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Should we reduce dairy foods to reduce the risk of prostate cancer?

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Every week there is a new story in the media about foods that cause or cure cancer. This week a study from the USA is reported to show that dairy foods are causing prostate cancer. Is it time to throw away the cheese and milk?

A new review article has been published asking whether plant-based or animal-based foods change the risk of getting prostate cancer. This study provides information that is useful for men who don’t have prostate cancer and want to reduce their risks of getting this disease. It’s not applicable for men who already have prostate cancer.

In order to interpret this study, we need to first consider the causes of prostate cancer:

What causes prostate cancer?

The ultimate cause of cancer is changes to DNA inside cells that make them grow differently. There are multiple DNA changes needed to cause cancer, not just one. Prostate cancer is believed to be caused by numerous different alterations to DNA. There are different ways that DNA can be damaged, and this damage accumulates over time. What this means is that there is more than one cause of prostate cancer.

It’s very difficult to know what has caused one particular man’s prostate cancer. But by studying large groups of people with and without prostate cancer we can determine the risk factors for this disease. These risk factors are things that put men at higher risk of being diagnosed with prostate cancer. Risk factors are not necessarily the same as direct causes. For instance, as people age, their risk of cancer increases - because the longer someone lives, the more likely their cells will accumulate damage.

What are the main risk factors for prostate cancer?

Older age  -  Older men are more likely to be diagnosed with prostate cancer than younger men.

Family history of prostate cancer  -  Men who have one or more direct relatives diagnosed with prostate cancer are more likely to get this disease. PCFA recommends men with a family history of prostate cancer talk to their GP about getting tested from the age of 40.

Genetics  -  Having certain forms of some genes puts men at greater risk of being diagnosed. African American men are more likely to be diagnosed with prostate cancer, and more likely to have aggressive disease. The reasons for this are not well understood.

Unfortunately, the main risk factors for prostate cancer are things that can’t be changed.

Can food increase or reduce the risk of prostate cancer?

Many people would like to decrease their risk of cancer through their diet. But the messages about what we should and shouldn’t eat to reduce this risk are confusing.

In general, we know that people who have a healthy diet are less likely to get cancer compared to people who have an unhealthy diet. For prostate cancer, we know that men who eat a high-fat diet are more likely to get this disease. If they get prostate cancer, it is more likely to be high-risk disease.

What about other food types? It seems that every week a story comes out in the media telling us what foods cause prostate cancer or protect us from it. There are hundreds, if not thousands of scientific studies of the associations between foods and prostate cancer. Many have conflicting results. None of these studies are perfect; they all have some flaws. How can anyone make sense of all this information?

To make sense of many different studies, scientists perform literature reviews to bring together lots of different results for one topic. By performing these reviews, they look for trends and try to make conclusions that take into account findings from all the best studies.

Effect of plant and animal-based diets on prostate cancer risk

A new review article has asked whether plant and animal-based foods are associated with the risk of being diagnosed with prostate cancer. This study was performed by researchers at the Mayo clinic in Minnesota, USA.

The researchers started by identifying 297 studies of plant and animal-based diets and their effects on prostate cancer risk. They reduced these to 47 eligible studies based on quality and relevance to the question.

Plant-based diets  -  One question addressed by the study is whether plant-based diets were associated with a higher or lower risk of prostate cancer. Three studies showed that men with a vegetarian diet were less likely to get prostate cancer, but three others showed no difference. Another three studies showed that men who were vegans had a lower risk.

Meat-based diets  -  Four studies of men who had diets high in red meat showed no increase in risk, where as one showed an increased risk of getting prostate cancer. One study showed an increase in risk for men who ate a lot of processed meat.

Dairy-foods  -  Fourteen studies were reviewed that examined the risk for men from eating dairy-foods. Seven of these showed an increased risk, six showed no difference and one showed a decreased risk of prostate cancer.

Why do these studies have different results? Each study was quite different. They were performed in different countries, recruited different groups of people and measured food intake in different ways. In particular, the difference results from these studies shows that making decisions based on only one study would not be an ideal approach.

Conclusions from the review

The authors made conclusions that were mostly based on counting the number of studies that supported differences in risk. They stated that their results “suggest that consumption of higher amounts of plant-based foods may be associated with decreased prostate cancer risk, and consumption of higher amounts of dairy products may be associated with increased prostate cancer risk.” There are a lot of “maybe” statements in this long sentence, because they couldn’t reach particularly firm conclusions.

Care is needed when interpreting studies like these. The risk factors they study are not necessarily the causes of cancer. For instance, people who choose a vegan diet had a lower risk of prostate cancer in some studies. But it’s not necessarily the diet that is making a difference. People who make this choice may be making other healthy choices such as exercising more, smoking less and drinking less alcohol. We don’t know the extent to which the diet made a difference, given other potential healthy choices. In other words - correlation is not the same as causation.

This article has been featured in numerous media stories suggesting that consumption of dairy products is associated with higher risk of prostate cancer. This is not a new finding, we already know that some studies point to a slightly higher risk of getting prostate cancer for men who eat a lot of dairy products.

What does this mean for Australian men?

Given the results of this study, should men reduce dairy consumption to reduce their risk of getting prostate cancer? We asked A/Prof David Smith, and expert in prostate cancer epidemiology from Cancer Council NSW. He told us that:

“Evidence reviews conducted by the World Cancer Research Fund show that there is strong evidence that dairy foods can be protective against bowel cancer. There is some limited evidence that dairy foods are protective against breast cancer (pre-menopause) and conversely, there is also some limited evidence suggesting that dairy foods are associated with increased risk of prostate cancer. However, given that the evidence is not strong for the link between consumption of dairy foods and increase prostate cancer risk, Cancer Council supports the recommendations of the Australian Guide to Healthy Eating for adults to include at 2.5 to 4 serves of milk, yoghurt or cheese per day.”

PCFA recommends that men wishing to reduce their risk of getting prostate cancer maintain a healthy diet, based on the Australian Guide to Healthy Eating. However, men who have had specific advice from doctors or dietitians regarding their diets should certainly adhere to that advice.

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