Fish oils and prostate cancer. Are supplements dangerous?

I have been asked to explain the reported “findings” of the SELECT study. This is the study generating new headlines claiming that fish oil supplements are linked to an increased risk of aggressive prostate cancer. 

For starters, the men in the SELECT study were not given fish oil supplements. What the results were reporting were levels of EPA, DPA and DHA omega-3s in blood, and cancer risk. And it is very important to note that the difference of omega-3 levels when comparing the healthy (control) group and the combined cancers group was nearly insignificant.

That difference is 4.66% in the combined cancer group versus 4.48% in the control group. It is a minute variance. Eating one additional serving of fish compared to the control group could result in that difference. Again no fish oil supplements were given in the study, so to blame omega-3 supplements for the reported findings is totally irresponsible.

Another point to consider is this. The reported omega-3 levels in any of the SELECT participants are not high levels. They are only slightly higher than the levels in those who consume a western diet typically average. Therefore any country that consumes a diet based on fish would have incredibly high levels of aggressive prostate cancer, and that is not the case.

What does the science really say about fish consumption or omega-3 levels and risk of prostate cancer?

The studies almost uniformly show that eating fish or having higher plasma levels of omega-3 is protective.  Several large scale epidemiological studies, including those looking at native Japanese men, Inuit men and people that live in South America, have shown that a high intake of fish and a high omega-3 status is linked to a lower incidence of many cancers including prostate cancer.

In a 2010 meta-analysis published in the American Journal of Clinical Nutrition by Szymanski et al, the authors pooled data from four studies on fish consumption and death from prostate cancer and found a 63% decrease in risk for high fish consumption.

One must be very careful with making decisions based on the latest headlines. People do have a tendency to make decisions based on the last study or news advertised, rather than the sum of evidence of many studies published on a specific topic. But we must not do this. We must take into consideration the totality of evidence that we have on a topic, not just the latest headline on CNN.

In addition, studies should be read critically – there are inherent flaws in most of them.

The findings from this most recently reported study: Brasky et al. 2013 ( SELECT) are from an observational study (looking back in time) and observational studies do not prove cause and effect.

Scientific studies that are prospective in design are considered more important in weighing total evidence and give us more useful guidance in our choices on diet or supplements that might improve our health.

Are there more useful, prospective studies that demonstrate protective benefits of omega 3 fatty acids against prostate cancer?

Yes there are quite a few. Lets consider some of those.

Researchers investigated the effect of dietary fish intake amongst 6272 Swedish men who were followed-up for 30 years. That study reported that men who ate no fish had a two to three-fold increase in the risk of developing prostate cancer compared with those who consumed large amounts of fish in their diet.

TerryP, Lichtenstein P, Feychting M, Ahlbom A, Wolk A. Fatty fish consumption andrisk of prostate cancer. Lancet 2001; 357: 1764–6

Another prospective cohort study based on the Physician’s Health Study found that fish consumption (≥5 times per week) was not related to prostate cancer risk but was protective of prostate cancer–specific death.

ChavarroJE et al. A 22-y prospective study of fish intake in relation to prostatecancer incidence and mortality. Am J ClinNutr 2008; 88: 1297–303.

Other studies have suggested lower prostate cancer risk with Omega 3 fatty acids from fish in Swedish men.

NorrishAE, Skeaff CM, Arribas GLB, et al. Prostate cancer risk and consumption of fishoils: A dietary biomarker-based case-control study. Br J Cancer. 1999;81:1238–42.

And this study showed lower prostate cancer risk with Omega 3 fatty acids from fish in Japanese and Brazilian men..

KobayashiM, et al, Serum n-3 fatty acids, fishconsumption and cancer mortality in six Japanese populations in Japan andBrazil Japan J Cancer Res.1999 Sep;90(9):914-21.

A large prospective cohort established in 1986 looked at 51,529 American men, 40 – 75 years of age, completed a mailed questionnaire about demographic and medical information found that a high intake of fish was associated with a lower risk of metastatic prostate cancer. A similar association was also found for dietary marine fatty acids from food.

Leitzmann MF, Stampfer MJ, et al, Dietary intake of n-3 and n-6 fatty acids and the risk of prostatecancer. Am J Clin Nutr. 2004 Jul;80(1):204-16.

An important clinical study published by a group at the Harvard School of Public Health examined the link between dietary fish consumption and the risk of metastatic prostate cancer. This paper reported results from the Health Professionals Follow-up Study that involved 47,882 men over twelve years. During the twelve years, 2,483 cases of prostate cancer were identified. Of these, 617 were advanced and 278 were metastatic. Eating fish more than three times a week reduced the risk of prostate cancer but had an even greater impact on the risk of metastatic prostate cancer. For each additional 500 mg of marine omega-3 consumed, the risk of metastatic disease decreased by 24%!

Augustsson,K., et al., A prospective study of intake of fish and marine fatty acids andprostate cancer. Cancer EpidemiolBiomarkers Prev, 12(1): p. 64-7, 2003.

And again, in the 2010 meta-analysis (an analysis of multiple studies) Szymanski and his team found that a significant 63% reduction in prostate cancer-specific mortality in those that consumed fish but no link between eating lots of fish and men’s risk of developing prostate cancer.

SzymanskiKM, Wheeler DC, Mucci LA. Fish consumption and prostate cancer risk: a reviewand meta-analysis. Am J Clin Nutr.2010 Nov;92(5):1223-33.

When one considers the larger picture, the headlines claiming that men who consume omega-3 supplements are at increased risk of prostate cancer, based on one study where no one used omega-3 supplements, and where levels of omega-3s were only 0.02% in variance among cancer cases and healthy cases, seem preposterous.