“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.
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.
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:
- 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
Body size was self-estimated by looking at pictures on a scale of 1 to 9. All those selecting 1 to 3 were categorised as normal weight and those selecting 4 to 9 were considered overweight/obese.
From this study the researchers identified 2,939 men who had been diagnosed with prostate cancer and matched them with 20,781 randomly selected men by age and GP practice who did not have prostate cancer to act as controls. They then excluded anyone who did not return the questionnaires and those who did not provide all of the body metrics.
This gave a sample of 1,806 men with prostate cancer and 12,005 controls.
The dietary questionnaires assessed how frequently they had consumed 114 items of food over the previous 12 months. This included an estimate of portion sizes.
From this information, the men were assigned a score to reflect how well they had achieved the first six of the eight WCRF/AICR recommendations (they did not have enough information for “salt consumption” or “dietary supplements”).
Adherence to each recommendation was scored (1 – complete adherence, 0.5 – partial adherence or 0 – non-adherence), giving an overall score between 0 and 6.
The researchers also looked at the intake of components of the “prostate cancer dietary index”: calcium, selenium and tomato products which they used as an indicator of lycopene intake (tomato juice, tomato sauce, pizza and baked beans). To be scored as adherent, men had to:
- eat less than 1,500mg of calcium per day
- eat more than 10 servings of tomato and tomato products per week
- eat between 105 and 200µg of selenium per day
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:
- family history of prostate cancer
- self-reported diabetes
- ethnic group
- occupational class
- smoking status
- total energy intake
What were the basic results?
After adjusting for possible confounding factors:
- being adherent to the tomato and tomato product recommendation by eating 10 or more servings of tomatoes per week was associated with an 18% reduced risk of prostate cancer compared to eating less than 10 servings (odds ratio (OR) 0.82, 95% confidence interval (CI) 0.70 to 0.97)
- each component of the “prostate cancer dietary index” that the men adhered to was associated with a 9% reduction in risk of prostate cancer (OR 0.91, 95% CI 0.84 to 0.99)
- the overall WCFR/AICR adherence score was not associated with a decreased risk of prostate cancer (OR 0.99, 95% CI 0.94 to 1.05)
- every 0.25 increase in the score for adherence to the plant food recommendation was associated with a 6% reduced overall risk of prostate cancer (OR 0.94, 95% CI 0.89 to 0.99)
A 0.25 increase in adherence score could be achieved by increasing fruit and vegetable intake from less than 200g/day to between 200 and 400g/day, or by increasing fruit and vegetable intake from between 200 and 400g/day to 400g/day or more (400g is equivalent to five portions) or by changing intake of unprocessed cereals (grains) and/or pulses (legumes).
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”.
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:
- reliance on the accuracy of the dietary questionnaires
- broad categories for self-estimate of body size
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.
Following the eight WCRF/AICR recommendations as listed above should also help prevent against other types of cancer as well as chronic diseases such as obesity and type 2 diabetes.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
ITV News, 27 August 2014
The Independent, 27 August 2014
BBC News, 27 August 2014
Daily Mail, 28 August 2014
Links to the science
Cancer Epidemiology, Biomarkers and Prevention. Published online July 13 2014