Fish oil supplements linked to prostate cancer

Behind the Headlines

Thursday July 11 2013

There is little evidence to support the use of omega-3 supplements

"Taking omega-3 fish oil supplements may increase the risk of aggressive prostate cancer by 70%," the Daily Mail reports.

The story, covered widely in the media, comes from a large and well designed study that also found that high blood levels of omega-3 fatty acids were associated with a 44% increase in the risk of slow growing prostate cancer.

Supporters of fish oil supplements have claimed that they can reduce the risk of stroke, heart attack and dementia as well as improving cognitive function and mental health. But there is little conclusive evidence to justify these claims.

For more information see the Behind the Headlines special report on dietary supplements.

The findings match previous studies that have found a similar link between high blood levels of omega-3 fatty acids and prostate cancer.

It is worth bearing in mind that this study did not assess participants’ diet and use of supplements. Researchers measured blood levels of fatty acids and analysed the association with prostate cancer risk. However, it is likely that the very high levels of fatty acids found in some participants’ blood came from supplements.

If you are considering taking an omega-3 supplement get medical advice first.


Oily fish

Long-chain omega-3 fatty acids may be good for the heart, but you can obtain enough from your diet. It’s not common for someone to need supplements for their health.

A healthy diet should include at least two portions of fish a week, including one of oily fish (such as mackerel). Babies, children and women who are pregnant, breastfeeding or planning to have children should have no more than two portions of oily fish a week.

Those not in these groups can eat up to four portions a week. This maximum level is recommended to avoid overexposure to marine pollutants. Read more about eating fish and shellfish and your health.

Where did the story come from?

The study was carried out by researchers from the Fred Hutchinson Cancer Research Center, Ohio State University and was funded by the National Cancer Institute. It was published in the peer-reviewed Journal of the National Cancer Institute.

It was covered fairly in the papers, with the Daily Mail including comments from independent experts.


What kind of research was this?

This was a case-control study that looked at the association between blood levels of long-chain omega-3 fatty acids and the risk of prostate cancer. 

In this type of study, cases of people who have a particular outcome – in this case, prostate cancer – are matched against a random group of people who do not develop the condition.

The research was part of a large randomised controlled trial called SELECT, looking at whether selenium and vitamin E supplements reduced the risk of prostate cancer. (It found no benefit from selenium and an increase in prostate cancer in men who took vitamin E.)

The researchers point out that omega-3 supplements are widely used and that ongoing trials are looking at their possible benefits for cancer and heart disease prevention. Their previous study from 2011 suggested a link between high blood levels of long-chain omega-3 fatty acids and high grade (aggressive) prostate cancer. 


What did the research involve?

The researchers included 834 men from the original trial, who had been diagnosed with prostate cancer, of which 156 were diagnosed with high-grade (aggressive) cancer. 

The researchers randomly selected 1,393 men who matched the case subjects on age and race, with a ratio of 1:3 for black men and 1:1.5 for other men. The men completed questionnaires about their backgrounds and health at the start of the study, while staff measured height and weight to calculate body mass index (BMI). Blood samples were also collected and blood levels of long-chain omega-3 fatty acids (also called polyunsaturated fatty acids, or PUFAs) were assessed. These were:

  • eicosapentaenoic acid (EPA)
  • docosapentaenoic acid (DPA)
  • docosahexaenoic acid (DHA)

They categorised blood levels of these fatty acids into quartiles (four equal groups of 25% of the study group).

Researchers also looked at blood levels of omega-6 fatty acids – linoleic acid (LA) and arachidonic acid (AA) – and of trans fatty acids.

Using standard statistical methods, the researchers analysed the associations between overall blood levels of omega-3 fatty acids and prostate cancer risk overall, and by grade. They also looked at the association between prostate cancer risk and blood levels of individual omega-6 fatty acids. They adjusted their results for other confounders that might affect the risk of prostate cancer, such as family history.

They also carried out a meta-analysis to compare their results with similar studies.


What were the basic results?

The researchers found that, compared with men whose blood levels of omega-3 fatty acids were in the lowest quartile, men in the highest quartile had:

  • 44% increased risk of low grade prostate cancer (HR (hazard ratio) = 1.44, 95% CI (confidence interval) = 1.08 to 1.93)
  • 71% increased risk of high grade prostate cancer (HR = 1.71, 95% CI = 1.00 to 2.94)
  • 43% increased risk of total prostate cancer (HR = 1.43, 95% CI = 1.09 to 1.88)

These associations were similar for the individual omega-3 long-chain fatty acids, EPA, DPA and DHA.

A higher blood level of linoleic acid was associated with a reduced risk of low grade prostate cancer (HR = 0.75, 95% CI = 0.56 to 0.99) and total prostate cancer (HR = 0.77, 95% CI = 0.59 to 1.01). Linoleic acid is found in various vegetable oils.


How did the researchers interpret the results?

The researchers say their study confirms previous reports of increased prostate cancer risk among men with high blood concentrations of omega-3 fatty acids. They say the consistency of these findings suggests that these fatty acids are involved in the growth of prostate tumours.

Recommendations to increase omega-3 intake “should consider its potential risk”, they argue.

They also say that the findings are surprising, given that omega-3 fatty acids are thought to have anti-inflammatory properties, pointing out that inflammation plays a role in the development of many cancers. Further research is needed into the possible mechanisms, they say.



This was a large, well designed study that supports previous research linking high blood levels of omega-3 fatty acids with prostate cancer risk. However, it cannot show that fish oil supplements cause prostate cancer and it is possible that other confounders affected men’s risk (although the researchers tried to take these into account).

The research did not look at the participants’ diets or whether they took omega-3 supplements. Still, it is unlikely that the high levels of these fatty acids found in the highest quartile would be the result of diet alone. Adults are advised to eat two portions of fish a week, one of them oily, as part of a healthy balanced diet.

Despite claims that fish oil supplements can help prevent numerous conditions including cancer, dementia, arthritis and heart problems, there is little hard evidence for them. Although they are “natural” products (albeit in a processed form), this does not mean they are safe or suitable for everyone.

While omega-3 supplements are sometimes advised for people who have had a heart attack this supplement is typically used under supervision of a healthcare professional.

If you are thinking of taking omega-3 supplements, talk to your GP or the healthcare professional in charge of your care first.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.

Analysis by Bazian

Edited by NHS Choices


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The 5 comments posted are personal views. Any information they give has not been checked and may not be accurate.

simpletheory said on 13 March 2014

arthursyd, the article you linked was interesting but very very far from balanced. The author has cherry picked in the extreme. The main source he uses is a Dr W Harris. Unfortunately Dr Harris can hardly be taken as an unbiased source and his reaction to the study could be seen as damage limitation given his extensive financial interests in promoting omega-3:

There are actually even more companies he has been involved in which are involved in selling omega-3, but my original reply timed out. Suffice it to say his comments should be taken with a large pinch of salt, making the above review balanced and unbiased.

Having (quickly) looked at the research abstracts available, it would seem the best summary is "it's complicated". My reading is if a diet is high in red meat and animal fats (typical of most western diets) then the impact seems to be neutral but the same diet AND omega-3, then that would seem to indicate a risk . If a diet has minimal red meat (such as nordic and japanese diets), then it is protective.

I found this page looking for information for my husband and I will be strongly recommending he does not take omega-3.

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Abel49 said on 05 December 2013

I suffer from benign prostate enlargement (BPH) with a current query of Cancer following a high PSA score. So I am looking for useful information about things I can do to help myself. The above "behind the headlines" article is poor and has fallen - as I see it - into the same trap as the "headline grabbing media".

This link is a much better "behind the headlines" article for the same SELECT study.

Some extracts:
- headline-hungry media did not cover all of the facts
- that team and the media coverage had “extrapolated the findings far beyond the data themselves.”
- Correlation does not imply causation.
- With the Japanese intake of omega-3 fatty acids at about eight-fold that of Americans and with their blood levels twice as high, one would expect a higher risk. However, the Japanese prostate cancer rate of 22.7 per 100,000 in 2008 was dramatically lower than the U.S. rates of 83.8 per 100,000.
- the question of whether fish oil supplements or an intake of more oily fish increases prostate cancer risk was not tested.

The last point is the most significant. The article above, the media and the study conclusions were *not* based on the evidence - that taking more fish, or omega 3 supplements increases prostate cancer risk.

Read the linked article above. I found it a more balanced behind the headlines analysis. More useful.

I will keep taking my O3 and fish.

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arthursyd said on 24 July 2013

in reply to Rosemary
Hello! You can easily search online for international organisations that report on fish consumption. The countries are as many people would expect - Nordic Countries such as Iceland and Finland; Asian countries such as Japan - all where fish consumption is much higher than the country where the research took part.

Not sure about your comment - "Using the term 'omega 3' is no more use than talking about 'vitamin'"? The research specifically sites omega 3, as does this NHS analysis, as does all the corresponding media hype. So its not omega 6 or a more generalised 'essential fatty acids' that is attributed but omega 3.

All the best

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Rosemary Stanton said on 16 July 2013

in reply to arthursyd
Using the term 'omega 3' is no more use than talking about 'vitamin'. Which countries do you include in your statement of 'countries where fish is eaten in quantity' ?

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arthursyd said on 15 July 2013

I have to say I am most disappointed with this review by Bazian. I am a huge fan of 'behind the headline' and to-date think their reviews and conclusions of stories are spot on. But here you seem unprofessionally to jump to un-scientific conclusions, even more so than the researchers themselves.

The research did not include any Omega 3 factors, the researcher's had no idea what the participants diet was (did any of them eat fish or take fish oil supplements?) yet you state "The research did not look at the participants’ diets or whether they took omega-3 supplements. Still, it is unlikely that the high levels of these fatty acids found in the highest quartile would be the result of diet alone." - What?! How can you jump to this conclusion. Also the research made no note of supplements yet your review directly criticises Omega 3 supplements - why?

Omega 3 has a number of EFSA (European Food Safety Authority) proven health claims and it has been pointed out that if Omega 3 raised prostate cancer risks then it would be a huge problem in countries where fish is eaten in quantity - and this is simply not the case?

I think sadly your review is (unusually) unhelpful to the public and a disservice to the fish industry in the UK and reputable UK supplement companies too.

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