My take on the fish oil and prostate cancer study

Every so often, a piece of health news will make a splash and trigger a mini-deluge of emails in my inbox. This was certainly the case last week, when a study linking certain omega-3 fats (‘fish oils’) with an increased risk of prostate cancer. The study, published in the Journal of the National Cancer Institute, assessed the level omega-3 fat in the blood of 2227 men and risk of prostate cancer [1]. Men with the highest levels of omega-3 fats had a statistically significant increased risk of prostate cancer compared to those with the lowest levels. Specifically, increased risk of ‘low-grade’ (early) and ‘high-grade’ (more advanced) prostate cancer was found to be 44 and 71 per cent respectively.

One good thing about this study is that it assessed omega-3 fat levels in the blood. This, I think, is better than relying, say, on people’s memories of how much oily fish they ate over the preceding year or whatever. However, one fundamental deficiency of this study is the fact that it is epidemiological in nature. These studies can tell us about the relationships between things, but not that one thing is causing another.

Also, it often makes sense to view the results of any new study in the context of what has come before.

The authors themselves present data from similar studies grouped together in the form of what is known as a ‘meta-analysis’. Meta-analyses are not perfect (for example, sometimes the methods used vary between studies which can make interpretation difficult), but they generally provide a better overview of the evidence than single studies. The meta-analyses (which included the present study) revealed:

  • A link between high omega-3 levels and increased risk of high-grade prostate cancer (higher levels were associate with a 51 per cent increased risk)
  • NO link between high omega-3 levels and increased risk of low-grade prostate cancer
  • NO link between high omega-3 levels and increased risk of prostate cancer overall (high- and low-grade prostate cancers combined)

Another way of getting an overview of the evidence is to broaden the enquiry beyond a single condition. Let’s use sunlight as an example. If we were, say, to focus purely on the relationship between sunlight exposure and certain skin cancers, we may be tempted to never go out in the sun unprotected. However, what is often missed is that sunlight exposure is also associated with a reduced risk of many different forms of cancer. Overall, the relationship between sunlight and cancer is one of protection. Also, sunlight is associated with a reduce risk of other conditions including heart disease and multiple sclerosis.

Probably the best and broadest way to judge the relationship between any factor and health is to assess its relationship with overall risk of death. Although still epidemiological in nature, these studies encompass all conditions. Do we have such evidence for omega-3 fats? Indeed we do.

In a study published earlier this year, researchers from the Harvard School of Public Health in the US measured blood levels of omega-3 fats in 2,692 adults and then followed them for an average of 11½ years [2].

Those with the highest levels of omega-3 fats (compared to those with the lowest levels) were 27 per cent less likely to die over the course of the study.

Those with the highest levels lived an average of 26½ months more than those with the lowest levels.

It would be difficult to make a case for this study not to trump the one from last week (which looked at risk of prostate cancer alone, and not even risk of death from this).

I saw the Harvard School of Public Health study when it came out but let it float by. Why? Primarily because it’s epidemiological in nature, and I’ve made it an unofficial policy of mine not to put too much store in research which, at the end of the day, doesn’t tell us that much.


1. Brasky TM, et al. Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial. J Natl Cancer Inst. 2013 Jul 10. [Epub ahead of print]

2. Mozaffarian D, et al. Plasma phospholipid long-chain ω-3 fatty acids and total and cause-specific mortality in older adults: a cohort study. Ann Intern Med. 2013;158(7):515-25

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14 Responses to My take on the fish oil and prostate cancer study

  1. Stephen Rhodes 16 July 2013 at 3:56 pm #

    I was a little surprised by the articles in the papers but then I checked and saw that the paper ( from which the alarmist headlines were taken was from 2011 so I had a look and found that Dr Alan Kristal has also carried out research ( showing that Omega 3 reduces risk of obesity related disease in Eskimos.

    In that paper they also note that the incidence of prostate cancer in the studied Eskimo are negligible and that the protection against cancer must result from some other factor.

  2. Dr. Stephen Sinatra 16 July 2013 at 5:59 pm #

    In my opinion, I felt that the researchers in this study—as well as others they reported on–had a negative bias against nutritional supplements. This wasn’t a double-blind, placebo controlled trial about omega-3s—in fact, we don’t even know if the participants in this study took omega-3s. Instead, the researchers drew a conclusion based on a .2% difference in omega-3s—one that can show association, but not causation.

  3. George @ the High Fat Hep C Diet 16 July 2013 at 7:50 pm #

    I wonder if omega-3 levels tended to be higher in people who felt AT RISK for some reason – family history, BHP, etc – and therefore followed advice to eat more fish/oil prior to the appearance of cancer.
    Also, surely it is common to suspect cancer, and try to do something to avoid it, before getting it diagnosed, which may take time.

  4. Robert Park 19 July 2013 at 8:12 pm #

    Just a thought here; if PUFAs are potent immunosuppressive (which they are recognised to be) and those who take supplementation of omega 3s from fish oil, surely then they would be placing their health at risk and not solely of prostate cancer?

  5. davidchandler 20 July 2013 at 8:52 am #

    Had the ones with high grade prostate cancer already been diagnosed,so were taking omega 3 as a nutritional supplement to help overcome the disease.
    If the fish oil in supplements was rancid,would this affect the omega 3s health wise

  6. Hilda Glickman 20 July 2013 at 1:26 pm #

    It may have nothing to do with the omega 3. If they had eaten lots of fish it could be the oestrogens and pollution that are in the fish.

  7. William L. Wilson, M.D. 20 July 2013 at 6:26 pm #

    I like Barry Sears’ analysis of this study:

    It’s a shame when reporters splash these headlines around the world without actually analyzing the study generating the headlines.

  8. Everett 20 July 2013 at 9:29 pm #

    Ever since reading Ray Peat, I’ve steered clear of PUFAs, save for some organic avocados.

    Guess we will see how it all goes down.

  9. donald tiso 23 July 2013 at 11:42 pm #

    ANH produced an interseting report on this study

  10. Anon 5 August 2013 at 9:44 am #

    There may be a possibility of getting cancer when taking large amounts (>10g / day) of polyunsaturated (omega 3 or omega 6) oils, due to the proneness of those oils to oxidation.

    How the oil became part of the diet (straight from the bottle or subject to the high temperatures of frying or baking) could make a difference.

    Best take ’em cold – as salad dressing or tonic straight from the bottle.

  11. David 3 September 2013 at 7:34 am #

    Hi, Dr. John Briffa
    Thanks for your sunlight example, it makes me brave. We know the relationship between sunlight contact and skin cancers, but we never stopped to go out in the sun. However, sunlight also allied with a reduced risk of different forms of cancer. Meta-analyses discovered a link between high omega-3 levels and increased risk of high-grade prostate cancer 51% but we know fish oil has many benefits for vision, brain, skin health, joints, blood pressure, lowering triglycerides, reducing arrhythmias, and yes, even cancer.
    Actually, we always fight against our worst nature & your article make us brave to live.


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