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Does diet influence cancer? The answer to this question is not simple.

The relationship between nutrition and disease is a complex one, but there is a tendency to simplify things to provide easily comprehensible information to the public.

That’s why we regularly see newspaper headlines linking individual food items to particular diseases.

But while the relationship between diet and cancer is far more complicated than is often made out, the large variation in cancer rates worldwide, and the links between dietary changes and cancer incidence among migrating populations, suggest that what we eat does have a part to play.

Although many studies have investigated the link between diet and cancer, the evidence is conflicting, and results do not apply to all cancer types. One of the big challenges is disentangling the impact of diet from other lifestyle factors – and this always has to be taken into consideration when interpreting results.

Furthermore, our diet consists of many different food items comprising many different nutrients and ‘phytochemicals’, all working together at the same time. It is difficult to pinpoint the effect of one single food item or group given such a convoluted situation.

Current recommendations from the World Cancer Research Fund/American Institute of Cancer Research suggest that sticking to a healthy diet – rich in fruit, vegetables, wholegrains and pulses, and low in processed or red meat – may lower the risk of certain cancers, whereas an unhealthy diet – high in salt, sugar, alcohol and processed or red meat, and low in fruit and vegetables – may increase cancer risk. Moreover, a balanced diet could help to maintain a healthy body weight, thereby reducing the risk of cancer.

But what about people who have survived cancer? Although cancer rates are increasing, survival rates are going up as well. Is it still useful for cancer survivors to change their diet after diagnosis? Currently, no specific dietary guidelines exist for improving cancer prognosis after therapy, and survivors are advised to follow the same dietary recommendations for cancer prevention.

There is limited evidence that cancer survivors could benefit from a healthy diet. Indeed, many survivors are highly motivated to make lifestyle changes to reduce their risk of recurrence. Future research should focus on identifying modifiable risk factors among cancer survivors – especially in bladder cancer, which demonstrates both a high survival and recurrence rate.

Approximately 10,000 people in the UK were diagnosed with bladder cancer in 2011, and it has been suggested that 42% of all bladder cancer cases each year are linked to modifiable risk factors.

In the West Midlands, the Bladder Cancer Prognosis Programme (BCPP) is investigating the influence of lifestyle factors on recurrence and progression in bladder cancer survivors. BCPP, which is run by the University of Birmingham, has the potential to promote healthy behaviour among bladder cancer survivors, reducing both recurrence rates and costs for the NHS.

Results from this study will help clarify the relationship between diet and bladder cancer survival.

Sylvia Jochems, PhD candidate in Cancer Sciences, University of Birmingham and Maastricht University