Breast cancer: weak tea study

Thursday January 22 2009

“Drinking three cups of tea a day could slash the risk of women developing breast cancer by two thirds” reports the Daily Express. The newspaper says that research has shown that younger women who consumed large amounts of tea each day cut their chances of developing any type of breast tumour by about 37%.

The claim is based on a  case-control study on 5,000 women aged 20 to 74 years who had been treated for breast cancer. Researchers interviewed these women on their consumption of tea over the five years prior to their cancer, and compared their responses to 4,500 healthy women who were also interviewed. The results showed a lower risk of developing cancer in women under 50 who drank three or more cups a day.

However, this reduced risk was seen only in a particular sub-group of the study, and when looking at the results from the study as a whole, levels of tea drinking were not related to a reduced risk of breast cancer. Other limitations within this study mean the results may have occurred by chance, and that this research should only be seen as low-level evidence. 

Overall this study, though interesting, should not be the basis by which people decide what to drink.

Where did the story come from?

This research was conducted by Dr Nagi Kumar of the Cancer Center and Research Institute in Tampa, Florida, and five colleagues from various other institutions. The study was supported by several grants from the National Institutes of Health and published in Cancer Epidemiology, Biomarkers and Prevention, a peer-reviewed medical journal.

What kind of scientific study was this?

This was a case control study in which the researchers examined the association of regular tea consumption with the risk of breast cancer.

The study analysed data on 5,082 women who had been treated for breast cancer (the cases) and data on 4,501 women without cancer (the control group). The case group was sourced using data from cancer registries in Wisconsin, Massachusetts, and New Hampshire: this also provided details on subjects’ cancer histories.

The data on case and control groups was sourced from a previous study. To be eligible as a control the women had to be between the ages of 20 and 74 and have no previous breast cancer.

Tea consumption was assessed through a structured telephone interview. The women were asked how often, on average, they consumed a cup of black or green tea (not herbal infusions commonly called tea) approximately five years before the breast diagnosis in cases, or at a comparable date in controls. Usual consumption could be recorded as per day, week, month, or year.

Tea drinking was reported by 44.7% of women in the case group  and 45.7% in the control group. Only 23 cases and 15 controls had missing tea consumption data.

During the interview the researchers also asked about known and suspected breast cancer risk factors, such as number of children, family history of breast cancer and whether they had had a screening mammography or not.

The researchers then used statistical techniques to estimate the odds (chance) of breast cancer in relation to the quantities of tea consumed. They adjusted their analysis to take into account other factors that might affect their results,  and analysed results within various age groups and sub-types of breast cancer.

What were the results of the study?

The authors say that tea consumption was not related to breast cancer risk overall. However, they report that in their sub-group analyses women under 50 years old who consumed three or more cups per day had a 37% reduced breast cancer risk when compared with women reporting no tea consumption.The adjusted odds ratio was 0.63 (95% confidence interval 0.44-0.89).

This inverse association noted among younger women was consistent for all the sub-types of breast cancer (in situ and invasive breast cancer, ductal and lobular breast cancer). The results were unchanged when analysis took other known risk factors into account.

What interpretations did the researchers draw from these results?

The researchers say that their results provide support for the theory that “regular tea consumption, particularly at moderately high levels, might reduce breast cancer risk in younger women”. They also say that further research is needed to confirm this association.

What does the NHS Knowledge Service make of this study?

There are several points to note when interpreting this study, some of which the authors do acknowledge as limitations. Importantly, although the authors are quoted as saying that regular tea consumption might reduce breast cancer risk in younger women they cautiously call for more studies.

It is not clear that the differences seen in sub-groups of women when examined retrospectively can be explained by the differences in tea drinking habits.

Other points to note:

  • There were significant differences between the case and control individuals on features such as number of children, family history of breast cancer and frequency of screening. These factors could all have accounted for part of the difference observed.
  • The main result of this study was not significant, with no association found between tea drinking and overall risk of breast cancer. Therefore all subsequent sub-group analysis should be treated with caution; in fact only one of the 12 sub-groups analysed showed statistical significance, and this formed the basis of the Daily Express story.
  • The authors report that “within the study the number of younger women consuming large quantities of tea was limited in all analyses” and the confidence intervals are wide in the sub-group analysis. This means that there is a good chance that these results have arisen by chance.
  • Other biases may have affected the results. For example, there is a possibility that cases and controls reported their tea consumption differently based on errors in recall or on their prior beliefs about the benefits of tea on health.

Overall this study, though interesting, should not be the basis by which people decide what to drink.

Analysis by Bazian
Edited by NHS Choices