Field of Science

Those fish oil supplements might cause cancer

Eating fish is good for you, especially fish that contain omega-3 fatty acids.  So I was surprised last week to read a new study in the Journal of the National Cancer Institute that found that omega-3 fatty acids increase the risk of prostate cancer.  The risk for both high-grade and low-grade cancer was increased with higher levels of omega-3 fatty acids.  This is a carefully done study, and the results should make anyone who is taking fish oil pills reconsider.

One reason this study caught many people off guard is that there has been much evidence showing that a diet rich in fish that contain omega-3 oils is good for you.  The Mayo Clinic says that  "eating fish helps your heart", especially fish like salmon that contain omega-3 fatty acids.  The American Heart Association (AHA) elaborates:
"Omega-3 fatty acids benefit the heart of healthy people, and those at high risk of — or who have — cardiovascular disease. Research has shown that omega-3 fatty acids decrease risk of arrhythmias (abnormal heartbeats), which can lead to sudden death. Omega-3 fatty acids also decrease triglyceride levels, slow growth rate of atherosclerotic plaque, and lower blood pressure (slightly)."
This all sounds great.  Because of the evidence about the benefits of fatty fish, supplement manufacturers have been marketing and selling fish oil pills for years, with great success.  As I described back in 2010, GlaxoSmithKline even created a high-dose omega-3 fatty acid pill called Lovaza that has FDA approval.

But the evidence for that you can get the same benefit from supplemental omega-3 fatty acids — taking a pill, that is — is much weaker.  In fact, a large review published last year in the Journal of the American Medical Association found no connection at all between supplemental omega-3 and a lower risk of heart attacks, strokes, or death in general.  Other studies have reported similarly negative results.  So it appears that fish oil pills may not have any heart benefits.

And now, with this new study, we learn that supplemental fish oil might increase the risk of prostate cancer.

The bottom line: the AHA recommendations about eating fish are probably still good ones.  The AHA website says:
"We recommend eating fish (particularly fatty fish) at least two times (two servings) a week. Each serving is 3.5 oz. cooked, or about ¾ cup of flaked fish.  Enjoy fish baked or grilled, not fried." 
But popping a fish oil pill is not going to cut it. As we've seen before, supplements often fail to show the benefits that a healthy diet offers.  So save your money and stop buying those fish oil pills — and fire up the grill and throw on a few salmon fillets for this weekend's barbecue.


  1. The thing to notice is the SERVING SIZE--this is what thwarts so many people's weight loss effort. 3.5 oz is not very much--but we find it saves a lot of money to stick to it! I point this out when people complain about the expense of salmon. Same with fruit. I search for SMALL apples and oranges (and bananas) which can be difficult to find in our "big is better" world. Starting with small is easier than trying to only eat half.

  2. Trouble is that the study you are referring to was not looking at supplements specifically but blood levels of omega-3, the authors do not know the source of the omega-3. As far as they know the risk is from any omega-3 fish or supps

    Another problem - the association was significant only with high-grade cancer and when you start slicing up all the numbers to look at quartiles on omega-3 and grade of cancer the numbers become very small (n=26, 35, 52 and 43, low to high levels - so, only 26 subjects in the referent group) - a big drop from the headline numbers in the abstract

    Also did they look for any benefits? Even if true that there was a small increase in prostate cancer, would that have been offset by possible health benefits in other areas? After all even seat belts in cars are responsible for increasing the risk of certain types of death and injury...

    (and grilled fish may not be the best way to get regular fish oil!)

    1. I read the whole paper looking for major flaws, and couldn't find any - and I tried. They cited several earlier studies that reported the same increase in prostate cancer risk, so this isn't the first one. The evidence is building that this is a real effect. They also cited (as did I) some large recent studies showing that fish oil *pills* have no benefit. There are earlier studies showing that a diet high in fatty fish is correlated with better heart health, hence their conclusion that there's no benefit from fish oil supplements, despite the apparent benefit from eating fish.

  3. it all looks a bit random though. in the present study they claim to have repeated results of a previous study ( But they did not. In the previous study the only significant association was found with DHA (and only, oddly, in the 2nd and 4th quartiles - in the 3rd quartile it was not significant). In the present study the only significant association was with total omega3, it was not significant for DHA and it was not significant for total omega3 in the previous study. Also previously they reported that trans fats were protective, in this study there was no association.

    I don't think the study was flawed but I think some of the conclusion were, at least they were too strong and not really supported by the data. Especially (as they also say) the mechanisms are unknown - so to even suggest causality they need to rule out more simple possibilities (e.g. other lifestyle behaviours associated with taking supplements)

  4. The study was well done in general, but not when it came to addressing Omega three fatty acids. The initial intent of this study was not to evaluate omega-3 intake and prostate cancer risk. The results may be confounded by the initial intended treatments: selenium and Vitamin E. There is no documentation of fish oil or dietary fish intake in the study group, which is a major methodological flaw. Researchers did not discuss diet or supplementation at the beginning, during, or end of the study. But most important, is that the measure of evaluating omega-3 intake in this study was plasma phospholipid
    levels, which is not a good gauge of long-term omega-3 consumption. Plasma phospholipid levels can be influenced dramatically by a single meal, or even the timing of a fish oil dose. These levels can increase or decrease, based on a single meal within a 48-hour period. Furthermore, fatty acid levels were taken from just one blood
    sample provided by participants upon enlistment in the study, not at the time of diagnosis, and certainly not at the end of the study five years later. What these men ate within 48 hours of study enrollment hardly justifies sound science. No measurement was ever taken again at any point in the study. The statistical model that was used to draw conclusions is appropriate for drug consumption at regular daily intervals, where levels stay fairly constant, but not for omega-3 blood serum levels, which may vary considerably based on diet.
    Finally, the researchers base their conclusions on very small differences in mean omega-3 blood plasma phospholipid levels. The increased prostate cancer risks they ascribe to men who consume large amounts of omega-3 is based on a level of 4.66% in the cancer group, versus a level of 4.48% in the control group. This is the difference of eating maybe an ounce of fish. The bottom line, is that they irresponsibly reported a singular data point and made a very wild non-scientific assumption based on this.

  5. From all I've read Omegas can be a bit unstable and require antioxidants such as natural Vitamin E (not synthetic -dl) to counteract the oxidation effects. For that reason I take ground flax seed and flax oil along with a minimal amount of Omegas. This allows my body to convert the flax to the EPA/DHA that it needs and reduces the amount of pure omegas that my body may require. Should I have difficulty with the conversion of flax I still get my omegas as a safety net. This way I get the best supplementation and I am not in a holding pattern until all the data shakes out.
    It is my understanding that krill has naturally occurring antioxidants and thereby may be a better source of omegas.
    I am in full agreement that the establishment Is much too quick to discredit supplementation with questionable data while totally overlooking the preponderance of evidence on the beneficial aspects from past studies and demographic analysis.
    This leads me to question the integrity of the scientists doing these studies and their motives as well as the media reporting it.

    - james from

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