DHA Supplements and Prostate Cancer
A study was released this month that correlated a higher percentage of omega-3 fatty acids in the blood, especially DPA and DHA, with an increased risk for prostate cancer. This made big news and the merits of the study have been debated and people have been asking me if it would make me change my recommendations for taking DHA.
The study by Braskey et al (1) was funded by the National Cancer Institute and was a case-control analysis. It was also an offshoot of the Selenium and Vitamin E Cancer Prevention Trial (SELECT). There were numerous statistically significant findings with the strongest being for DHA. A DHA percentage of >3.62 was associated with a 39% increased risk of prostate cancer when compared to a DHA percentage of < 2.33 (1.39, 1.06-1.82). The finding was stronger for low-grade prostate cancer while insignificant for high-grade.
Interestingly, ALA levels were not associated with an increased risk of prostate cancer and the omega-6 fatty acid, linoleic acid (LA), was slightly associated with a reduced risk.
Additionally, the researchers performed a meta-analysis of 7 similar studies and again found that DHA was associated with an increased risk of prostate cancer (1.16, 1.03-1.31). In the 5 studies that separated low-grade versus high-grade, both low (1.20, 1.04-1.38) and high (1.48, 1.10-1.99) were associated with an increased risk.
As I see it, the strengths of the study are:
– Percentage of fatty acids in the blood is possibly a better indicator of actual intake (or absorption) than a food frequency questionnaire.
– The follow-up was about 8 years, which seems long enough to eliminate the possibility of reverse causation in which DHA levels might be elevated due to the prostate cancer.
The potential weaknesses of the study are:
– Measuring fatty acids as a percentage rather than an absolute amount could be misleading. One person might have a higher percentage of DHA, but a lower total amount of DHA. For example, if you eat a very low-fat diet but with a bit of fish, then your DHA will be high as a percentage, but could possibly be lower, in total, than someone who eats a large amount of fat including fish. In other words, it could be that people with a higher percentage of DHA actually have a lower absolute amount of DHA. I don’t know how likely this is, but it doesn’t seem to be out of the question if many fish-eaters are on low-fat diets. Given some of the findings from other studies I’m about to discuss, this might be plausible.
– The whole project of associating the percentage of various fatty acids with various cancers has been fraught with null, inconsistent, and weak findings. When this is the case, it’s a good indication, in my opinion, that there is nothing really there.
This study was criticized by Dr. Michael Murray in his July 22, 2013 article, Does Fish Oil Really Boost Prostate Cancer? One interesting point he makes is that the omega-3 levels in those with prostate cancer were not very high. Those without cancer had an average DHA level of 2.91% (95% CI: 2.86-2.96) while those with cancer had a level of 3.01% (95% CI: 2.95-3.08). People taking fish oil supplements tend to have amounts quite a bit higher than that.
I will go a step further than Dr. Murray and suggest that the differences between the two groups, while statistically significant, are so minor as to be practically meaningless: a DHA level of 2.91% is pretty much the same as 3.01%. Even when you add EPA + DPA + DHA together, the difference is hardly anything (4.48% for the no cancer group vs. 4.66% for the cancer group).
What about studies measuring intake?
A 2006 meta-analysis of 7 cohort studies found no significant association with the incidence of prostate cancer and intake of EPA or DHA (2). A 2010 meta-analysis of fish consumption and prostate cancer, which included most of the studies from the 2006 meta-analysis plus many more, found no association between fish consumption and prostate cancer incidence, and a reduced risk for prostate consumption mortality (3).
Speaking of fish, there is a question as to whether the association between omega-3 percentage and prostate cancer found in Braskey et al could be explained by mercury contamination of fish. While I found a study indicating that mercury levels in humans are correlated with fish consumption (4), I could not find any examining an association between mercury and prostate cancer. I did find two sources which indicate that if mercury causes prostate cancer, no one seems to know about it yet (5, 6).
So where does this leave us? I have a friend who pointed out that the mere idea that DHA could cause prostate cancer is ridiculous given what an important component of brain tissue DHA is. That’s an interesting perspective, though something good for the brain is not necessarily good for the prostate. I do think his point should increase the burden of proof needed before concluding that DHA causes prostate cancer.
One final point is that vegans who do not supplement with DHA tend to have DHA levels around 1%. As I reported recently, we have an anecdotal report of a 60-year old vegan man who raised his DHA levels from 1.0% to 4.8% using 320 mg of DHA plus 130 mg of EPA per day. My recommendations are for vegetarians 60 and under to supplement with 200-300 mg every other day and for vegans over 60 to supplement with that much every day. If you are someone who is at high risk for prostate cancer, I can see supplementing on the lower side of these recommendations (though I cannot tell any given individual what amount they personally need) to keep your levels below the 3.6% cutoff in Braskey et al.
In summary, I’m skeptical that DHA causes prostate cancer, but if you are at high risk, it might be prudent to stick to the lower end of the daily recommendations until more is known.
1. Brasky TM, Darke AK, Song X, Tangen CM, Goodman PJ, Thompson IM, Meyskens FL Jr, Goodman GE, Minasian LM, Parnes HL, Klein EA, Kristal AR. Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial. J Natl Cancer Inst. 2013 Jul 10. [Epub ahead of print] | link
2. MacLean CH, Newberry SJ, Mojica WA, Khanna P, Issa AM, Suttorp MJ, Lim YW, Traina SB, Hilton L, Garland R, Morton SC. Effects of omega-3 fatty acids on cancer risk: a systematic review. JAMA. 2006 Jan 25;295(4):403-15. Review. Erratum in: JAMA. 2006 Apr 26;295(16):1900. | link
3. Szymanski KM, Wheeler DC, Mucci LA. Fish consumption and prostate cancer risk: a review and meta-analysis. Am J Clin Nutr. 2010 Nov;92(5):1223-33. doi: 10.3945/ajcn.2010.29530. Epub 2010 Sep 15. | link
4. Yoshizawa K, Rimm EB, Morris JS, Spate VL, Hsieh CC, Spiegelman D, Stampfer MJ, Willett WC. Mercury and the risk of coronary heart disease in men. N Engl J Med. 2002 Nov 28;347(22):1755-60. (I only read the abstract.) | link
5. Landrigan PJ. Occupational and community exposures to toxic metals: lead, cadmium, mercury and arsenic. West J Med. 1982 Dec;137(6):531-9. | link
6. Järup L. Hazards of heavy metal contamination. Br Med Bull. 2003;68:167-82. | link