17 November 2015
What is the evidence for the benefit of tomatoes for prostate cancer?
By Dr Wendy Winnall - PCFA Research Team
Everyone knows that eating tomatoes is good for prostate cancer – right? The internet is full of this advice coming from multiple scientific studies. An example is this news article from last year that tells us eating 10 serves of tomatoes a week was beneficial, according to a British study. Tomatoes are rich in lycopene, an antioxidant, that many believe is responsible for their beneficial effects. If this is such a "scientific fact" then why is this not supported by the US FDA (Food and Drug Administration) and by a well-respected Cochrane review into the subject? Ultimately the answer to this question is that no "gold standard" clinical trials have been done to show that tomatoes or lycopene prevent or treat prostate cancer in humans. Why don’t scientists just do such a study? Unfortunately the gold standard study, a phase III randomised controlled trial (RCT), costs many millions, and takes a very long time. Before such a study would be funded, smaller studies, such as phase II RCTs, need to be very promising. In the case of tomatoes and prostate cancer, the evidence from the preliminary studies is inconsistent.
Results from a phase II trial of tomato extract to prevent the development of prostate cancer have recently been published. First we will visit the case in favour of the benefit of tomatoes in order to appreciate the context of this trial.
In the case of tomatoes, the evidence in favour of their benefit for prostate cancer comes in two forms. The first is from many studies of human groups (epidemiological studies), showing that men who ate a diet rich in tomatoes were less likely to develop prostate cancer. Most of these studies are retrospective – that is, they involve asking people about their diet in the past. These types of study, although useful, suffer from issues that make it very difficult to make conclusions about the exact cause of the differences seen. For instance, someone who eats more tomatoes, is more likely to eat a healthier diet rich in fruit and vegetables at the same time, therefore it may be the overall effects of a healthy diet, rather than just tomatoes, that are beneficial.
The second line of evidence for the benefit of tomatoes is based on laboratory studies, on lab-grown cells and in animals. One example is that addition of lycopene to prostate cancer cells grown in the lab can reduce their ability to move. This indicates that lycopene may prevent the spreading of cancer. Good evidence comes from animal trials such as from a study showing a tomato-rich diet decreased the incidence of prostate cancer and slowed cancer progression in mice. Unfortunately it’s not unusual for promising results in animal studies to fail to hold up in humans trials.
A group of scientists from Illinois in the US have published the findings of their phase II randomised controlled trial (RCT) of lycopene-rich tomato extract for the prevention of prostate cancer development. This is a small scale version of the gold standard RCT trial. It’s not large enough to directly test whether eating tomatoes leads to less cases of prostate cancer in men. Instead this study looked at PSA levels and prostate biopsy scores after tomato consumption for 6 months. Men were recruited who had a prostate condition called high-grade prostatic intraepithelial neoplasis (HGPIN), considered to be at risk of developing prostate cancer. 58 men were randomly assigned into groups that ate either lycopene-rich tomato tablets each day or placebos (tablets that appeared similar but did not contain lycopene). PSA tests and prostate biopsies were compared before and after six months of treatment. Unfortunately there were no differences in the PSA levels or markers of cancer growth between the two groups. This trial was very well designed, but had a relatively small number of participants. Unfortunately the results of smaller trials like this make it less likely that a gold standard phase III RCT will be done.
So what's the bottom line? Should men with prostate cancer eat a lot of tomatoes? Yes, if tomatoes are part of a diet rich in fruit and vegetables, which we know is beneficial for the prevention and treatment of cancer. There remains a considerable body of evidence in favour of tomatoes and lycopene in the prevention or treatment of prostate cancer, not enough to be irrefutable, but enough for the idea to remain plausible.