Thursday December 20 2007
Green tea contains high levels of antioxidants
“Men who drink lots of green tea could cut their chances of getting prostate cancer by half,” reported the Daily Express today.
The newspaper said that a study of 50,000 Japanese men, aged between 40 and 69, found that those who drank five cups of green tea were less likely to develop prostate cancer than those who drank only one.
The authors of the study acknowledge that green tea is “not a guaranteed way of preventing the disease", but say it should give hope that a treatment can be found. The paper also points out that not all previous research has found that green tea reduces the risk of prostate cancer.
This story is based on a large, relatively well-designed, study in Japan. However, despite this, the study does not provide strong enough evidence that green tea definitely reduces the risk of prostate cancer. This is because there were many other differences between the people who drank green tea frequently and those who drank it infrequently, and it is these factors that could be affecting the results.
It is important to note that the study found no association between green tea consumption and the overall risk of prostate cancer, and there was only an association between green tea drinking and a reduced risk of advanced prostate cancer. As the authors of the research acknowledge, until a good quality clinical trial is carried out on the effects of green tea on prostate cancer rates, there is no certainty of its effects.
Where did the story come from?
Dr Norie Kurahashi from the National Cancer Centre in Japan and colleagues in the Japan Public Health Centre study group carried out the research. The study was funded by the Ministry of Health, Labour, and Welfare and the Ministry of Education, Culture, Sports, Science, and Technology in Japan. The study was published in the peer-reviewed medical publication American Journal of Epidemiology.
What kind of scientific study was this?
This was a prospective cohort study – the Japan Public Health Center-based Prospective study.
The researchers enrolled 65,802 men aged 40 to 69 from 10 regions across Japan in 1990 and 1993. Men who reported already having prostate cancer were excluded.
When they enrolled, the participants filled out questionnaires about their health and lifestyles, including questions about how many days a week they drank green tea, and how many cups they drank a day. The researchers assessed how accurate these reports were by asking some of the men to fill out a diary of what they ate and drank for 28 days, and compared this with their original questionnaire answers.
The men were then followed until 2004 to see if they developed prostate cancer, and how advanced this cancer was. The researchers obtained this data from local hospital records, national cancer registries, and death certificates. Diagnoses and the stage of the cancer were confirmed through medical records.
Cancers that had not spread outside of the prostate were referred to as localised cancers, while those that had spread were described as advanced. The researchers collected data on deaths and cause of death, as well as determining whether men had moved away from the areas where they enrolled. The researchers then compared the risk of developing prostate cancer in men who drank differing amounts of green tea. Their analyses took into account other factors that might affect the results, including age, whether the men lived with their wives, body mass index, smoking, and consumption of alcohol, coffee, black tea, miso soup, soy, and fruit and vegetables.
The researchers were able to include 49,920 men in their analyses. These were the men who had completed their questionnaires about green tea consumption, and who had been successfully followed up to 2004 by the researchers.
What were the results of the study?
The researchers followed up the men for about 14 years on average, and during this period 404 men (just under 1 per cent) developed prostate cancer, with 114 cases of advanced cancer, 271 of localised cancer, and 19 where the stage of the cancer was not known.
There was no difference in the overall risk of developing prostate cancer among people who drank differing amounts of green tea. However, the more green tea men drank, the less likely they were to develop advanced prostate cancer during follow up. Men who drank five cups a day were about half as likely to develop advanced prostate cancer than those who only drank one cup a day.
What interpretations did the researchers draw from these results?
The researchers concluded, “Green tea may be associated with a decreased risk of advanced prostate cancer.” They suggest that further well-designed trials in humans are needed before we can be certain than green tea can prevent prostate cancer.
What does the NHS Knowledge Service make of this study?
The study was a very large prospective study, and it is its size that gives its results more weight. However, there are some points to bear in mind when interpreting the results, most of which the authors acknowledge:
- The difficulty with interpreting this type of study is that the men who have different exposures to the factor being investigated (in this case green tea) may also have different exposure to many other factors. For example, men who drink lots of green tea might adhere to a more traditional Japanese diet than those who drink less. In fact, the researchers found that men who drank more green tea tended to be older, to live with their wives, to smoke more, and to eat more miso soup, soy, and fruit and vegetables. Because of this, when the researchers compare what has happened to groups with differing levels of green tea consumption, it is difficult to say for certain that it is the green tea that is responsible for the effect, rather than any of the other factors. The authors of this study have tried to take into account potentially confounding factors, but there is no certainty that all the factors have been sufficiently accounted for. As the authors suggest, a well-designed randomised controlled trial is needed for a reliable confirmation that green tea has an effect on the risk of prostate cancer. This is because a randomised trial would be much better than a cohort study at eliminating the differences (other than green tea consumption) between the men who were studied.
- It is unknown if the men had undergone screening for prostate cancer. If one group of men (frequent or infrequent green tea drinkers) underwent screening more frequently, this would affect the results, because those who are screened are more likely to have their cancers identified.
- The authors themselves acknowledge that not all previous studies have found that tea drinking reduces the risk of prostate cancer. However, they suggest that not all of these studies have distinguished between green and black tea, and that not all of them have analysed results by cancer stage.
- This study relied on men to self-report how much green tea they drank when they enrolled. These measures may lack accuracy, as they may not have given an accurate estimation of how much green tea they drank on average, and because of variations in cup size. In fact, when the researchers compared the questionnaire answers with the 28 day dietary diaries in a sample of men, there was not a very good correlation between the two reports. In addition, the men’s green tea consumption may have changed over the follow up period, which could have affected results.
- The men may have already had undiagnosed prostate cancer when they enrolled, as they did not all undergo screening at enrolment, and had to self-report any prostate cancer diagnoses.
- This study was conducted in Japanese men and because of the differences in genetic make up and environmental exposures in men from different countries, these results may not be indicative of the effects that drinking green tea would have in men in other countries.
Sir Muir Gray adds…
About one-third of cancers are caused by diet and so dietary studies are important. However, a single study is much less important than a systematic review of all the relevant studies.