Men savouring a cuppa this morning could be forgiven for spitting out their brew in alarm after reading that seven cups of tea a day “raises risk of prostate cancer by 50%” (Daily Mail). Similar headlines elsewhere in the media repeated the message that male tea drinkers are at a “greater risk of prostate cancer”.
This headline is based on findings from a large and long-term Scottish cohort study that found those men who slurped the most tea (more than seven cups a day) were 50% more likely to develop prostate cancer than those who sipped the least (0-3 cups a day). Overall, 6.4% of those who drank the most tea developed prostate cancer during the study period, compared with 4.6% of those who drank the least. Those drinking a moderate level of four to six cups of tea a day were not at any increased risk compared with those who drank the least.
Despite its size and long duration, this study had many limitations that call into question the reliability of its results. Information on tea consumption and other lifestyle factors was only collected at the start of the study. Given the average follow-up was 28 years, it is unlikely that tea drinking habits, and other behaviours such as alcohol and smoking levels, remained stable over this entire period. This could have affected the results.
The results of this study should not alarm male tea drinkers. However, men should remain alert to the signs and symptoms of prostate cancer regardless of their tea-drinking habits.
Where did the story come from?
The study was carried out by a collaboration of cancer researchers based in Glasgow, Scotland and was published in the peer-reviewed science journal Nutrition and Cancer. No source of funding was reported.
The media generally reported the 50% increase in relative risk of prostate cancer between the highest and lowest tea consumption group. They failed to mention that the other groups were found to have no increased risk, as well as other important limitations associated with the research methodology.
What kind of research was this?
This was an analysis of data collected as part of the Collaborative Cohort Study, which enrolled employed men and women (aged 21 to 75 years) from 27 workplaces in Scotland in the early 1970s. The study had collected extensive lifestyle, social and medical data from participants at the time of enrolment, though the specific aims of the original cohort are not reported in this paper.
For the purposes of this study, the researchers used data collected from the men to investigate the potential link between tea consumption and overall risk of developing prostate cancer. The researchers were also interested in the link between tea consumption and the development of different severities of prostate cancer, known as ‘grade-specific risk’.
Prostate cancer is the most common cancer diagnosed in men, and the researchers report that previous research has found inconsistent results regarding the link between black tea and prostate cancer. As tea is one of the most common drinks in the world, the researchers thought it important to assess whether there was any link between tea drinking and prostate cancer.
A cohort study is a useful study design to answer this research question as a randomised controlled trial may not be practical. The main limitation of cohort studies is that they show associations rather than prove causes. They can potentially show tea to be associated with cancer, but will never be able to prove that tea causes cancer, because many other factors may be involved in this link.
What did the research involve?
A group of 6,016 employed Scottish men who had been enrolled in the Collaborative Cohort Study between 1970 and 1973 were followed up until December 2007 – a period of up to 37 years.
On enrolment, participants filled in a questionnaire. This asked them for details including their height, weight, blood pressure, social class, years of full-time education, occupation and lifestyle habits including smoking and alcohol use. Daily tea intake reported by the participants was categorised into four groups based on roughly equal numbers of participants in each group (0-3 cups, 4-5 cups, six cups and seven or more cups of tea a day).
The participants were flagged within an NHS registration system so the researchers were notified when participants were diagnosed with cancer or died.
The researchers then analysed how the different categories of tea consumption were related to the chance of being diagnosed with prostate cancer later in life. This was done for all cases of prostate cancer and also different severities of prostate cancer.
What were the basic results?
Information from 6,016 men was analysed, with an average (median) follow-up period of 28 years and a maximum of 37 years. The average (median) age of the group on enrolment in the 1970s was 48 years old (range 21-75 years). Key results are as follows:
- 318 men were diagnosed with prostate cancer in the follow-up period
- the authors reported that participants almost exclusively drank black tea (as opposed to green tea) but they did not specify whether this was with or without milk
- individuals in the highest tea consumption group (seven or more cups a day) were older, more likely to be smokers, non-alcohol drinkers, non-coffee drinkers and had a healthy weight, compared with men drinking 0-3 cups a day
- middle-class men, and those with 7-9 years of full-time education, were more likely to drink seven or more cups of tea a day
- individuals in the highest tea consumption group (≥7 cups a day) were 50% more likely to develop prostate cancer than the lowest (0-3 cups a day), after adjusting for a range of other factors including coffee consumption, alcohol intake and smoking status
- the 50% increased relative risk was based on the observation that 6.4% of those in the highest tea consumption group developed prostate cancer during the study period compared with 4.6% in the lowest consumption group
- those who drank four to six cups of tea a day were not at a significantly higher risk of developing prostate cancer when compared with the 0-3 cups a day group
- no evidence was found for a relationship between tea consumption and grade-specific prostate cancer based on information from 186 prostate cancers with details about their severity at diagnosis
How did the researchers interpret the results?
The authors concluded that men who consumed high amounts of tea “experienced the highest risk of prostate cancer; however, no association was observed for high- or low-grade disease”. They stated their findings were important given the “poorly understood natural history and the lack of known modiﬁable risk factors of prostate cancer”.
This large cohort study tracking Scottish men over a period of 28 years showed that those with the highest tea consumption levels (more than seven cups a day) were 50% more likely to develop prostate cancer than those in the lowest consumption groups (0-3 cups a day). Those drinking fewer than seven cups a day were not at any increased risk compared with the lowest consumption group.
The strengths of this study are its size and long follow-up period, but it also has significant limitations that should be considered when judging the reliability and relevance of these findings.
Lifestyle factors were recorded at a single time
Information on tea consumption and other lifestyle factors was only collected at the start of the study. Given the long average follow-up period of 28 years, tea habits and other behaviours such as alcohol and smoking levels are likely to have varied over this period. This could mean tea habits and other lifestyle factors were incorrectly classified, which could significantly affect the conclusions drawn from this study.
Tea drinkers may live longer, allowing cancers to develop
The authors of the study highlight that many healthy behaviours, such as having a healthy weight, not drinking alcohol and having optimum cholesterol levels, were more common in those in the highest tea consumption group. They raised the possibility that these men, who were generally healthier, may have lived for longer, allowing more time for prostate cancer to develop. As prostate cancer risk is known to increase with age, those living longer are more likely to develop the condition, which could explain this result. The researchers did attempt to adjust for this age-related effect, but this may not have been completely successful with residual effects playing a part.
Only a small number of the men developed prostate cancer
Though this was a large study, only 318 men developed prostate cancer during the follow-up period. If these men are further subdivided according to the amount of tea they drank, smaller sample sizes are created that can affect the reliability of risk estimates (only 92 men with prostate cancer drank seven or more cups a day).
The study measured cancer diagnoses not cancer deaths
This study looked at the risk of tea consumption on being diagnosed with prostate cancer rather than risk of dying from it. A large proportion of those diagnosed with prostate cancer will die with the disease but from other unrelated causes, rather than directly from prostate cancer itself.
The types of tea drunk are unclear
The study authors state that most of the study participants were drinking black tea (as opposed to green tea) in their discussion section. However, the results of the study by tea type are not reported. It is unclear whether tea type was measured at the start of the study or was assumed to be black tea by the authors due to the trends in tea drinking at the time. This is important as different types of tea vary in their constituents and could potentially affect the body in different ways. It is also not clear whether tea was taken with or without milk, which could further influence this potential link between tea and prostate cancer.
Family history is missing
The study did not collect data on family history of prostate cancer and other potential dietary factors that have been linked to prostate cancer in previous research. Not adjusting for these factors in the analysis may have biased the findings of this study.
Cancer grading difficulties
The amount of information on grade-specific prostate cancer was small and severely limited the power of the study to detect a potential link between the condition and tea consumption.
Cohort study design
The main limitation of cohort studies is that they show associations rather than prove causes. Hence, this study doesn’t demonstrate that tea causes prostate cancer, only that those who drink the most tea generally develop prostate cancer more often. Other influencing factors are likely to be involved in explaining this potential causal link.
In summary, men who are tea drinkers should not be alarmed by the results of this study as it has many limitations that cast doubt on the reliability of the findings. However, men should remain alert to the signs and symptoms of prostate and other forms of cancer, regardless of their tea habits.