“Though many laud the health benefits of omega-3 fatty acids, one study is showing that eating the fatty acids found in fish oils may increase the risk of prostate cancer for men.”
The media has been ablaze this week with the results from Brasky et al. (1) published in the Journal of the National Cancer Institute linking high long-chain omega-3 blood levels with increased risk of advanced prostate cancer. But, once again, this is another example of the media confusing correlation with causation. Here’s why we don’t suggest you give up fish just yet, regardless of your gender…
What’s interesting is that the Brasky et al. (1) results are nothing new – they had similar findings published in 2011 (2) showing an increased risk of advanced prostate cancer among men with very high blood levels of omega-3 fatty acids, namely DHA, EPA and DPA (the one’s found in fish). But that’s not the whole picture – time to look more closely at the study design and conclusions before we hang fish out to dry!
Both Brasky et al. (1,2) studies were “nested” and pulled their data from large-scale randomized controlled trials – the SELenium and vitamin E Cancer prevention Trial (SELECT Trial) and the Prostate Cancer Prevention (PCPT) Trial. Here’s some background on the trials themselves:
- SELECT: looked at selenium and vitamin E supplements and risk of prostate cancer. Was discontinued early in 2011 when prostate cancer cases were significantly MORE common in men taking vitamin E.
- PCPT: looked at finasteride drug treatment and risk of prostate cancer. Was discontinued early in 2003 when prostate cancer cases were significantly LESS common in men taking finasteride. Interestingly, men who developed prostate cancer while taking finasteride experienced a slightly higher incidence of high-grade tumors (owing to smaller prostate size).
How is this relevant? Well, the two nested studies linking omega-3 levels and prostate cancer were not designed to answer this specific question; the data pulled from the two larger studies was cross-sectional (that is, from one time-point) and cannot establish causation, only correlation. Just because two things are linked, does not mean one is causing the other. For more information on correlation vs. causation, check out our previous posts: The War on Spurious Science – Cause and Effect?, Again with the Egg Yolks! and Calcium Supplements and CVD: What is the Evidence?
Importantly, they did not report on measuring men’s dietary or supplement omega-3 intake (3). While very high dietary intake through food or supplements would most certainly boost omega-3 levels, the authors did not directly link dietary intake of omega-3s to an increased risk of prostate cancer.
The conclusions made by Brasky et al. (1) are more modest compared with the media coverage – there is no mention of avoiding fish though they state that recommendations to increase DHA and EPA intake “should consider its potential risks”. In several media releases from the authors, they note that the difference between the lowest and highest circulating levels of omega-3s was akin to “somewhat larger than the effect of eating salmon twice a week,” which is equivalent to at least 3000 mg per week.
Does this mean you cannot eat fish more than twice per week? Nope* – there is evidence from dietary studies that fish intake is actually linked with REDUCED risk of prostate cancer (4-6). No, fish and chips is not going to cut it! The only fish linked with increased risk of prostate cancer? White fish “cooked with high-temperature methods (pan-frying, oven-broiling and grilling) until fish was well done” (6).
No Baloney’s advice? No need to toss your poached salmon for dinner tonight! The findings of these studies definitely raise some questions about the relationship between circulating omega-3s and prostate cancer development/progression, but mechanisms remain unknown. Circulating levels of nutrients are affected by numerous factors like gut absorption, smoking, exercise and disease status. Perhaps advanced prostate cancer changes the body’s utilization of omega-3s, thereby increasing blood levels?
Until there are more clear answers, we vote to keep fish on the menu. As for supplements, a recent meta-analysis found no improvements in heart disease risk with supplements (7) so we say go with tried-and-true food first! If you choose to take supplements, use caution – low-dose is always preferable to mega-dosing. Too much of a good thing is still a bad thing! When in doubt, talk to your doctor.
*We also think that given they did not measure dietary intake, it is inappropriate to make these kinds of specific statements inferring risk with food intake. But maybe that’s just us!
- Brasky TM, Darke AK, Song X, 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]
- Brasky TM, Till C, White E, et al. Serum phospholipid fatty acids and prostate cancer risk: results from the Prostate Cancer Prevention Trial. Am J Epidemiol 2011; 173: 1429
- Chua ME, Sio MC, Sorongon MC, Morales ML Jr. The relevance of serum levels of long chain omega-3 polyunsaturated fatty acids and prostate cancer risk: A meta-analysis. Can Urol Assoc J 2013; 7:E333-43.
- Torfadottir JE, Valdimarsdottir UA, Mucci LA, et al. Consumption of fish products across the lifespan and prostate cancer risk. PLoS One 2013; 8:e59799.
- Bosire C, Stampfer MJ, Subar AF, et al. Index-based dietary patterns and the risk of prostate cancer in the NIH-AARP diet and health study. Am J Epidemiol 2013; 177:504-13.
- Joshi AD, John EM, Koo J, Ingles SA, Stern MC. Fish intake, cooking practices, and risk of prostate cancer: results from a multi-ethnic case-control study. Cancer Causes Control 2012; 23:405-20.
- Rizos EC, Ntzani EE, Bika E, Kostapanos MS, Elisaf MS. Association between omega-3 fatty acid supplementation and risk of major cardiovascular disease events: a systematic review and meta-analysis. JAMA 2012; 308:1024-33.