Introduction
Prostate cancer remains one of the most common cancers afflicting men today. It is one of the most frequently diagnosed malignancies among men in the world [
1]. Although the etiology of this malignancy remains unclear, both hereditary and environmental components are considered to contribute to prostate carcinogenesis, with age, race, and family history being the only well-established risk factors [
2]. Recently, ecologic studies have offered substantial evidence that dietary factors may play a role in the etiology of prostate cancer [
3]. As one of the most common beverages consumed worldwide, tea has gained increasing attention.
Tea is generally consumed in the form of black (fermented), green (unfermented) and oolong (partially fermented), all of which originate from the leaves of the plant
Camellia sinensis[
4]. Cellular study and animal xenograft research have showed that tea or the active ingredient in tea, polyphenols, could afford protection against a variety of cancer types [
5]. Numerous epidemiological evidences support a protective effect of tea towards cancers [
6‐
8]. However, the results based on epidemiological studies on the association of tea consumption with prostate cancer risk were inconsistent. A previous meta-analysis conducted by Zheng et al. [
9] suggested that green tea but not black tea may have a protective effect on prostate cancer, especially in Asian populations. However, most recent epidemiological studies since the previous meta-analysis revealed inconclusive results [
10,
11]. Even more, a recent cohort study of Scottish [
12] revealed that men with higher intake of tea are at greater risk of developing prostate cancer. Thus, we aimed to update and quantitatively re-assess the association between tea consumption and the risk of prostate cancer by summarizing the results of latest published studies. As such, we intended to provide the best available evidence as to whether tea consumption can reduce prostate cancer risk.
Materials and methods
Literature search
We initially identified publications in the PubMed database up to July 2013 using keywords ‘tea’ and ‘prostate cancer’. Additional studies were identified from references cited in retrieved articles. Eligible studies should fulfill all the following inclusion criteria: (1) studies evaluating the tea consumption and prostate cancer risk; (2) the consumption of the natural green tea product, not of green tea extracts or supplements, were recorded; (3) the outcome of interest should be an incidence of prostate cancer; (4) odds ratio (OR) or relative risk (RR) estimates with corresponding 95% CIs (or sufficient data allowing us to compute them) were provided. In studies with overlapping patients or controls, the latest study with the largest sample size should be included.
Two investigators independently extracted the name of the first author, the year of publication, the population in which the study was conducted, study design, adjusted effects estimates, exposure assessment and adjusted covariates. Disagreement was resolved by either discussion or the third-party resolution. Considering that prostate cancer was a rare disease, the RR was assumed approximately the same as OR, and the OR was used as the study outcome. If a study provided separate adjusted ORs for different tea types or age stratifications, we treated them as independent studies.
Statistical analysis
The adjusted ORs/RRs and the corresponding 95% CI for highest
versus non/lowest tea consumption levels were pooled, as only those with highest tea consumption level were considered as habitual consumer. Between-study heterogeneity across the eligible comparisons was measured using the chi-square-based Q test and
I-square test [
13]. Heterogeneity was considered significant if
P <0.05. According to the significance of heterogeneity, the data from a single study were combined using either the fixed effect or random effect models [
14,
15]. In addition, subgroup in terms of study population (Asian
vs. non-Asian), type of tea (black
vs. green) and study design (cohort
vs. case–control) were stratified. The potential publication bias was assessed graphically by funnel plot and estimated statistically by both Begg’s and Egger’s test [
16].
P <0.05 was considered significant for publication bias. In the overall study, sensitivity analysis would be performed to evaluate the effect of a single study contributing to overall summary and the Duval and Tweedie non-parametric ‘trim and fill’ method would be used to account for publication bias if any publication bias appeared to exist [
17]. All statistical analyses were performed with STATA (version 8.0, StataCorp, College Station, TX, USA). All
P values were two-sided.
Discussion
Numerous studies have indicated daily tea consumption could be a non-invasive, economical and valuable means to prevent cancer. However, our current meta-analysis found that there was no significant association between tea consumption and reduced prostate cancer risk. Compared with the previous meta-analysis, our study has appended eight additional articles published after 2010 that were not included in the previous meta-analysis, which would strengthen the current conclusion.
Additionally, stratified analysis was performed according to population, tea type and study design in the studies to further verify our conclusion. Consistent with the overall summary, the most stratified analysis showed that there was no association between tea consumption and prostate cancer, except the summary of case–control studies which demonstrated a borderline inverse association between tea intake and the risk of prostate cancer. Also, it should be noted that both green tea and black tea consumption did not reduce prostate cancer risk, even though green tea was generally considered having a stronger anti-cancer effect.
The non-significant findings regarding the effects of tea consumption on prostate cancer in our meta-analysis contradicted the results of previous laboratory studies. Molecular study in cell-culture system and animal xenograft model research showed that both green tea polyphenol and black tea extracts [
38‐
40] could afford anticancer potential towards prostate cancer. The compounds from tea could modulate carcinogenic signal pathway and induce apoptosis in prostate cancer cell. However, our meta-analysis failed to unveil the chemopreventive role of tea towards prostate cancer in epidemiological aspect. The contradiction was likely to be due to the lower quantities of human tea consumption compared to the doses used in experimental studies, while most cellular or animal xenograft studies used tea extract, such as catechins and flavonols, which had higher bioavailability. Also, the blood-prostate barrier could probably impair the penetration of active compounds from tea [
41]. Therefore, the anti-cancer effect of tea would be weakened compared with cancer arising from the digestive tract. Besides, confounding effects of socioeconomic, dietary pattern and lifestyle factors probably masked the protective effect of tea consumption [
42].
As is often the case with meta-analysis, several potential limitations in our study should also be taken into account. First, we did not attempt to include unpublished observations and studies with insufficient information to estimate an OR/RR, which probably would bias our results. Second, most included studies did not provided detail information on tea consumption duration or drinking pattern, for example, number of new batches brewed each day, quantity of tea leaves used per batch [
24]. These parameters can influence the association between tea consumption and prostate cancer risk profoundly. Consequently, we did not have sufficient data to evaluate the risk of prostate cancer associated with these other dimensions of tea consumption. Third, even though we have used the maximal adjusted estimates, the adjusted criteria for OR/RR varied between studies. These probably would also bias our results.