By Clare Dix
Contributing writer for Real News Australia.
Nourishment Dietetics | Nutrition. 

In July this year there were several news reports stating that a study in the USA had found fish oil was linked to prostate cancer risk.

USA http://www.9news.com/moms/article/344827/499/Study-finds-correlation-between-fish-oil-supplements-and-prostate-cancer

Australia http://au.news.yahoo.com/video/national/watch/17961141/link-between-cancer-and-fish-oils/.

I thought it would be interesting to have a critical read of the original research and outline the study and my professional opinion on how to interpret it without the mainstream media hype.

Advertisement

The research data comes from the SELECT trial, a study designed to assess the effect of supplementation of vitamin E and/or selenium on prostate cancer risk.  Over 3 years 35,533 men took part in the trial, 1674 of the subjects with prostate cancer, split into two groups: low or high grade cancer, and 1393 of the controls in the study had their blood fatty acid levels measured and compared at the beginning of the trial. Specifically the levels of DHA, DPA and EPA (omega-3 fatty acids), linoleic and arachidonic acids (omega-6 fatty acids) and trans fats (16:1, 18:1, 18:2) were measured and compared between the groups.

Results

They found:

• Levels of DHA, DPA and EPA were higher among subjects with low grade prostate cancer compared with the control subjects
• Only levels of DHA were higher among subjects with high grade prostate cancer compared with the control subjects
• Levels of all trans fats were higher in subjects with low grade prostate cancer
• Only levels of trans fat 16:1 were higher in subjects with high grade prostate cancer compared to the controls subjects

This means that the DHA, DPA and EPA fatty acid levels in the blood were higher in the subjects with low-grade prostate cancer than the control group subjects, and that only the levels of DHA in the blood were higher in the subjects with high-grade prostate cancer than in the control group subjects.  It also means that levels of trans fats were higher in subjects with low-grade prostate cancer, but only trans fat 16:1 was higher in subjects with high-grade prostate cancer.

The authors conducted a search of other studies and determined that due to inconsistent results they could not conclude that trans fats or omega-6 fatty acids contribute to prostate cancer risk. But did conclude that omega-3 fatty acids, particularly DHA and DPA are associated with increases in prostate cancer risk.

What does it mean?

At first glance this study seems to contradict everything that we are told to do for heart health, i.e. eat more fish and cut out the trans fats. And of course this is the headline-grabbing angle the mainstream media took, but there are a few things to keep in mind. Firstly it is important to remember this study is identifying links between fatty acids and prostate cancer risk, not causes of prostate cancer. This is not a cause and effect study, and so you cannot use it as evidence that fish oil causes prostate cancer. Overall the difference between omega-3 levels in the control group and the cancer group was only 0.2%. The results for omega-3 levels for all groups were within the normal ranges and could easily be achieved through diet alone, which does not back the author’s claims that this study indicates we should consider the potential risks of fish oil supplementation. If high levels of omega-3s did cause higher risk of prostate cancer then wouldn’t we be seeing high levels of prostate cancer in countries with fish based diets (Japan, Iceland)?

Secondly, this study was not designed to look at the effects of fish oil on prostate cancer risk, and so has the potential to mislead through poor study design.  The statistical analysis methods used are not particularly robust, even the authors admit this. Thirdly, there is a wealth of research showing the protective, anti-inflammatory, and anti-tumorigenic properties of fish oil. I personally recommend getting your omega-3s through food sources to maximize the nutrients you are getting (calcium, protein, vitamins A and D). It is not wise to disregard years of robust research based on three studies that have reported association between omega-3s and prostate cancer risk (two of which are by the authors of this most recent study).

I think this study more reflects the complexity of interactions between nutrients and chronic disease. We need a more comprehensive understanding of the effects of nutrients on disease before we can start recommending dietary changes. This is an excellent example of the mainstream media using a poor quality study to make sensationalist headlines and ultimately confuse us and compromise our health.

Take home message

Don’t throw out your fish oil supplements or stop eating fish; if you are worried about prostate cancer please go see your doctor. The interactions and effects of nutrients in chronic disease are complex and not completely understood…. yet.

About the Author

Nourishment is run by Accredited Practising Dietitian Clare Dix.
Nourishment provides one-on-one, group and online advice for a variety of nutritional issues, including weight management, general healthy eating and chronic disease management.
http://nourishmentdietetics.weebly.com