150)
349 (47.81%) 99 (55.93%) 117 (53.92%)
381 (52.19%) 78 (44.07%) 100 (46.08%)
0.074
Intake of tea as a wholea None 189 (46.10%) Low (⩽300) 143 (50.71%) High (4300) 233 (53.94%)
221 (53.90%) 139 (49.29%) 199 (46.06%)
0.074
Variables Intake of green tea None Low (⩽150) High (4150)
a
Alcohol Not drinking Drinking
459 (49.68%) 106 (53.00%)
465 (50.32%) 94 (47.00%)
0.394
Exercise No Yes
156 (47.13%) 409 (51.58%)
175 (52.87%) 384 (48.42%)
0.174
Coffee Not drinking Drinking
152 (58.46%) 413 (47.80%)
108 (41.54%) 451 (52.20%)
0.003
Intake of green teab No Yes
354 (47.39%) 211 (55.97%)
393 (52.61%) 166 (44.03%)
0.007
Salt intake Low high
165 (50.61%) 400 (50.13%)
161 (49.39%) 398 (49.87%)
0.882
Intake of oolong teab No Yes
372 (49.47%) 193 (51.88%)
380 (50.53%) 179 (48.12%)
0.446
Vegetable and fruit Not taking Taking
10 (43.48%) 555 (50.41%)
13 (56.52%) 546 (49.59%)
0.511
Intake of British teab No Yes
349 (47.81%) 216 (54.82%)
381 (52.19%) 178 (45.18%)
0.025
Intake of tea as a wholeb No 189 (46.10%) Yes 376 (52.66%)
221 (53.90%) 338 (47.34%)
0.034
Statistical analysis Univariate analysis was employed to evaluate between the two groups with hypertension and without hypertension, respectively, by using a chi-square test for dichotomous variables or by using analysis of variance for continuous variables. All variables which were significant in the previous test were enrolled in logistic regression analysis to examine the relation between tea intake and other correlated factors and prevalence of hypertension. Three models were utilized to analyze the influence of tea intake. Model 1 included the tea intake (yes or no) and other significant univariables. Model 2 included the consumption level of different types of tea (none, low or high level) and other significant univariables. Model 3 included single or synergetic effects of green tea and British tea (none, green tea only, British tea only or a combination of both) and other significant univariables. All the statistical tests were two-sided, and Po0.05 was regarded as significant. All data were double entered by Epidata (version 3.1; The EpiData Association, Odense, Denmark) and analyzed by STATA respectively (version 11; published by Stata Press, College Station, TX, USA).
RESULTS During the study period, a total of 1184 participants were recruited. Among them, 60 subjects were omitted because of missing data and 1124 participants were analyzed for the present study (Table 1). As shown in Table 1, in the univariate analysis of the socio-demographic factors and hypertension, the subjects with hypertension indicated a higher average age than the nonhypertensive subjects (Po 0.01). Data also indicated higher ratio of hypertension in the older age groups (Po 0.01) when the age group was stratified (P o0.01). Among all the 1124 Singaporean Chinese subjects, there were more male hypertensive subjects (55.65% vs 45.50%, Po 0.01). High body weight was highly associated with hypertension. In the subjects ranging from underweight to obesity, the ratio of hypertension increased from © 2015 Macmillan Publishers Limited
a
Model 1: categorical tea intake evaluation models. bModel 2: binominal tea intake evaluation models.
37.10% to 69.49% (P o 0.01). Prevalence of hypertension also closely associated with the family history of hypertension (P o 0.01), history of hyperlipidemia (Po 0.01) and history of diabetes mellitus (Po 0.01). Hypertension was more common in retirees than those still working (58.74% vs 43.14%, P o0.01). For the univariate analysis of the effects of supplements and living habits on the prevalence of hypertension, we only found that drinking of coffee had higher correlation with hypertension (52.20% vs 41.54%, P o 0.01; Table 2). To understand the effect of tea intake on hypertension, the categorical intake and binominal intake were analyzed together (Table 3). Higher category intake of green tea was more likely to be associated with reducing the hypertension prevalence (high intake, low intake and non-intake: 41.40, 46.60 and 52.61%, respectively, P o 0.05). Comparing with non-drinkers, drinkers of green tea, British tea or tea as a whole had lower hypertension ratio (P o0.05). Tea intake habits among subjects with various backgrounds were analyzed and only results with significant differences are listed in Table 4. Higher portion of subjects in the 50–59 age group were more frequent tea drinkers than any other age groups (43%, P o 0.01). In addition, larger percentage of this group was high-volume green tea drinkers (19%, P o 0.01) than others. Most subjects aged ⩾ 80 years did not drink tea (51%, P o 0.01). Most hypertensive subjects did not drink green tea (70%, Po 0.05). More retired people were non-tea-drinker than those still working (41% vs 33%, P o 0.05). Alcohol drinkers tended to consume larger volume of tea than the alcohol abstainers (50% vs 36%, Po 0.01). Journal of Human Hypertension (2015), 1 – 7
Journal of Human Hypertension (2015), 1 – 7
142 (15) 49 (25)
637 (69) 110 (55)
233 (70) 514 (65)
Alcohol No Yes
Exercise No Yes
34 (13) 157 (18)
58 (18) 133 (17)
(18) (19) (15) (13) (13)
51 (20) 135 (16)
40 (12) 146 (18)
145 (16) 41 (21)
139 (17) 47 (15)
62 (13) 124 (19)
109 (19) 77 (14)
15 98 47 20 6
0.082
0.033
0.001
0.048
0.001
0.015
0.001
P
(73) (61) (71) (72) (68)
183 (70) 569 (66)
242 (73) 510 (64)
651 (70) 101 (51)
549 (68) 203 (63)
334 (70) 418 (64)
372 (66) 380 (68)
82 309 220 109 32
None
High
(15) (18) (13) (13) (13)
32 (12) 138 (16)
41 (12) 129 (16)
125 (14) 45 (23)
121 (15) 49 (15)
59 (12) 111 (17)
81 (14) 89 (16)
17 89 39 19 6
(12) (21) (16) (16) (19)
45 (17) 157 (18)
48 (15) 154 (19)
148 (16) 54 (27)
134 (17) 68 (21)
82 (17) 120 (18)
112 (20) 90 (16)
13 107 49 24 9
Number (% of subgroup)
Low
Oolong tea
0.291
0.017
0.000
0.175
0.060
0.243
0.052
P
(56) (64) (69) (64) (74)
179 (69) 551 (64)
230 (69) 500 (63)
617 (67) 113 (57)
526 (65) 204 (64)
313 (66) 417 (64)
349 (62) 381 (68)
63 322 212 98 35
None
High
(26) (17) (13) (13) (6)
27 (10) 150 (17)
56 (17) 121 (15)
133 (14) 44 (22)
120 (15) 57 (18)
74 (16) 103 (16)
99 (18) 78 (14)
29 85 40 20 3
(18) (19) (18) (22) (19)
54 (21) 163 (19)
45 (14) 172 (22)
174 (19) 43 (22)
158 (20) 59 (18)
88 (19) 129 (20)
117 (21) 100 (18)
20 98 56 34 9
Number (% of subgroup)
Low
British tea
All records are presented as 'actual number (percentage of the related subgroup)'. 68 (82) means 68 cases, which accounts for 82% of its subgroup.
175 (67) 572 (66)
123 (15) 68 (21)
Functional supplement No 542 (67) Yes 205 (64)
Coffee No Yes
67 (14) 124 (19)
346 (73) 401 (62)
Occupation Retired Working
102 (18) 89 (16)
(19) (17) (10) (4)
354 (63) 393 (70)
94 51 15 2
Hypertension No Yes
(82) (62) (68) (77) (83)
High
Number (% of subgroup)
Low
68 313 210 117 39
None
Green tea
Correlation between tea intake and various study factors
Age (years) o50 50–59 60–69 70–79 ⩾ 80
Variables
Table 4.
0.025
0.007
0.009
0.476
0.824
0.074
0.038
P
(32) (32) (40) (42) (51)
100 (38) 310 (36)
144 (44) 266 (34)
362 (39) 48 (24)
295 (37) 115 (36)
197 (41) 213 (33)
189 (33) 221 (40)
36 164 122 64 24
None
High
(35) (24) (25) (26) (13)
55 (21) 227 (26)
79 (24) 203 (26)
229 (25) 53 (27)
199 (25) 83 (26)
114 (24) 168 (26)
143 (25) 139 (25)
39 122 76 39 6
(33) (43) (36) (32) (36)
105 (40) 327 (38)
108 (33) 324 (41)
333 (36) 99 (50)
310 (39) 122 (38)
164 (35) 268 (41)
233 (41) 199 (36)
37 219 110 49 17
Number (% of subgroup)
Low
Tea total
0.248
0.005
0.000
0.916
0.010
0.074
0.007
P
Tea consumption and Singaporean Chinese hypertension W Li et al
4
© 2015 Macmillan Publishers Limited
Tea consumption and Singaporean Chinese hypertension W Li et al
5 Table 5.
Logistic regression analysis of the outcomes in the three models (models 1–3) OR
s.e.
P-value
95% CI
Age460a Family history of Hypertension BMI 425e History of diabetes History of hyperlipidemia Male Coffee intake Tea intakeb
4.19b,c, 4.15d 2.69b, 2.76c, 2.72d 2.11b, 2.10c,d 2.29b, 2.33c, 2.32d 1.8b, 1.79c,d 1.59b, 1.56c,d 1.45b, 1.44c, 1.46d 0.74b
0.6b,c, 0.59d 0.39b, 0.40c,d 0.32 0.48b, 0.49c,d 0.3 0.22 b,c 0.23 , 0.24d 0.1b
0 0 0 0 0 0 0.02 0.03b
3.17–5.54b, 3.16–5.55c, 3.14–5.49d 2.02–3.58b, 2.07–3.68c, 2.04–3.62d 1.56–2.84b, 1.55–2.83c, 1.56–2.83d 1.52–3.44b, 1.55–3.52c, 1.54–3.51d 1.31–2.49b, 1.30–2.48c,d 1.21–2.08b, 1.18–2.04c, 1.19–2.05d 1.06–1.99b,1.05–1.98c, 1.07–2.01d 0.56–0.97b
Green tea intake (ml per week)c,f 0 0–150 4150
1.00 (referent)c 0.91c 0.63c
0.17c 0.12c
0.60c 0.01c
0.63–1.31c 0.43–0.91c
British tea intake (ml per week)c,g 0 0–150 >150
1.00 (referent)c 0.76c 0.74c
0.15c 0.13c
0.16c 0.09c
0.51–1.11c 0.52–1.05c
Tea typed,h None Green tea only British tea only Both green and British tea
1.00 (referent)d 0.76d 0.76d 0.58d
0.14d 0.14d 0.11d
0.14d 0.13d 0.01d
0.52–1.09d 0.53–1.09d 0.39–0.85d
Subgroups
Abbreviations: BMI, body mass index; CI, confidence interval; OR, odds ratio. aPatients with age ⩽ 60 were reference group. bModel 1: total tea intake evaluation models. cModel 2: green tea and British tea intake evaluation models. dModel 3: one type tea intake (either green tea or black tea) or combination intake of both green tea and black tea evaluation models. ePatients with BMI ⩽ 25 were reference group. fPatients without green tea consumption were the reference group. gPatients without British tea consumption were the reference group. hPatients without tea consumption were reference group.
Exercisers consumed higher volume of tea than the non-exercisers (41% vs 33%, P o 0.01). The results of the multivariate analyses after control for the confounding effects of other risk factors are shown in Table 5. All significant variables in univariate test were presented. Among all the three multivariable regression models, older age (age 460 years, odds ratio (OR): 4.15–4.19, Po 0.01), obesity (BMI425 kg m − 2, OR: 2.10–2.11, P o0.01), family history of hypertension (OR: 2.69–2.76, P o0.01), diabetes history (OR: 2.29–2.33, P o0.01), hyperlipidemia history (OR: 1.79–1.80, P o0.01), male (OR: 1.56–1.59, P o0.01) and coffee intake (OR: 1.44–1.46, Po 0.05) were all associated with higher risk in the prevalence of hypertension. Model 1 examined the correlation of hypertension and intake of tea and showed that tea consumption (of any kinds) was associated with a reduction in the risk of hypertension (model 1, OR: 0.74, 95% confidence interval (CI): 0.56–0.97, P o 0.05). Model 2 investigated the influence of the tea intake amount and indicated that green tea consumption (can blend with other kind of tea) at high volume (4150 ml per week) had good potential of lowering blood pressure (model 2, OR: 0.63, 95% CI: 0.43–0.91, P o 0.05). Model 3 analyzed the effect of tea combination and demonstrated that combination of drinking green tea and British tea was associated with a greater reduction in the risk of hypertension (model 3, OR: 0.58, 95% CI: 0.39–0.85, P o0.05). DISCUSSION In our study, the prevalence of hypertension for the subjects aged ⩾ 40 years and ⩾ 60 years was 49.73% and 66.47%, respectively (calculated from Table 1) and were congruent with the Singapore Ministry of Health 2010 statistics10 and the survey by Malhotra et al.11 The prevalence among subjects aged 70–79 years and ⩾ 80 years were 76.97% and 80.85%, respectively, further showing the seriousness of the situation among the elderly in Singapore. © 2015 Macmillan Publishers Limited
Our study also produced some interesting observations about the potential effect of tea consumption and hypertension. The major constituents of tea are flavonoids and polyphenols, including catechin, epicatechin, epigalloca-techin, epicatechin gallate, epigallocate-chin gallate, quercetin, kaempferol and so on.28 The objective in the production of green tea is to avoid the oxidation of those polyphenols. In contrast, black tea is produced by promoting enzymatic oxidation of tea polyphenols. During this process, catechins are oxidized to complex theaflavins. The partially fermented oolong tea represents an intermediate between black tea and green tea.29 In this large population study of Singaporean Chinese adults aged ⩾ 40 years, we found that more frequent tea consumption, especially consumption of the combination of green tea and British tea, was associated with a lower risk of hypertension (Table 5, model 3, OR: 0.58, 95% CI: 0.39–0.85, P o 0.05). Drinking green tea at least one cup every week (4150 ml per week) for 12 months was associated with lowering the risk of hypertension (Table 5, model 2, OR: 0.63, 95% CI: 0.43–0.91, P o0.05). Hence, if this association can be further confirmed by large-scale, longer-term cohort study or proven as causal relationship by randomized controlled clinical trial, it may be advisable to encourage subjects with low-tea consumption, such as elderly subjects (age 480 years), non-exercisers and retirees in our study, to increase tea consumption. In addition, our results indicated coffee intake was associated with a slightly increased risk of hypertension (OR: 1.44–1.46, P o0.05). This finding has been shown by other researchers as well. Hu et al.30 also found coffee drinking had the potential to increase the risk of compromising the effects of antihypertensive drug treatment, and likewise, Zhang et al.31 indicated that slightly elevated hypertension risk was associated with light-to-moderate consumption of coffee. Hence, patients with high risk of hypertension should limit the intake of coffee. As caffeine, the major active ingredient in coffee, is also one important component in tea, there may also be worries about the risk of tea in raising blood pressure. According to the report by Hodgson Journal of Human Hypertension (2015), 1 – 7
Tea consumption and Singaporean Chinese hypertension W Li et al
6
et al.,32 ingestion of green tea or black tea did cause greater acute increase in blood pressure than taking caffeine alone. Nevertheless, any acute effects of tea on blood pressure was found not to translate into significant alterations in ambulatory blood pressure during regular tea consumption.32 This is probably due to the effect of flavonoids or polyphenols.33–35 However, longterm cohort study and/or randomized control trials are needed to confirm this hypothesis. Finally, our study did not find any correlation of hypertension prevalence with consumption of supplements, cigarette, alcohol, salt, vegetable and fruit and exercising. One most plausible explanation would be due to the sample size or sampling limitation. Certainly, there are other limitations in our current study. Being a cross-sectional study, it was not possible to establish a causal relationship between the factors evaluated and the risk of hypertension. Another intrinsic limitation due to our study design was that the temporal relationship between tea drinking and appearance of hypertension could not be established. Hence, our observations would need to be verified by longer-term cohort study and better still confirmed by a randomized control trial. In conclusion, this cross-sectional study suggests an association between the consumption of tea, especially green tea and British tea, and lowering of the risk of having hypertension, while the consumption of coffee may have the opposite effect. These findings may provide insights to develop public health-promotion strategies to reduce the risk of hypertension and warrant further studies to confirm the observed association.
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8 9
10
11
12 13
14
What is known about this topic ● Hypertension is a global public health issue and associated closely with the incidence of cardiovascular and cerebrovascular diseases. The prevalence and risk of hypertension increased greatly from age 40 years. ● Clinical studies indicating that tea consumption could lower blood pressure in countries other than Singapore. What this study adds ● This is the first study examining the correlation between tea consumption and hypertension among Singaporean Chinese residents. ● We found an association between consumption of tea with reduced prevalence of hypertension and a stronger association between high tea consumption and prevalence of hypertension. ● We found no association between supplement intake and other known risk factors with the prevalence of hypertension in Singaporean Chinese residents aged ⩾ 40 years.
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CONFLICT OF INTEREST
22
The authors declare no conflict of interest. 23
ACKNOWLEDGEMENTS The research was sponsored by a seed grant from the Singapore College of Traditional Chinese Medicine (STCM 2013-1). The authors are grateful for the support and help from Chung Hwa Medical Institution, Singapore. The support and research assistance from the students of the STCM are highly appreciated.
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