"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.
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.
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.