What your patients need to know about diet and prostate cancer
No specific food can cause or cure cancer, but some may be associated with its development. Various dietary factors have been suggested to influence prostate cancer risk, but currently there is not sufficient evidence to support a direct causative association. In the future specific recommendations may be available to reduce prostate cancer risk, but for now people should adopt a healthy diet and maintain a healthy weight.
- There is no proven association between any specific diet and prostate cancer risk.
- There is no evidence to support recommendations for any dietary supplements to reduce prostate cancer risk.
- A healthy diet with a high intake of vegetables and fruits, a moderate intake of fat, and which avoids high energy foods and excessive meat and dairy products, is recommended. Although conclusive evidence on the effectiveness of this in reducing cancer risk is limited.1
Food and cancer
The food we eat can influence our risk of developing certain types of cancer – both directly and indirectly. In general high energy and high fat diets can lead to obesity and are thought to increase risk for some cancers. A healthy diet, high in fresh fruits, vegetables and wholegrain foods may reduce cancer risk.1
Evidence supports a link between some common cancers and specific food groups. For example, eating red and processed meats increases the risk of colorectal cancer.2 Based on this evidence, the World Cancer Research Fund advises to limit red meat intake (< 500 g per week) and avoid processed meats.3
However, more research is required to clearly understand the role of specific aspects of the diet to cancer risk, including prostate cancer.
Influence of lifestyle factors on prostate cancer
The only well-established risk factors for prostate cancer are non-modifiable – older age, race and family history.4 Some modifiable lifestyle factors may increase risk, but the extent to which they do so is yet to be determined.
One of the key indicators that risk of prostate cancer may be influenced by lifestyle factors, including diet, is the wide global variation in incidence (Figure 1). More than a 25-fold difference in age-adjusted incidence exists between countries, with Australia and New Zealand among the highest and South-Central Asia the lowest.5 Additional support for the influence of lifestyle factors comes from migration studies where men moving from low-risk to high-risk countries had increased rates of prostate cancer compared to those in their native countries.6,7
Some of the increased incidence may be explained by differences in prostate cancer screening practices, as this can lead to diagnosis of a significant proportion of otherwise latent prostate cancers.4
Read more information about testing for prostate cancer.
Figure 1. Estimated prostate cancer incidence worldwide in 20085
Adapted from Ferlay J, Shin HR, Bray F, Forman D, Mathers C and Parkin DM. GLOBOCAN 2008, Cancer Incidence and Mortality Worldwide: IARC CancerBase No. 10. [Internet]. Lyon, France: International Agency for Research on Cancer; 2010.
Lifestyle risk factors may contribute to prostate cancer risk
Several lifestyle factors are suggested to influence the risk of developing prostate cancer, including diet, obesity, smoking and physical activity. However, the evidence is not sufficient to estimate the benefit gained by modifying these factors.
- The relationship between obesity and the incidence of prostate cancer is complex, but there is consistent evidence that obesity is associated with worse outcomes and increased mortality, especially in aggressive disease.8
- There is no consistent evidence that physical activity protects against prostate cancer, but several studies have reported that being active may reduce the risk of advanced disease and reduce mortality from prostate cancer.9,10
- Smoking is known to increase risk of developing some cancers, but the link to prostate cancer risk is not consistent. In men with prostate cancer, smoking is associated with increased cancer mortality, with more recent smoking being more strongly associated.4
The contribution of diet to prostate cancer risk is less clear, but several foods have been investigated as being potentially protective or harmful. The dietary components that have received the greatest attention are: dietary fat, antioxidants (carotenoids) and dietary supplements including vitamin E, vitamin D, calcium and selenium.11
Dietary fat intake and prostate cancer risk
A diet with high fat intake
A high fat diet comprising mostly animal fat sources (such as dairy products and fatty meats) is suggested to increase the risk of developing prostate cancer and prostate cancer mortality.12,13 However, the overall findings are not consistent and many trials have not found evidence of a link.14-16
Polyunsaturated fatty acids
Several investigations have identified particular types of dietary fat that alter prostate cancer risk. Some studies suggest that a diet with a low omega-6 to omega-3 fatty acid ratio may reduce prostate cancer, but the evidence is not consistent.17,18 More recent evidence suggests that high blood concentrations of omega-3 polyunsaturated fatty acids is associated with increased prostate cancer risk.19
Prospective clinical trials have not reported a link between risk for prostate cancer and dietary fatty acid intake.20 However, data about plasma phospholipid levels may provide a greater insight into prostate cancer risk. The Melbourne Collaborative Cohort Study involving over 41,000 people suggested that people with high plasma phospholipid concentrations of saturated fatty acids were at increased risk of developing prostate cancer.20
Given the weakness of the association and reported inconsistencies, it has been suggested that it is unlikely that dietary fat is an important contributor to prostate cancer risk.20 However, given recent results 19 consider the potential risk of excessive plasma concentrations of omega-3 polyunsaturated fatty acids before recommending a diet high in omega-3 fatty acids.
Read more evidence about the recommended use of fish oil supplements containing omega-3 for secondary prevention of cardiovascular disease.
Are antioxidants beneficial?
In recent years, antioxidants have received much attention as potential anti-cancer compounds. Antioxidants reduce oxidative stress which may damage molecules including proteins and DNA, and has been implicated in carcinogenesis.4
Several dietary antioxidants, including isoflavones, selenium, vitamin E and lycopene have been investigated as nutritional factors with potential to protect against the development of prostate cancer.4 However there is little evidence from clinical trials to support this proposition.
Antioxidants are provided by a healthy diet that includes a variety of fruits and vegetables. A diet high in vegetables (soy, in particular) is thought to decrease the risk of prostate cancer or benign prostatic hyperplasia.21 Prostate problems are reportedly less frequent or less severe in areas in which a plant-based diet is predominant.12
Selenium and vitamin E
Vitamin E is a fat-soluble antioxidant found in many foods, such as nuts, seeds and vegetable oil. Selenium is a trace element that is naturally present in many foods, including seafood, meat and cereals.22 These two antioxidant supplements have attracted a great deal of attention as having a possible preventive role against prostate cancer.
However, evidence from a large clinical trial suggests that selenium or vitamin E, alone or in combination, do not prevent prostate cancer in relatively healthy men.23 The SELECT clinical trial that was planned for 7–12 years was stopped after 5.5 years due to lack of efficacy for risk reduction.23 The data did reveal a trend that suggests there may be an increased prostate cancer risk among men receiving 400 IU/day of vitamin E.23
Lycopene is a potent antioxidant found in tomatoes, tomato-based products, watermelon and strawberries that is proposed to reduce the risk of prostate cancer. Many trials have investigated the role of tomatoes in prostate cancer risk, but the association remains controversial. A meta-analysis showed that high levels of tomato in the diet may play a role in the prevention of prostate cancer.24 However, at present the data do not support the clinical use of lycopene in a preventive setting.25
Tea and coffee
Tea contains polyphenol compounds which are antioxidants proposed to play a role in cancer prevention. Consumption of tea, particularly green tea, is suggested to reduce prostate cancer risk.26,27 Most of the evidence comes from epidemiologic studies and few clinical trials have been conducted. Overall the results are inconsistent and it is not known if consumption of tea will reduce risk of prostate cancer.
Coffee contains many biological compounds that have potent antioxidant activities that are suggested may reduce risk of prostate cancer. A recent prospective clinical trial of almost 48,000 men demonstrated a small reduction in prostate cancer risk in men who consumed 6 or more cups per day.28 However, the results are not consistent and other trials have not shown an altered risk profile with coffee consumption.26
Isoflavones are organic compounds that occur naturally in some foods, especially legumes such as soybeans. It is proposed that the high intake of soy-derived food products and the metabolism of the isoflavones they contain is one of the factors which accounts for the significant difference in the incidence of prostate cancer between Asian and European/North American populations.29 Epidemiological studies suggest that increasing soy intake decreases prostate cancer risk.30 But there is little evidence from clinical trials to suggest that dietary isoflavones can alter the risk of prostate cancer.31
Supplements should not be relied on to compensate for a poor diet. Antioxidant supplements do not appear to offer additional benefits in preventive prostate cancer over a well-balanced diet, possibly because of the choice of the substances tested or of an excessive dosage.32 The majority of studies investigating antioxidant supplements do not show a variation in general mortality or cancer incidence.32
Fish oil supplements are one of the most commonly used medicines. For example, a national census of medicines use in Australia estimated that 1 in 4 people over 50 years of age regularly take omega-3 fish oil supplements.33 Bearing in mind the data recently reported on potential increase in prostate cancer risk in men with high blood concentrations of omega-3 polyunsaturated fatty acids,19 additional research is required to determine how fish oil supplements may impact on prostate cancer risk.
Healthy advice for life
To date publications about the effect of diet on prostate cancer risk have been highly heterogeneous and variable in quality and design. More robust, high quality randomised controlled clinical trials are needed to improve the understanding of the complex relationship between diet and prostate cancer.
There are many ongoing clinical trials focusing on particular aspects of the diet that may allow future recommendations for risk prevention. But for now, advise men to eat a well-balanced diet including fruit and vegetables, maintain a healthy weight and undertake regular physical exercise.