Eating cauliflower and broccoli can half the risk of prostate cancer reported several newspapers. The Daily Telegraph also reported that the research behind the story found that eating tomatoes did not protect against cancer.
Most of the reports stated that eating the vegetables once or twice a week, would cut the risk of getting cancer by 52%, for cauliflower, and 45%, for broccoli. The anti-cancer properties were said to be explained by the fact that the vegetables are rich in compounds that prevent damage to DNA.
This research does not provide conclusive evidence that these vegetables protect against cancer. The study looked only for correlations between the consumption of different food types (established through analysis of eating habits) and cancer (identified through screening) of volunteers.
Although is is advisable to eat a variety of fruit and vegetables as part of a balanced diet, it is premature to change eating habits based on this research.
Where did the story come from?
Doctors from the Prostate, Lung, Colorectal and Ovarian Screening Trial group published this research. The lead author, Victoria Kirch, is based in Toronto, Canada. The Division of Cancer Prevention Programme at the National Institute of Health in the US funded the research.
The study was published in the peer-reviewed Journal of the National Cancer Institute .
What kind of scientific study was this?
This study was a retrospective analysis from one arm of a large ongoing randomised screening trial. Researchers took the data on 1,338 men who had developed prostate cancer among the screened men in this study, and looked back at their eating habits and other factors.
Men were recruited to the study by advertisement and direct mailing. More than 29,000 men were recruited across 10 US screening centres between 1993 and 2001. They completed a general risk factor questionnaire and a 137-item food frequency questionnaire at the start of the study. The group who were randomised to be screened for cancer were included in the study and from these the 1,338 men who went on to develop prostate cancer had their food intake analysed in detail.
What were the results of the study?
The researchers reported that vegetable and fruit consumption was not related to overall prostate cancer risk in this study.
What interpretations did the researchers draw from these results?
Despite the lack of an association to overall prostate cancer, the data was analysed in more detail. The researchers then concluded that a high intake of cruciferous vegetables, including broccoli and cauliflower, may be associated with reduced risk of aggressive prostate cancer, particularly disease outside the prostate.
What does the NHS Knowledge Service make of this study?
This study has looked at a large amount of data and reported the main result accurately. The authors have selectively reported on the only two significant links out of over 100 links examined for food groups, and three out of 54 links between vegetable types and cancer. We cannot be sure that these results have not arisen by chance.
- This was a study of observed links and was not designed to test causes.
- The fact that individuals with higher intakes of fruit and vegetables may also be healthier in other ways may not have been adequately adjusted for in the analysis.
- Those with higher intakes of fruit and vegetables are very likely to have lower rates of smoking or be more physically active than those with low intakes, and this may account for some of the observed effect.
It seems reasonable to base decisions on what vegetables to buy on factors other than prostate cancer risk.