Researchers have found evidence of a reduced risk of bladder cancer associated with tea drinking and report that it was significant among men and former smokers but not among women and those who had never smoked.
They say the statistically significant association and dose-response supports a causal relationship but wrote: However, inconsistency between males and females and the absence of association between never smokers weakens the evidence of causality and may indicate residual confounding
Further studies are needed to investigate more deeply the causality and mechanism of interaction between tea consumption and sex and smoking status, they said.
Tea has been shown to be associated with reduced risk of several diseases including cardiovascular diseases, stroke, metabolic syndrome and obesity. However, the authors say the results on the relationship between tea drinking and risk of bladder cancer are conflicting.
They said: This research aimed to assess the association between tea consumption and risk of bladder cancer using a pooled analysis of prospective cohort data. Secondary aims include stratified analysis by sex and smoking status, stratified analysis according to bladder cancer and stage groups and dose-response analysis
The team made up from researchers from France, the Netherlands, Sweden, Norway, the US, China, Japan and Saudi Arabia analysed data from 532, 949 participants in 12 cohort studies.
The Bladder-cancer-Epidemiology and Nutritional Determinants study (BLEND) is an international consortium formed to investigate the effect of dietary factors on the risk of bladder cancer development by pooling and standardising data from worldwide epidemiological studies. In this study, cohort studies from BLEND were included if they provided data on tea consumption and covariates of interest. Out of the 15 in BLEND, 12 studies satisfied the inclusion criteria.
The data involved study characteristics including location and assessment method, participant characteristics including age, sex, ethnicity, disease characteristics - stage and metastasis, smoking status and dietary measurements.
Tea consumption was defined as consuming drinks based on tea tree leaves so excluded herbal tea and other infusions.
During the follow-up period (median 11.44 years), 2,915 incident bladder cancer cases were detected. Among participants 67.8% were females, 91.8% Caucasian and the mean age was 52.5 years with a range of 19.3 - 98.5 years. Around one third of all subjects were never tea drinkers compared to former and never smokers.
The researchers noted: A higher level of tea consumption was associated with lower risk of bladder cancer. The present pooled analysis of 12 cohort studies showed a statistically significant reduced bladder cancer risk associated with tea consumption in the overall population.
"A similar inverse was shown for males and current and former smokers while no evidence of association was observed for females and never smokers".
While our findings of a reduced bladder cancer risk with higher tea consumption are in line with previous studies, some meta-analysis failed to show a significant association between tea consumption and bladder cancer
They said that a recent study by Zhao et al included five cohort studies and found a statistically significant 5% reduction in bladder cancer risk for each one cup increment of tea consumption while no significant reduced risk could be observed by comparing the highest vs the lowest tea consumption.
One possible explanation for this observed null finding is the high variation in defining the categories of tea consumption between the included primary studies.
Our analysis showed an interaction between tea consumption and smoking. All levels of tea consumption were significantly associated with a reduced bladder cancer risk among current and former smokers, while never smokers showed a null association.
The authors say this present study has many strengths included a large sample size allowing for more precise estimates and thorough analysis including stratification by sex and smoking status.
Limitations included detailed characteristics of tea consumption including number of years and concentration of tea plus green vs black tea. Other factors potentially correlated to both bladder cancer and tea consumption including physical activity, body weight, education and income as well as environmental exposure to carcinogenic chemicals were not available for the analysis.
The study was partly funded by the World Cancer Research Fund.