REVIEW
Tai Lim Kim,1 Gwang Hun Jeong,2 Jae Won Yang,3 Keum Hwa Lee,4,5 Andreas Kronbichler,6 Hans J van der Vliet,7
Giuseppe Grosso,8 Fabio Galvano,8 Dagfinn Aune,9,10,11 Jong Yeob Kim,1 Nicola Veronese,12 Brendon Stubbs,13,14,15
Marco Solmi,16 Ai Koyanagi,17,18 Sung Hwi Hong,1,19 Elena Dragioti,20 Eunyoung Cho,21,22
Leandro FM de Rezende,23 Edward L Giovannucci,22,24 Jae Il Shin,4,5 and Gabriele Gamerith25
1 Yonsei University College of Medicine, Severance Hospital, Seoul, Korea; 2 College of Medicine, Gyeongsang National University, Jinju, Korea; 3 Department
of Nephrology, Yonsei University Wonju College of Medicine, Wonju, Korea; 4 Department of Pediatrics, Yonsei University College of Medicine, Seoul, Korea;
5 Division of Pediatric Nephrology, Severance Children’s Hospital, Seoul, Korea; 6 Department of Internal Medicine IV (Nephrology and Hypertension), Medical
University Innsbruck, Innsbruck, Austria; 7 Department of Medical Oncology, Amsterdam UMC, VU University, Cancer Center Amsterdam, Amsterdam,
The Netherlands; 8 Department of Biomedical and Biotechnological Science, School of Medicine, University of Catania, Catania, Italy; 9 Department of
Epidemiology and Biostatistics, School of Public Health, Imperial College London, London, UK; 10 Department of Nutrition, Bjørknes University College, Oslo,
Norway; 11 Department of Endocrinology, Morbid Obesity and Preventive Medicine, Oslo University Hospital, Oslo, Norway; 12 National Research Council,
Neuroscience Institute, Aging Branch, Padova, Italy; 13 Department of Psychological Medicine, Institute of Psychiatry, Psychology and Neuroscience, King’s
College London, London, UK; 14 South London and Maudsley NHS Foundation Trust, London, UK; 15 Positive Ageing Research Institute, Faculty of Health, Social
Care, Medicine and Education, Anglia Ruskin University, Chelmsford, UK; 16 Department of Neuroscience, University of Padova, Padova, Italy; 17 Parc Sanitari
Sant Joan de Déu/CIBERSAM, Universitat de Barcelona, Barcelona, Spain; 18 ICREA, Barcelona, Spain; 19 Department of Global Health and Population, Harvard
T.H. Chan School of Public Health, Boston, MA, USA; 20 Pain and Rehabilitation Centre, and Department of Health, Medicine and Caring Sciences, Linköping
University, Linköping, Sweden; 21 Department of Dermatology, The Warren Alpert Medical School, Brown University, Providence, RI, USA; 22 Channing Division
of Network Medicine, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA, USA; 23 Universidade Federal de São
Paulo, Escola Paulista de Medicina, Departamento de Medicina Preventiva, São Paulo, Brazil; 24 Department of Nutrition, Harvard T.H. Chan School of Public
Health, Boston, MA, USA; and 25 Internal Medicine V, Department of Hematology & Oncology, Medical University Innsbruck, Innsbruck, Austria
ABSTRACT
Tea is one of the most widely consumed beverages, but its association with cancer risk remains controversial and unclear. We performed an umbrella
review to clarify and determine the associations between tea consumption and various types of cancer by summarizing and recalculating the
existing meta-analyses. Meta-analyses of observational studies reporting associations between tea consumption and cancer risk were searched
on PubMed and Embase. Associations found to be statistically significant were further classified into levels of evidence (convincing, suggestive, or
weak), based on P value, between-study heterogeneity, prediction intervals, and small study effects. Sixty-four observational studies (case-control
or cohort) corresponding to 154 effect sizes on the incidence of 25 types of cancer were included. Forty-three (27.9%) results in 15 different types
of cancer were statistically significant. When combining all studies on the same type of cancer, 19 results in 11 different types of cancer showed
significant associations with lower risk of gastrointestinal tract organ cancer (oral, gastric, colorectal, biliary tract, and liver cancer), breast cancer, and
gynecological cancer (endometrial and ovarian cancer) as well as leukemia, lung cancer, and thyroid cancer. Only the reduced risk of oral cancer
in tea-consuming populations (OR = 0.62; 95% CI: 0.55, 0.72; P value < 10−6 ) was supported by convincing evidence. Suggestive evidence was
found for 6 results on biliary tract, breast, endometrial, liver, and oral cancer. To summarize, tea consumption was shown to have protective effects
on some types of cancer, particularly oral cancer. More well-designed prospective studies are needed with consideration of other factors that can
cause biases. Adv Nutr 2020;11:1437–1452.
Keywords: tea, cancer, oral cancer, meta-analysis, umbrella review
Introduction
Tea produced from the leaves of the plant Camellia sinensis
has been cultivated and consumed for centuries, and is
still one of the most widely consumed beverages worldwide
(1). Tea components vary with factors such as tea variety,
climate, season, agricultural practices, the age of the leaf,
and manufacturing processes (2). Green tea manufacturing
involves steaming or pan-frying fresh tea leaves, thereby
C The Author(s) on behalf of the American Society for Nutrition 2020. Adv Nutr 2020;11:1437–1452; doi: https://doi.org/10.1093/advances/nmaa077.
Copyright
1437
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Tea Consumption and Risk of Cancer: An Umbrella
Review and Meta-Analysis of Observational
Studies
Methods
Data sources and searches
Three investigators (TLK, GHJ, and JIS) independently
searched PubMed and Embase databases for meta-analyses
on the effect of tea consumption on different types of cancers.
Articles were limited to those written in English published
up to April 30, 2019. Keywords used in the search were
The authors reported no funding received for this study. BS is supported by a Clinical
Lectureship (ICA-CL-2017-03-001) jointly funded by Health Education England (HEE) and the
National Institute for Health Research (NIHR). BS is part funded by the NIHR Biomedical
Research Centre at South London and Maudsley NHS Foundation Trust. BS is also supported by
the Maudsley Charity, King’s College London, and the NIHR South London Collaboration for
Leadership in Applied Health Research and Care (CLAHRC) funding.
Author disclosures: The authors report no conflicts of interest.
This article presents independent research. The views expressed in this publication are those of
the authors and not necessarily those of the acknowledged institutions.
Supplemental Tables 1–4 and Supplemental References are available from the “Supplementary
data” link in the online posting of the article and from the same link in the online table of
contents at https://academic.oup.com/advances.
TLK and GHJ contributed equally to this work.
Address correspondence to JIS (e-mail: shinji@yuhs.ac).
Abbreviations used: AICR, American Institute for Cancer Research; EGCG, epigallocatechin
gallate; ES, excess of significance; IARC, International Agency for Research on Cancer; PI,
prediction interval; WCRF, World Cancer Research Fund Network.
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Kim et al.
“(Tea) AND (cancer OR carcinoma OR tumor) AND (metaanalysis OR systematic review).” The articles found using
the two databases were screened and selected for eligibility
based on examination of titles, abstracts, and full texts. Metaanalyses included prospective cohort studies, case-control
studies (hospital-based and population-based), or both study
designs (hereinafter referred to combined observational
studies). Studies of unrelated topics, letters, and case reports
were excluded while screening by title.
Eligibility criteria and extraction of data
Only systematic reviews and meta-analyses investigating
the association between tea consumption and cancer were
eligible for inclusion. Studies that did not specifically include
tea as an independent exposure, such as combined caffeine
exposure or maté tea, were not included. Tea consumption
was divided into consumption of 2 specific types of tea (green
tea and black tea) or consumption of any tea (regardless
of type). The comparison groups of tea exposure were
subclassified as high compared with low, any compared with
none, and increments of 1–3 cups/d. The definition of criteria
of high compared with low consumption of tea and size of a
cup followed that of the original meta-analysis included in
our review. Only meta-analyses that reported outcomes with
metrics that were relevant to the risk of cancer, such as RR,
OR, or HR, were included.
From the eligible meta-analyses, the following data were
extracted: title, first author, year of publication, number
of studies included, type of study (case-control, cohort,
or observational studies including both case-control and
cohort), type of tea, comparison groups of tea consumption, type of cancer, number of cancer cases/total number
of participants, type of outcome metrics (RR or OR),
meta-analysis model, effect size and its 95% CI, and
largest effect size among included studies from each metaanalysis.
Statistical analysis
The primary studies obtained from the original articles were
recalculated to receive additional information to evaluate
the evidence level of meta-analyses. Comprehensive MetaAnalysis (v. 3.3.070; Biostat) and Microsoft Excel (v. 16.0)
were used for the recalculation. The summary effect size, 95%
CI, and P values were calculated under both random- and
fixed-effects models using the identical type of metrics used
as in the original meta-analyses. The summary effect size
(represented as RR, HR, or OR) and 95% CI were recalculated
using meta-analysis with both random-effects and fixedeffects models.
The between-study heterogeneity was recalculated using
the I2 statistic and the P value from the χ 2 -based Cochran
Q test. The I2 statistic describes the percentage of variation
among studies that is due to heterogeneity rather than due
to chance. I2 <50% is considered as low-to-moderate heterogeneity between studies, whereas I2 > 50% is considered as
large and I2 > 75% as very large heterogeneity, respectively
(12). If the heterogeneity between studies was large or very
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rapidly inactivating enzymes and preventing the oxidation
of polyphenols, mainly catechins (3). Black tea is made
by rolling the tea leaves to promote oxidation, followed
by fermenting the leaves, which forms compounds such as
theaflavins and thearubigins (4).
Historically tea has been claimed to have various beneficial health benefits and used for medical purposes (5).
The compounds of tea have been suggested to have cancerpreventive effects in several studies (6–8). However, there
has been no clear consensus in epidemiological literature
about whether tea consumption is beneficial to health or not,
especially concerning cancer (8). Because a large population
consumes tea regularly throughout adult life, potential minor
health benefits or risks associated with its consumption can
have profound health implications at the population level.
There are multiple quantitative studies on the association
between tea and different types of cancer; however, there
is still a need for a comprehensive appraisal of uncertainty
and/or biases in the claimed associations. Recently, a new
quantitative approach called “umbrella reviews” has been
developed to understand the epidemiological credibility of
complex health areas such as cardiovascular diseases, cancer,
and multiple health outcomes (9–11).
Using existing meta-analyses of observational studies, we
conducted an umbrella review of the meta-analyses and
critically appraised the strengths and breadth of claimed
associations between tea consumption and risk of cancer.
In this study, we summarized the results from previously
published meta-analyses and also performed the most
updated meta-analysis by combining individual studies or
the same subject (same type of cancer). To the best of our
knowledge, this study is the first umbrella review to consider
the whole breadth of evidence concerning tea consumption
and cancer incidence.
Determination of the level of evidence in meta-analyses
Associations between tea consumption and the risk of
different types of cancer were classified into 5 levels of
evidence strength in accordance with grading schemes
applied in previously published umbrella reviews (16–18).
Evidence of strong statistical significance using randomeffects meta-analyses at P value <10−6 (19), magnitude of
between-study heterogeneity (I2 < 50%), absence of small
study effects (Egger P value >0.10), and 95% PI excluded the
null.
The criteria for determining the level of evidence were as
follows:
Nonsignificant association: random-effects P value did
not meet the significance threshold (randomeffects P value >0.05).
Weak evidence: result was significant (random-effects P
value <0.05), but there was evidence of betweenstudy heterogeneity (I2 > 50 and 95% PI included
the null) or small study effect.
Suggestive evidence: result was significant (randomeffects P value <0.05), and there was no evidence
of both between-study heterogeneity (I2 < 50) and
small study effect, number of cases >1000, but 95%
PI failed to reject the null hypothesis.
Convincing evidence: result was highly significant for
random-effects P value <10−6 , low to moderate
heterogeneity (I2 < 50), 95% PI rejected the null
hypothesis, no evidence of small study effect,
number of cases >1000, and the largest study was
concordant in terms of statistical significance with
the random-effects result.
In case of inadequate number of individual studies or
unavailable information for calculating 95% PI, I2 , and Egger
P value, we determined that the evidence was insufficient to
state conclusions (see Supplemental Table 1).
In addition, we performed random-effects meta-analysis
under a credibility ceiling for associations that satisfy the
criteria of convincing level of evidence to determine the
robustness of the associations. Credibility ceilings account
for inherent methodological bias that can result in spurious
significant results of the meta-analyses due to reporting
of exaggerated associations in small studies (20, 21). We
checked whether statistical significance was retained under
a credibility ceiling of 10%, which is considered to be
relatively lenient, to adjust each study included in the
meta-analysis so as not to exceed a maximum certainty
of 90%.
Meta-analysis combining all individual studies of the
meta-analyses
To account for the inconsistencies of the results between
multiple meta-analyses studying the same subject (same
type of cancer) but consisting of different individual studies, we combined all the individual studies of the metaanalyses of the same subjects and performed “the most
updated” meta-analysis. While combining the meta-analyses,
we identified and excluded the individual studies duplicated
in >1 meta-analysis. If ≥2 individual studies based on
identical population groups were identified, only the most
recently published studies were included. We then metaanalyzed this new set of individual studies (the most updated
meta-analysis) and evaluated the level of evidence of the
associations. Finally, we performed subset analyses of casecontrol and cohort studies, with respect to the statistically
significant results of meta-analyses. We also compared the
results with those of meta-analyses of overall studies and
cohort studies with the highest number of individual studies,
respectively. The flowchart of the analysis is presented
in Figure 1.
Results
Characteristics of studies included in the final analyses
Initially 556 unique articles were screened, and 64 original
articles corresponding to 154 effect sizes (25 case-control
studies, 24 cohort studies, 105 combined observational
study effect sizes) met the eligibility criteria, as shown
in the flowchart in Figure 2. Of the 154 effect sizes
including 25 different types of cancer, 25 (16.2%) effect sizes
were estimated from case-control studies (hospital-based
or population-based), 24 (15.6%) from prospective cohort
studies, and 105 (68.2%) from both case-control and cohort
studies (combined observational studies) (see Table 1 and
Supplemental references).
Tea consumption and risk of cancer 1439
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large, the meta-analysis was re-examined to determine if
the heterogeneity was due to differences in the size of the
association or due to differences in the direction of the effect.
Using the recalculated data, the 95% prediction interval (PI)
was also estimated. A 95% PI represents the distribution of
true effects in which 95% of new and unique studies on
the same subject will fall (13). Therefore, 95% PI further
signifies between-study heterogeneity, whereas a 95% CI of
each meta-analysis represents the accuracy of the summary
effect size (14).
The P value of the Egger regression test was also calculated
to evaluate small study effects. The Egger test assumes
that when meta-analyses are based on a limited number
of small trials the results are more prone to bias than
larger studies (15). The threshold for the implications of
small study effects was P < 0.10 from the Egger test.
The random-effects summary effect size of the largest
component study of each meta-analysis was compared with
the random-effect summary effect size of each recalculated
meta-analysis to evaluate whether the 2 effect sizes were
concordant or discordant. Moreover, within each metaanalysis, we recorded the number of component studies
that were statistically significantly associated with decreased
risk, not statistically significant, or statistically significantly
associated with increased risk—D (decreased risk), N (no
association), I (increased risk), respectively.
Summary of individual meta-analyses under
conventional interpretation of meta-analysis criteria
(random-effects P value <0.05)
We evaluated 154 meta-analyses including tests for bias
and heterogeneity (see Table 1 and Supplemental Tables 2
and 3). Under conventional thresholds of statistical significance (random-effects P value <0.05), 43 (27.9%) metaanalyses on 15 types of cancer were significant and adequately
assessed, and 42 (27.2%) showed decreased associations
between tea consumption and risks of cancer incidence. The
only original meta-analysis that showed significant increased
risk of cancer was for breast cancer (high compared with low
black tea consumption). Within 43 significant associations,
7 (16.3%) meta-analyses were significant at P < 0.001 using
random-effects model.
Results of meta-analyses combining all individual
studies under conventional interpretation of
meta-analysis criteria (random-effects P value <0.05)
The original studies from each of the meta-analyses were
combined for a comprehensive umbrella review comprising
all the studies that were on the comparison regarding tea
consumption and type of cancer. This resulted in 66 results
on 25 types of cancer comparing different patterns of tea
consumption (see Table 2 and Supplemental Tables 3 and 4).
Within 66 results, 19 (28.8%) showed significant results
(random-effects P value <0.05) between tea consumption
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Kim et al.
and decreased risk of 11 different types of cancer. The
19 statistically significant results were as follows: biliary
tract cancer (any tea, any compared with none), breast
cancer (green tea, any compared with none; green tea, high
compared with low; any tea, any compared with none),
colorectal cancer (green tea, high compared with low; any
tea, high compared with low), endometrial cancer (green
tea, high compared with low), gastric cancer (any tea, any
compared with none), leukemia (any tea, high compared with
low; any tea, any compared with none), liver cancer (green
tea, any compared with none; green tea, high compared
with low), lung cancer (any tea, any compared with none),
oral cancer (green tea, high compared with low; any tea,
high compared with low; any tea, any compared with none),
ovarian cancer (any tea, any compared with none), and
thyroid cancer (any tea, high compared with low) (see
Table 2).
Level of evidence
After recalculating the data by considering heterogeneity
between estimates and biases in the literature, 2 results (1.3%)
were supported by convincing evidence. Sixteen results
(10.4%) were supported by suggestive evidence, 25 results
(16.2%) showed weak evidence, 107 results (69.5%) were
nonsignificant, and 4 results (2.6%) were not adequately
assessed due to insufficient information (see Supplemental
Table 2).
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FIGURE 1 Flow diagram of our umbrella review.
From the 19 statistically significant results of updated
meta-analyses combining all the individual studies, reduction in the incidence of oral cancer was found to have convincing evidence for any compared with none (OR = 0.62;
95% CI: 0.55, 0.72; P < 10−6 ) consumption of any type
of tea. Under the consideration of credibility ceilings, the
result with convincing level of evidence preserved statistical
significance with a ceiling of 10%. Six results were found
to have suggestive levels of evidence. Consumption of any
type of tea showed a lowered risk of biliary tract cancers
(RR = 0.77; 95% CI: 0.64, 0.92; P = 0.004) compared with no
tea consumption. Also, the reduced risk of oral cancer with a
high dose of tea consumption (RR = 0.86; 95% CI: 0.79, 0.93;
P = 0.00024) showed a suggestive level of evidence. High
consumption compared with low green tea consumption
significantly lowered the risk of breast cancer (RR = 0.75;
95% CI: 0.61, 0.92; P = 0.006), liver cancer (RR = 0.87;
95% CI: 0.78, 0.98; P = 0.026), and oral cancer (RR = 0.82;
95% CI: 0.69, 0.96; P = 0.015). High consumption of green
tea reduced the risk of endometrial cancer (RR = 0.78; 95%
CI: 0.61, 1.00; P = 0.046) compared with low consumption
of green tea. Twelve results associated with breast cancer,
colorectal cancer, gastric cancer, leukemia, liver cancer, lung
cancer, ovarian cancer, and thyroid cancer were classified to
have weak evidence.
Summary of meta-analyses separated by study design
In the case of oral cancer, high consumption of any kind of
tea showed suggestive evidence in observational studies due
to the threshold P value being unsatisfied; also, the outcomes
in both case-control and cohort studies showed suggestive
evidence because their 95% PI included null. Further, the
meta-analysis with the largest number of individual studies
showed suggestive evidence. Among the 5 results with
Tea consumption and risk of cancer 1441
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FIGURE 2 Flowchart of literature search.
TABLE 1 Summary of individual effect sizes from original meta-analyses of the associations on tea
consumption and risk of cancer included in the study
Comparison
details, %
154
100.0
78
50.7
Black tea
19
12.3
Green tea
57
37.0
25
24
105
16.2
15.6
68.2
1
7
3
31
8
1
1
11
6
17
2
2
1
4
5
8
2
12
5
7
3
3
5
7
1
1
0.6
4.5
1.9
20.1
5.2
0.6
0.6
7.1
3.9
11.0
1.3
1.3
0.6
2.6
3.2
5.2
1.3
7.8
3.2
4.5
1.9
1.9
3.2
4.5
0.6
0.6
2
16
25
107
4
1.3
10.4
16.2
69.5
2.6
Total
By exposure (tea type)
Any tea
By study type
Case-control
Cohort
Observational (combined)
By cancer type
Biliary tract cancer
Bladder cancer
Brain cancer
Breast cancer
Colorectal cancer
Colon cancer
Rectal cancer
Endometrial cancer
Esophageal cancer
Gastric cancer
Gallbladder cancer
Glioma
Renal cell carcinoma
Liver cancer
Lung cancer
Leukemia (childhood)
Leukemia (adult)
Ovarian cancer
Laryngeal cancer
Oral cancer
Oropharyngeal cancer
Pharyngeal cancer
Pancreatic cancer
Prostate cancer
Thyroid cancer
Skin cancer (nonmelanoma)
By level of evidence
Convincing
Suggestive
Weak
Nonsignificant
Not adequately assessed
suggestive evidence, results on biliary tract cancer showed
suggestive evidence in cohort studies but failed to show
significance in case-control studies. In case of endometrial
cancer, both cohort and case-control studies were not
statistically significant. Besides the case of colorectal cancer
with high compared with low tea consumption, all results
that showed weak evidence presented nonsignificant results
in cohort studies but showed significance in case-control
studies (1 suggestive, 10 weak). In case of colorectal cancer
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Kim et al.
n
38 Any vs. none
32 High vs. low
8 Increment of 1–3 cups/d
5 Any vs. none
11 High vs. low
3 Increment of 1–2 cups/d
12 Any vs. none
41 High vs. low
4 Increment of 1–2 cups/d
with high compared with low tea consumption, the result
of meta-analyses with both cohort and case-control studies
failed to show its significance (see Table 3 and Figure 3).
Discussion
In this study, we summarized and analyzed original metaanalyses to critically appraise the strength and breadth of
claimed associations between tea consumption and risk of
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Number of effect
sizes
Category
TABLE 2 The results and the level of evidence of the effect of tea and risk of cancer1
Outcome
Study
type
Small study
effect4
Concordance
Metrics
Summary effect
(randomeffects)5
Any vs. none
Any tea
<10−6
No
Not large
6/0/0
No
Yes
OR
0.62 (0.55, 0.72)
Any vs. none
High vs. low
High vs. low
High vs. low
High vs. low
High vs. low
Any tea
Green tea
Green tea
Green tea
Green tea
Any tea
<0.05 but >10−6
<0.05 but >10−6
<0.05 but >10−6
<0.05 but >10−6
<0.05 but >10−6
<0.05 but >10−6
Yes
Yes
Yes
Yes
Yes
No
Not large
Large6
Not large
Not large
Not large
Not large
4/4/0
6/5/0
2/4/0
2/9/0
1/4/0
5/26/0
No
No
No
No
No
No
Yes
Yes
Yes
No
No
Yes
RR
RR
RR
RR
RR
RR
0.77 (0.64, 0.92)
0.75 (0.61, 0.92)
0.78 (0.61, 1.00)7
0.87 (0.78, 0.98)
0.82 (0.69, 0.96)
0.86 (0.80, 0.91)
High vs. low
Any vs. none
Any vs. none
High vs. low
High vs. low
Any vs. none
High vs. low
Any vs. none
Any vs. none
Any vs. none
Any vs. none
High vs. low
Green tea
Green tea
Any tea
Any tea
Green tea
Any tea
Any tea
Any tea
Green tea
Any tea
Any tea
Any tea
<0.05 but >10−6
<0.05 but >10−6
<0.05 but >10−6
<0.05 but >10−6
<0.05 but >10−6
<0.001
<0.001
<0.05 but >10−6
<0.05 but >10−6
<0.001
<0.05 but >10−6
<0.05 but >10−6
Yes
Yes
Yes
Yes
Yes
Yes
No
No
Yes
Yes
Yes
Yes
Large
Large
Very large
Not large
Large
Very large
Not large
Not large
Large
Very large
Very large
Not large
6/10/0
3/11/0
6/20/0
6/45/2
4/11/0
23/30/3
4/4/0
1/7/0
3/7/0
18/24/1
8/22/1
1/13/0
No
No
No
Yes
No
Yes
Yes
Yes
No
No
No
Yes
No
Yes
No
No
No
No
No
No
No
Yes
No
No
RR
OR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
0.82 (0.71, 0.96)
0.87 (0.76, 0.99)
0.81 (0.71, 0.94)
0.93 (0.87, 0.99)
0.87 (0.75, 1.00)7
0.78 (0.70, 0.86)
0.55 (0.43, 0.72)
0.76 (0.65, 0.89)
0.65 (0.48, 0.88)
0.76 (0.67, 0.86)
0.82 (0.71, 0.94)
0.77 (0.61, 0.97)
High vs. low
Any vs. none
High vs. low
High vs. low
High vs. low
High vs. low
Any vs. none
High vs. low
High vs. low
High vs. low
High vs. low
Any vs. none
Any tea
Any tea
Any tea
Any tea
Any tea
Green tea
Any tea
Black tea
Black tea
Black tea
Green tea
Green tea
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Not large
Not large
Large
Large
Large
Not large
Not large
Not large
Large
Large
Not large
Not large
0/8/1
0/14/0
1/22/2
2/29/2
1/7/0
0/5/0
2/6/0
0/15/0
1/12/0
1/27/0
0/5/0
0/9/0
No
No
No
No
No
No
No
No
No
No
No
No
Yes
No
Yes
Yes
Yes
Yes
No
No
No
Yes
Yes
Yes
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
OR
0.93 (0.74, 1.18)
0.93 (0.82, 1.05)
0.97 (0.87, 1.09)
0.95 (0.86, 1.06)
0.86 (0.65, 1.13)
1.03 (0.82, 1.31)
0.90 (0.74, 1.09)
1.04 (0.97, 1.12)
0.91 (0.80, 1.03)
0.98 (0.91, 1.06)
0.99 (0.83, 1.77)
0.94 (0.83, 1.05)
(Continued)
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Tea consumption and risk of cancer 1443
Associations supported by convincing evidence
Oral cancer
CC
Associations supported by suggestive evidence
Biliary tract cancer
Obs
Breast cancer
CC
Endometrial cancer
Obs
Liver cancer
Obs
Oral cancer
Obs
Oral cancer
Obs
Associations supported by weak evidence
Breast cancer
Obs
Breast cancer
Obs
Breast cancer
Obs
Colorectal cancer
Obs
Colorectal cancer
Obs
Gastric cancer
Obs
Leukemia
Obs
Leukemia
Obs
Liver cancer
Obs
Lung cancer
Obs
Ovarian cancer
Obs
Thyroid cancer
Obs
Nonsignificant associations
Acute leukemia (childhood)
Obs
Acute leukemia (childhood)
Obs
Bladder cancer
CC
Bladder cancer
Obs
Bladder cancer
Co
Bladder cancer
Obs
Brain cancer
Obs
Breast cancer
Co
Breast cancer
CC
Breast cancer
Obs
Breast cancer
Co
Breast cancer
Co
95% PI
Effect-size
including Heterogeneity distribution
P value
null
(I2 )2
(D/N/I)3
Comparison Type of tea (random-effects)2
Kim et al.
Outcome
Study
type
Breast cancer
Breast cancer
Breast cancer
Breast cancer
Colon cancer
Colorectal cancer
Endometrial cancer
Endometrial cancer
CC
Co
CC
Obs
Obs
Obs
Obs
Obs
Endometrial cancer
Esophageal cancer
Gallbladder cancer
Gallbladder cancer
Gastric cancer
Gastric cancer
Gastric cancer
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Glioma
Glioma
Laryngeal cancer
Liver cancer
Lung cancer
Lung cancer
Oropharyngeal cancer
Ovarian cancer
Ovarian cancer
Pancreatic cancer
Pancreatic cancer
Pancreatic cancer
Pharyngeal cancer
Prostate cancer
Prostate cancer
Prostate cancer
Prostate cancer
Rectal cancer
Renal cell carcinoma
Skin cancer (non-melanoma)
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
Obs
95% PI
Effect-size
including Heterogeneity distribution
P value
null
(I2 )2
(D/N/I)3
Comparison Type of tea (random-effects)2
Any vs. none
High vs. low
High vs. low
High vs. low
High vs. low
High vs. low
High vs. low
Increment of
1cup/d
High vs. low
High vs. low
High vs. low
Any vs. none
High vs. low
High vs. low
Increment of
3cups/d
Any vs. none
High vs. low
High vs. low
Any vs. none
High vs. low
High vs. low
Any vs. none
Any vs. none
Any vs. none
High vs. low
Any vs. none
High vs. low
Any vs. none
High vs. low
High vs. low
Any vs. none
High vs. low
High vs. low
Any vs. none
Any vs. none
Small study
effect4
Concordance
Metrics
Summary effect
(randomeffects)5
Green tea
Any tea
Any tea
Any tea
Green tea
Black tea
Any tea
Any tea
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Very large
Not large
Large
Large
Not large
Large
Large
Large
3/2/0
0/12/2
1/8/0
1/20/2
1/9/1
2/14/4
3/12/1
0/4/1
No
No
No
No
Yes
No
Yes
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
OR
RR
OR
RR
RR
RR
RR
RR
0.83 (0.62, 1.10)
1.03 (0.96, 1.10)
0.90 (0.75, 1.10)
0.98 (0.90, 1.06)
0.98 (0.85, 1.12)
0.99 (0.87, 1.13)
0.90 (0.75, 1.09)
1.04 (0.98, 1.10)
Green tea
Green tea
Any tea
Any tea
Green tea
Black tea
Any tea
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Large
Large
Very large
Very large
Large
Not large
Not large
1/8/1
9/11/2
2/2/0
3/3/0
3/25/2
0/4/1
0/5/0
No
No
No
No
Yes
No
No
No
Yes
No
No
Yes
Yes
Yes
RR
RR
RR
RR
RR
RR
RR
0.99 (0.79, 1.23)
0.81 (0.62, 1.06)
0.57 (0.25, 1.30)
0.67 (0.40, 1.12)
0.93 (0.84, 1.04)
1.18 (0.79, 1.77)
0.98 (0.89, 1.08)
Any tea
Any tea
Any tea
Any tea
Green tea
Black tea
Any tea
Green tea
Black tea
Any tea
Any tea
Green tea
Any tea
Green tea
Any tea
Any tea
Black tea
Green tea
Any tea
Any tea
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
>0.05
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Very large
Very large
Large
Very large
Very large
Large
Large
Very large
Large
Not large
Large
Large
Not large
Very large
Large
Large
Not large
Large
Very large
Not large
0/4/0
1/3/0
2/5/1
3/9/0
4/7/1
4/10/0
2/4/0
3/5/1
4/12/0
1/20/1
2/25/2
1/6/1
0/4/0
3/6/0
6/15/2
7/20/2
1/9/1
3/6/0
1/11/0
4/4/0
No
No
No
No
No
No
No
No
Yes
No
No
No
No
Yes
Yes
Yes
No
No
No
No
No
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
RR
OR
0.67 (0.40, 1.12)
0.57 (0.25, 1.30)
0.91 (0.67, 1.23)
0.77 (0.57, 1.03)
0.78 (0.61, 1.01)
0.86 (0.70, 1.05)
0.68 (0.45, 1.03)
0.76 (0.57, 1.02)
0.90 (0.78, 1.04)
0.97 (0.85, 1.10)
0.99 (0.89, 1.10)
0.99 (0.78, 1.25)
0.88 (0.74, 1.04)
0.73 (0.51, 1.06)
0.86 (0.71, 1.04)
0.87 (0.75, 1.01)
0.99 (0.82, 1.20)
0.97 (0.77, 1.22)
1.03 (0.88, 1.21)
0.88 (0.76, 1.02)
1
CC, case-control studies; Co, cohort studies; Obs, observational studies; PI, prediction interval.
Heterogeneity is defined as “Very large” when I2 > 75%, “Large” when 50% < I2 < 75%, and “Not large” when I2 < 50%.
3
Number of individual studies of effect size with statistical significance in direction of decreased cancer risk(D)/no association(N)/increased cancer risk(I).
4
The presence of small study effects is determined if the Egger P value is <0.10.
5
Summary effect with 95% CI value obtained from umbrella review combining meta-analyses of the same comparison.
6
Although heterogeneity is large, the distribution of the effect sizes was considered over the I2 metrics.
7
The value is rounded up (to 2 decimal places), and hence is statistically significant.
2
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1444
TABLE 2 (Continued)
TABLE 3 Summary of results of the associations on tea consumption and risk of cancer outlined by study design, largest meta-analysis of observational studies, and largest meta-analysis of
cohort studies1
Oral cancer (any tea; any vs. none)
Oral cancer (any tea; high vs. low)
Biliary tract cancer (any tea; any vs. none)
Endometrial cancer (green tea; high vs. low)
Liver cancer (green tea; high vs. low)
Oral cancer (green tea; high vs. low)
Breast cancer (green tea; high vs. low)
Tea consumption and risk of cancer 1445
Breast cancer (green tea; any vs. none)
Breast cancer (any tea; any vs. none)
Study design
Metrics
Summary effect
(random-effects)6
P value
(random-effects)
95% PI
including null
Heterogeneity
(I2 )2
Small study
effect3
Concordance
Level of evidence4
CC
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
OR
RR
RR
RR
RR
0.62 (0.55, 0.72)
0.86 (0.79, 0.93)
0.80 (0.67, 0.94)
0.87 (0.79, 0.96)
0.84 (0.75, 0.94)
<0.001
<0.001
0.007
0.004
0.002
No
No
Yes
Yes
Yes
Not large
Not large
Not large
Not large
Not large
No
No
No
No
No
Yes
Yes
Yes
No
No
Convincing
Suggestive
Suggestive
Suggestive
Suggestive
RR
RR
RR
RR
0.77 (0.64, 0.92)
0.82 (0.70, 0.95)
0.66 (0.42, 1.03)
0.77 (0.64, 0.92)
0.004
0.008
0.068
0.004
Yes
Yes
Yes
Yes
Not large
Not large
Not large
Not large
No
No
No
No
Yes
Yes
No
Yes
Suggestive
Suggestive
Nonsignificant
Suggestive
RR
RR
RR
RR
0.78 (0.61, 1.00)5
0.75 (0.44, 1.30)
0.78 (0.57, 1.06)
0.78 (0.61, 1.00)5
0.046
0.298
0.108
0.046
Yes
N/A
Yes
Yes
Not large
N/A
Not large
Not large
No
N/A
No
No
No
N/A
Yes
No
Suggestive
Nonsignificant
Nonsignificant
Suggestive
RR
RR
0.87 (0.78, 0.98)
0.87 (0.78, 0.98)
0.026
0.026
Yes
Yes
Not large
Not large
No
No
No
No
Suggestive
Suggestive
RR
RR
RR
RR
RR
RR
0.87 (0.78, 0.98)
0.87 (0.78, 0.98)
0.82 (0.69, 0.96)
0.44 (0.19, 1.04)
0.84 (0.72, 0.98)
0.82 (0.69, 0.96)
0.026
0.026
0.015
0.058
0.030
0.015
Yes
Yes
Yes
N/A
Yes
Yes
Not large
Not large
Not large
N/A
Not large
Not large
No
No
No
N/A
No
No
No
No
No
N/A
No
No
Suggestive
Suggestive
Suggestive
N/A
Suggestive
Suggestive
RR
RR
RR
RR
RR
OR
OR
OR
OR
OR
RR
RR
RR
RR
0.82 (0.71, 0.96)
0.99 (0.83, 1.18)
0.75 (0.61, 0.92)
0.81 (0.67, 0.98)
0.98 (0.83, 1.16)
0.87 (0.76, 0.99)
0.94 (0.83, 1.05)
0.83 (0.62, 1.10)
0.87 (0.76, 0.99)
0.94 (0.83, 1.05)
0.81 (0.71, 0.94)
1.06 (0.91, 1.23)
0.74 (0.63, 0.88)
0.79 (0.65, 0.95)
0.015
0.895
0.006
<0.001
0.821
0.040
0.278
0.196
0.040
0.278
<0.001
0.451
<0.001
0.012
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Large
Not large
Large
Large
Not large
Very large
Not large
Very large
Very large
Not large
Very large
Not large
Very large
Very large
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
No
Yes
No
No
Weak
Nonsignificant
Weak
Weak
Nonsignificant
Weak
Nonsignificant
Nonsignificant
Weak
Nonsignificant
Weak
Nonsignificant
Weak
Weak
NR
NR
NR
NR
NR
NR
(Continued)
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Cancer type
TABLE 3 (Continued)
1446
Cancer type
Colorectal cancer (green tea; high vs. low)
Gastric cancer (any tea; any vs. none)
Leukemia (any tea; high vs. low)
Leukemia (any tea; any vs. none)
Liver cancer (green tea; any vs. none)
Lung cancer (any tea; any vs. none)
Ovarian cancer (any tea; any vs. none)
Thyroid cancer (any tea; high vs. low)
Metrics
Summary effect
(random-effects)6
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
Obs
Co
CC
Largest MA (Obs)
Largest MA (Co)
RR
RR
RR
RR
RR
RR
RR
RR
RR
0.93 (0.87, 0.99)
0.94 (0.88, 1.01)
0.91 (0.80, 1.03)
0.93 (0.87, 0.99)
0.94 (0.88, 1.01)
0.87 (0.75, 1.00)5
0.93 (0.79, 1.10)
0.73 (0.60, 0.90)
0.95 (0.81, 1.11)
RR
RR
RR
RR
RR
P value
(random-effects)
95% PI
including null
Heterogeneity
(I2 )2
Small study
effect3
Concordance
Level of evidence4
0.031
0.108
0.117
0.031
0.108
0.050
0.408
0.003
0.493
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Not large
Not large
Large
Not large
Not large
Large
Large
Not large
Very large
Yes
No
Yes
Yes
No
No
No
No
Yes
No
Yes
Yes
No
Yes
No
Yes
Yes
Yes
Weak
Nonsignificant
Nonsignificant
Weak
Nonsignificant
Weak
Nonsignificant
Weak
Nonsignificant
0.78 (0.70, 0.86)
1.01 (0.91, 1.13)
0.70 (0.62, 0.80)
0.76 (0.72, 0.80)
<0.001
0.791
<0.001
0.010
Yes
Yes
Yes
Yes
Very large
Not large
Very large
Very large
Yes
No
Yes
Yes
No
Yes
Yes
Yes
Weak
Nonsignificant
Weak
Weak
0.55 (0.43, 0.72)
<0.001
No
Not large
No
No
Weak
Not large
Not large
No
No
No
No
Weak
Weak
Not large
No
No
Weak
Not large
Not large
No
No
No
No
Weak
Weak
Large
Large
Large
Large
No
No
No
No
No
Yes
Yes
No
Weak
Nonsignificant
Weak
Weak
Very large
Very large
Very large
Very large
No
No
No
No
Yes
No
No
No
Weak
Nonsignificant
Weak
Weak
Very large
Not large
Very large
Not large
Not large
Not large
N/A
Large
Not large
No
Yes
No
Yes
No
Yes
N/A
Yes
Yes
No
Yes
No
Yes
Yes
No
Yes
No
No
Weak
Nonsignificant
Weak
Weak
Suggestive
Weak
N/A
Weak
Weak
NR
NR
NR
RR
RR
0.52 (0.38, 0.72)
0.55 (0.43, 0.72)
<0.001
<0.001
No
No
RR
0.76 (0.65, 0.89)
<0.001
No
NR
NR
RR
RR
0.69 (0.54, 0.87)
0.76 (0.65, 0.89)
0.002
<0.001
No
No
RR
RR
RR
RR
0.65 (0.48, 0.88)
0.83 (0.61, 1.11)
0.46 (0.29, 0.74)
0.65 (0.48, 0.88)
0.004
0.205
0.001
0.004
Yes
Yes
Yes
Yes
RR
RR
RR
RR
0.76 (0.67, 0.86)
0.91 (0.76, 1.08)
0.69 (0.59, 0.79)
0.77 (0.68, 0.88)
<0.001
0.273
<0.001
<0.001
Yes
Yes
Yes
Yes
RR
RR
RR
RR
RR
RR
RR
RR
RR
0.82 (0.71, 0.94)
0.92 (0.79, 1.06)
0.76 (0.61, 0.95)
0.89 (0.80, 1.00)5
0.71 (0.55, 0.93)
0.76 (0.61, 0.96)
0.88 (0.56, 1.37)
0.74 (0.56, 0.97)
0.76 (0.61, 0.96)
0.006
0.260
0.014
0.045
0.013
0.024
0.568
0.029
0.024
Yes
Yes
Yes
Yes
Yes
Yes
N/A
Yes
Yes
NR
NR
NR
NR
1
CC, case-control studies; Co, cohort studies; MA, meta-analysis; N/A, not applicable; NR, not reported; Obs, observational studies; PI, prediction interval.
Heterogeneity is defined as “Very large” when I2 > 75%, “Large” when 50% < I2 < 75%, and “Not large” when I2 < 50%.
3
The presence of small study effects is determined if the Egger P value is <0.10.
4
The definition of each category of the level of evidence is presented in Supplemental Table 1.
5
The value is rounded up (to 2 decimal places), and hence is statistically significant.
6
Summary effect with 95% CI value obtained from umbrella review combining meta-analyses of the same comparison.
2
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Kim et al.
Colorectal cancer (any tea; high vs. low)
Study design
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FIGURE 3 Statistically significant associations between cancer and tea exposure from umbrella review outlined by study design. The
definition of each category of the level of evidence is presented in Supplemental Table 1. CC, case-control studies; Obs, observational
studies.
Tea consumption and risk of cancer 1447
1448
Kim et al.
have higher levels of evidence than case-control studies. In
general, case-control studies are prone to biases, including
the possibility of recall bias and the presence of selection
bias. Thus, we can assume that there might be a spurious
association in meta-analyses of case-control studies.
Furthermore, we compared the relative risks and level of
evidence from our study with reports published by the WHO
International Agency for Research on Cancer (IARC) and the
World Cancer Research Fund Network/American Institute
for Cancer Research (WCRF/AICR). The IARC report states
that there is inadequate evidence for the carcinogenicity of
tea consumption in humans, and hence states that tea is not
classifiable as to its carcinogenicity (40, 41). Our study is in
line with this statement, because no result showed that tea
consumption was associated with an increased risk of cancer.
Moreover, the WCRF/AICR reports have stated that the
evidence is limited and no firm conclusions can be drawn for
any type of cancer (see Table 4). This includes all cancer types
that were found to have decreased associations in our study
(42). Especially for oral cancer, where our analyses revealed
convincing evidence, the WCRF states there is no evidence
for this association. Also, the limited suggestive evidence
reported by the WCRF for reduced risk of bladder cancer by
tea consumption (RR = 0.94; 95% CI: 0.89, 0.98, for 1 cup/d
increment) was not reproduced in our analyses, because only
1 meta-analysis included in our study was significant and our
final meta-analysis remained nonsignificant in this context.
There are several reasons why our results differ from
those of the WCRF. First, the criteria for grading evidence
are different. According to the WCRF criteria, the evidence
level is determined by the presence of between-study heterogeneity, the quality of studies, biological rationale, and
the number of cohort studies included. However, except
for the statistical heterogeneity, the rest were not included
as criteria of our study. Second, the WCRF largely relied
on prospective cohort studies, whereas our review included
both cohort and case-control studies. Finally, the WCRF
attempted a dose–response meta-analysis of cohort studies
whenever possible and presented summary estimates in
continuous scale (e.g., 1 cup/d). However, we used the effect
estimates from each meta-analysis, which were largely based
on categorical comparisons of high compared with low or any
compared with none intakes instead of a continuous scale of
tea intake.
Despite the above differences, our study has several strong
points compared with the results from the WCRF. First,
our study not only summarized the existing meta-analyses
of the same subject but also performed the most updated
meta-analyses with combined primary studies. This made it
possible to understand the effects of tea consumption over
a wider range and expanded statistical power due to the
inclusion of overall studies. Second, the WCRF separately
evaluated different sorts of tea (green and black tea), whereas
we included any of type of tea in our analyses. Therefore,
studies reporting the results of green or black tea but not
tea overall contributed to “any” tea in our review but were
excluded in the review performed by the WCRF. Again, this
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cancer incidence. We found that consumption of any type
of tea was associated with a lower risk of 11 types of cancer
(oral, biliary tract, breast, colorectal, endometrial, gastric,
leukemia, liver, lung, ovarian, and thyroid cancer). However,
only the association between tea consumption and lower
risk of oral cancer was supported by convincing evidence.
Suggestive evidence was found for lowering risk of biliary
tract, breast, endometrial, liver, and oral cancer.
The negative associations between tea and the risk of
specific cancers can be explained by several biological
mechanisms. In vitro and in vivo studies have suggested
that tea polyphenols have preventive effects against several
types of cancer, including oral (22), biliary tract (23), breast
(24), endometrial (25), liver (26), colorectal (27), gastric
(28), leukemia (29), lung (30), ovarian (31), and thyroid
cancer (32). As key antioxidants in tea, polyphenols or tea
catechins are thought to contribute to reducing the risk
of some cancers, acting as scavengers of reactive oxygen
species and potentially affecting transcription factors and
enzyme activities (33). Some important polyphenols are (−)epigallocatechin gallate (EGCG), (−)-epigallocatechin, (−)epicatechin gallate, and (−)-epicatechin (34). EGCG is the
most abundant tea catechin and is thought to play the most
important role in inhibiting cancer initiation and progression
(35). Tea polyphenols are thought to suppress the growth
of cancer cells by various proposed mechanisms, such as
inducing the apoptosis of cancer cells (36), suppression
of receptor-dependent signaling pathways and angiogenesis
(37), silencing genes related to epigenetic mechanisms such
as methylation of DNA (38), and inhibiting the activities of
enzymes (39). However, additional mechanistic studies and
more in-depth analyses focusing on molecular changes are
needed.
We found a total of 19 significant meta-analyses with
combined individual studies comprising 11 types of cancer.
Specific findings of our outcome must be interpreted with
caution. In case of some cancers such as endometrial cancer,
a suggestive level of evidence in combined observational
studies (cohort and case-control) was found, whereas the
results were nonsignificant in both cohort and case-control,
respectively. The combination of different study designs
possibly has an impact on the results due to the heterogeneity
between studies. The potential heterogeneity in nutritional
epidemiology comes from the difference in the definition
of the consumption amounts and follow-up periods. To
conclude, because the outcomes were nonsignificant in both
study designs, the outcome with suggestive evidence of
endometrial cancer could overestimate the true effect and
could thus be reconsidered. In addition, a convincing level
of evidence was derived from a single meta-analysis of
oral cancer including 8 individual case-control studies only.
In general, this is a small number for umbrella review,
further underlining our concern related to the level of
evidence.
In our findings, meta-analyses of cohort studies tended to
show null results whereas those of case-control studies were
statistically significant. Cohort studies are usually thought to
TABLE 4 Summary and comparison of individual meta-analysis articles, our umbrella review, and the WCRF report on associations of tea and cancer1
Meta-analyses from original articles
Cancer type
Tea type
Biliary tract
Brain cancer
Breast
Any
Any
Any
1
1
3
1/0/0
0/1/0
1/2/0
0/1/0/0/0
0/0/0/1/0
0/0/1/2/0
Black
9
0/8/1
0/0/1/8/0
Green
15
7/8/0
0/3/3/8/1
Any
5
0/5/0
0/0/0/5/0
Colon
Green
Any
Black
Green
Green
2
4
1
3
1
1/1/0
1/3/0
0/1/0
1/2/0
0/1/0
0/1/0/1/0
0/0/1/3/0
0/0/0/1/0
0/0/1/2/0
0/0/0/1/0
Any vs. none
Any vs. none
Any vs. none
High vs. low
High vs. low
High vs. low (cohort)
High vs. low (CC)
High vs. low (CC)
High vs. low
High vs. low (cohort)
Any vs. none (CC)
Any vs. none
Any vs. none (cohort)
High vs. low (CC)
High vs. low (cohort)
High vs. low
High vs. low
High vs. low
High vs. low
High vs. low
High vs. low
Rectal
Green
1
0/1/0
0/0/0/1/0
High vs. low
Endometrial
Any
5
3/2/0
0/3/0/2/0
Esophageal
Gastric
Black
Green
Green
Any
3
3
6
2
0/3/0
3/0/0
1/5/0
1/1/0
0/0/0/3/0
0/3/0/0/0
0/0/1/5/0
0/0/1/1/0
Gallbladder
Black
Green
Any
1
14
2
0/1/0
4/10/0
0/2/0
0/0/0/1/0
0/1/3/10/0
0/0/0/2/0
Any
2
1/1/0
0/0/1/1/0
High vs. low
Increment of 1 cup/d
High vs. low
High vs. low
High vs. low
Any vs. none
Increment of 3 cup/d
High vs. low
High vs. low
High vs. low
Any vs. none
Any vs. none
High vs. low
Colorectal
Tea consumption and risk of cancer 1449
Glioma
Comparison
No. of
studies
8
8
26
23
28
15
13
11
16
5
5
14
9
25
8
33
5
53
20
15
11
16
5
6
10
22
56
5
5
30
4
6
4
4
WCRF
D/N/I
RR
(95% CI)
Level
of evidence2
Level
of evidence
4/4/0
2/6/0
6/20/0
1/20/2
1/27/0
0/15/0
1/12/0
6/5/0
6/10/0
0/5/0
3/2/0
3/11/0
0/9/0
1/22/2
1/7/0
2/29/2
0/5/0
6/45/2
2/14/4
4/11/0
1/9/1
0.77 (0.64, 0.92)
0.89 (0.76, 1.05)
0.81 (0.71, 0.94)
0.98 (0.90, 1.06)
0.98 (0.91, 1.06)
1.04 (0.97, 1.12)
0.91 (0.80, 1.03)
0.75 (0.61, 0.92)
0.82 (0.71, 0.96)
0.99 (0.83, 1.77)
0.94 (0.83, 1.05)
0.87 (0.76, 0.99)
0.83 (0.62, 1.10)
0.97 (0.87, 1.09)
0.86 (0.65, 1.13)
0.95 (0.86, 1.06)
1.03 (0.82, 1.31)
0.93 (0.87, 0.99)
0.99 (0.87, 1.13)
0.87 (0.75, 1.00)3
0.98 (0.85, 1.12)
Suggestive
Nonsignificant
Weak
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Suggestive
Weak
Nonsignificant
Nonsignificant
Weak
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Weak
Nonsignificant
Weak
Nonsignificant
N/A
N/A
Limited—no
conclusion
3/6/0
0.97 (0.77, 1.22)
Nonsignificant
3/12/1
0/4/1
2/4/0
1/8/1
9/11/2
23/30/3
0/5/0
0/4/1
3/25/2
2/2/0
3/3/0
0/4/0
1/3/0
0.90 (0.75, 1.09)
1.04 (0.98, 1.10)
0.78 (0.61, 1.00)3
0.99 (0.79, 1.23)
0.81 (0.62, 1.06)
0.78 (0.70, 0.86)
0.98 (0.89, 1.08)
1.18 (0.79, 1.77)
0.93 (0.84, 1.04)
0.57 (0.25, 1.30)
0.67 (0.40, 1.12)
0.67 (0.40, 1.12)
0.57 (0.25, 1.30)
Nonsignificant
Nonsignificant
Suggestive
Nonsignificant
Nonsignificant
Weak
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Limited—suggestive:
decreases risk
RR = 0.94 (95% CI: 0.89,
0.98)
Limited—no
conclusion
Limited—no
conclusion
Limited—no
conclusion
Limited—no
conclusion
N/A
Limited—no
conclusion
Limited—no
conclusion
N/A
(Continued)
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Bladder
D/N/I
Evidence
C/S/W/N/X
Umbrella review
No. of
meta-analyses
1450
Meta-analyses from original articles
Umbrella review
WCRF
Tea type
No. of
meta-analyses
D/N/I
Evidence
C/S/W/N/X
Comparison
No. of
studies
D/N/I
RR
(95% CI)
Level
of evidence2
Level
of evidence
Any
1
0/1/0
0/0/0/1/0
Any vs. none
12
1/11/0
1.03 (0.88, 1.21)
Nonsignificant
Liver
Any
Green
2
2
0/2/0
2/0/0
0/0/0/2/0
0/1/1/0/0
Lung
Any
Black
Green
Any
1
2
2
8
1/0/0
0/2/0
1/1/0
0/8/0
0/0/1/0/0
0/0/0/2/0
0/0/1/1/0
0/0/0/8/0
Any
2
2/0/0
0/0/2/0/0
Any
Black
Green
Any
9
1
2
5
5/4/0
0/1/0
1/1/0
0/5/0
0/1/4/4/0
0/0/0/1/0
0/0/1/1/0
0/0/0/4/1
Any vs. none
High vs. low
Any vs. none
Any vs. none
High vs. low
High vs. low
High vs. low
Any vs. none
High vs. low
Any vs. none
Any vs. none
Any vs. none
Any vs. none
High vs. low
12
11
10
44
14
12
9
14
8
8
21
16
9
8
3/9/0
2/9/0
3/7/0
18/24/2
4/10/0
4/7/1
0/8/1
0/14/0
4/4/0
1/7/0
8/22/1
4/12/0
3/5/1
2/5/1
0.77 (0.57, 1.03)
0.87 (0.78, 0.98)
0.65 (0.48, 0.88)
0.76 (0.68, 0.86)
0.86 (0.70, 1.05)
0.78 (0.61, 1.01)
0.93 (0.74, 1.18)
0.93 (0.82, 1.05)
0.55 (0.43, 0.72)
0.76 (0.65, 0.89)
0.82 (0.71, 0.94)
0.90 (0.78, 1.04)
0.76 (0.57, 1.02)
0.91 (0.67, 1.23)
Nonsignificant
Suggestive
Weak
Weak
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Weak
Weak
Weak
Nonsignificant
Nonsignificant
Nonsignificant
Limited—no
conclusion
N/A
Any
Any
Green
Any
1
5
1
3
1/0/0
2/2/0
1/0/0
1/2/0
1/0/0/0/0
0/2/0/2/0
0/1/0/0/0
0/0/1/1/1
Any vs. none
High vs. low
High vs. low
Any vs. none
6
31
5
6
6/0/0
5/26/0
1/4/0
2/4/0
0.62 (0.55, 0.72)
0.86 (0.80, 0.91)
0.82 (0.69, 0.96)
0.68 (0.45, 1.03)
Convincing
Suggestive
Suggestive
Nonsignificant
Pharyngeal
Any
3
0/3/0
0/0/0/2/1
Any vs. none
4
0/4/0
0.88 (0.74, 1.04)
Nonsignificant
Pancreatic
Any
4
0/4/0
0/0/0/4/0
Prostate
Green
Any
1
1
0/1/0
0/1/0
0/0/0/1/0
0/0/0/1/0
Thyroid
Skin (nonmelanoma)
Black
Green
Any
Any
2
4
1
1
0/2/0
0/4/0
1/0/0
0/1/0
0/0/0/2/0
0/0/0/4/0
0/0/1/0/0
0/0/0/1/0
High vs. low
Any vs. none
High vs. low
High vs. low
Any vs. none
High vs. low
High vs. low
High vs. low
Any vs. none
22
29
8
23
29
11
9
14
8
1/20/1
2/25/2
1/6/1
6/15/2
7/20/2
1/9/1
3/6/0
1/13/0
4/4/0
0.97 (0.85, 1.10)
0.99 (0.89, 1.10)
0.99 (0.78, 1.25)
0.86 (0.71, 1.04)
0.87 (0.75, 1.01)
0.99 (0.82, 1.20)
0.73 (0.51, 1.06)
0.77 (0.61, 0.97)
0.88 (0.76, 1.02)
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Nonsignificant
Weak
Nonsignificant
Cancer type
Renal cell carcinoma
Leukemia (in childhood)
Leukemia
Ovarian
Laryngeal
Oral
Oropharyngeal
1
Limited—no
conclusion
N/A
N/A
Limited—no
conclusion
Limited—no
conclusion
Limited—no
conclusion
Limited—no
conclusion
Limited—no
conclusion
Limited—no
conclusion
Limited—no
conclusion
N/A
N/A
CC, case-control studies; C/S/W/N/D, convincing/suggestive/weak/nonsignificant/not adequately assessed; D/N/I, decrease in risk/no association/increase in risk; N/A, not applicable; WCRF, World Cancer Research Fund network.
The definition of each category of the level of evidence is presented in Supplemental Table 1.
3
The value is rounded up (to 2 decimal places), and hence is statistically significant.
2
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Kim et al.
TABLE 4 (Continued)
in the criteria for level of evidence might not be definitive.
We included meta-analyses with both case-control studies
and cohort studies. Because of the potential biases that can
affect case-control studies, such as recall bias and selection
bias, further prospective studies are needed before firm
conclusions can be drawn. Furthermore, the summary effects
of the meta-analyses about the same question might have
variations due to multiple reasons (46). Also, evaluating
any discrepancies or errors of individual meta-analyses was
beyond the scope of our review. Another problem is that the
summary effect size could be from a combination of studies
with different measures, such as OR, RR, and HR. OR is
statistically similar to RR when the outcome is uncommon
(47). Moreover, the main comparisons for tea exposure used
in this study (high compared with low, any compared with
none) can vary over a wide range. The exact amount of
tea polyphenol intake cannot be determined, because it
can be affected by multiple factors such as individual tea
preferences, the size of a cup, addition of sugar, different
cultural practices, natural variability in polyphenol concentration in tea sorts, and other possible organic influencing
factors.
Regardless of these limitations, the findings of this
study show health implications that could be beneficial to
individuals and populations. The association between tea
consumption and the risk of oral cancer was supported by
convincing evidence. It is possible that tea consumption can
reduce the risk of some other cancers, but further prospective
and mechanistic studies are needed before more robust
conclusions can be made.
Acknowledgments
The authors’ responsibilities were as follows—TLK, GHJ,
KHL, A Kronbichler, G Grosso, HJvdV, G Gamerith, FG,
DA, JYK, BS, A Koyanagi, MS, SHH, ED, EC, LFMdR, ELG,
JIS: contributed to the concept and design of the study;
TLK, GHJ, JIS: acquired, collected, and analyzed the data;
JIS: had final responsibility for the decision to submit for
publication; and all authors: had full access to all the study
data, participated in drafting the manuscript, and read and
approved the final manuscript.
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