“Tomatoes ‘cut risk of prostate cancer by 20%’,” the Daily Mail reports,
citing a study that found men who ate 10 or more portions a week had a reduced
risk of the disease.
The study in question gathered a year’s dietary information from 1,806 men
who were found to have prostate cancer and 12,005 who were clear
after random prostate checks. The researchers compared the diets and adjusted
the results to take into account factors such as age, family history of prostate
cancer and ethnicity.
They found that men who ate more than 10 portions of tomatoes or tomato
products per week have an 18% reduced risk of prostate cancer compared to men
who ate less than 10.
As this was a case controlled study, and not a randomised controlled trial,
it cannot prove that eating more tomatoes prevents prostate cancer. It can only
show an association.
The association is biologically plausible, because tomatoes are a rich source
of lycopene, a nutrient thought to protect against cell damage. However, the
jury is still out on whether it really does protect cells.
So a healthy, balanced diet, regular exercise and stopping smoking are still
the way to go. It’s unlikely that focusing on one particular food will improve
your health.
Where did the story come from?
The study was carried out by researchers from the University of Bristol, the
National Institute for Health Research (NIHR) Bristol Nutrition Biomedical
Research Unit, Addenbrooke’s Hospital in Cambridge and the University of Oxford.
It was funded by the NIHR and Cancer Research UK.
The study was published in the peer-reviewed medical journal Cancer
Epidemiology, Biomarkers and Prevention.
In general, the media reported the story accurately but also reported
different numbers of study participants, ranging from 1,800 to 20,000. This is
because out of the 23,720 men who were initially included in the study, a
proportion were excluded from the analyses due to missing questionnaires.
Several news sources have also reported that eating the recommended five
portions of fruit or veg per day reduced the risk of prostate cancer by 24%
compared to 2.5 servings or less per day. This seems to have come directly from
the lead researcher, but these figures are not clearly presented in the research
paper.
What kind of research was this?
This was a case-control study looking at the diet,
lifestyle and weight of men who had had a prostate check and were subsequently
diagnosed with (cases) and without (controls) prostate cancer. The researchers
wanted to see if there were any factors that reduced the risk of being diagnosed
with prostate cancer.
A previous systematic review suggested that a diet
high in calcium is associated with an increased risk of prostate cancer and that
diets high in selenium and lycopene are associated with reduced risk. Selenium
is a chemical element essential for life that is found in animals and plants,
but high levels are toxic. Lycopene is a nutrient found in red foods such as
tomatoes and pink grapefruit.
The researchers defined intake of selenium and lycopene as the “prostate
cancer dietary index”. They looked at whether there was an association between
men’s index scores and their risk of having prostate cancer.
In addition, in 2007, the World Cancer Research Fund (WCRF) and the American
Institute for Cancer Research (AICR) made eight recommendations on diet,
exercise and weight for cancer prevention.
The eight cancer prevention recommendations
The eight recommendations drawn up by the WCRF and AICR, some of which are harder to stick to than others, are:
- be as lean as possible without being underweight
- stay physically active every day
- limit consumption of energy-dense foods and avoid sugary drinks
- the diet should consist of foods mostly from plant origin
- limit intake of red and processed meat
- limit consumption of alcoholic drinks
- limit consumption of salty foods and foods processed with salt
- don’t take supplements in an attempt to prevent cancer
However, recent research has shown conflicting results as to whether these recommendations are applicable to prostate cancer. One large European study found that men who followed the recommendations did not have a lower general prostate cancer risk, and the other found that men did have a reduced risk of aggressive prostate cancer.
The researchers wanted to see if these recommendations should be changed to include any of the prostate cancer dietary index components for men and/or men at higher risk of prostate cancer.
What did the research involve?
The researchers used data collected from a large UK study called the ProtecT trial. In this trial, 227,300 randomly selected men aged 50 to 69 were invited to have a prostate check between 2001 and 2009.
Nearly half of the men then had a prostate specific antigen (PSA) test and 11% of them went on to have further investigations. Before the test they were asked to fill out questionnaires on:
- lifestyle
- diet
- alcohol intake
- medical history
- family history
They were also asked to provide information on their:
- physical activity level
- body mass index (BMI)
- waist circumference
- body size aged 20, 40 and at the time they entered the study
Statistical analyses were then performed to determine the risk of low or high grade prostate cancer according to adherence to the WCRF/AICR recommendations or intake of any of the three dietary components of the prostate cancer dietary index. The results were adjusted to take into account the following confounders:
- age
- family history of prostate cancer
- self-reported diabetes
- ethnic group
- occupational class
- smoking status
- total energy intake
- reliance on the accuracy of the dietary questionnaires
- broad categories for self-estimate of body size
How did the researchers interpret the results?
The researchers concluded that, “in addition to meeting the optimal intake for the three dietary factors associated with prostate cancer, men should maintain a healthy weight and an active lifestyle to reduce risk of developing prostate cancer, cardiovascular diseases and diabetes”. They also say that “high intake of plant foods and tomato products in particular may help protect against prostate cancer, which warrants further investigations”.
Conclusion
This large study has shown an association between the consumption of more than 10 portions of tomatoes per week and an 18% reduction in risk of prostate cancer. However, as this was a case controlled study, and not a randomised controlled trial, it cannot prove that eating more tomatoes prevents prostate cancer.
Strengths of the study include its large size and attempts to account for potential confounding factors, although there are some limitations to the study, including:
This study does not provide enough evidence to change the recommendations for reducing the risk of prostate cancer. A healthy, balanced diet, regular exercise and stopping smoking are still the way to go, rather than relying on eating one exclusive food type such as tomatoes
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