Prostate cancer and omega 3

By John Pickering 12/07/2013

The media is in a feeding frenzy with reports of a link between Omega 3 and Prostate Cancer.  Here’s a sample:

Link Between Omega-3 Fatty Acids and Increased Prostate Cancer Risk Confirmed (Science Daily)
Omega-3 supplements ‘could raise prostate cancer risk’ (Telegraph)
Omega-3 supplements linked to prostate cancer (Fox)
Omega 3 could increase cancer risk (TV3)

So, what’s the fuss?  The fuss is about a study published online yesterday in the Journal of the National Cancer Institute:

Brasky, T. M., Darke, A. K., Song, X., Tangen, C. M., Goodman, P. J., Thompson, I. M., et al. (2013). Plasma Phospholipid Fatty Acids and Prostate cancer Risk in the SELECT trial. Journal Of The National Cancer Institute, 1–10. doi:10.1093/jnci/djt174/-/DC1

The article is behind a paywall, so I’m not sure how many of the journalists have bothered to read it instead of relying on press releases.  I’ve access to the paper through my university, so here is a synopsis for the lay reader (bearing in mind I am not an expert in either omega 3 or cancer).

The thinking in the general public: Prostate cancer bad, Omega 3 good, therefore Omega 3 may prevent/delay prostate cancer

The thinking of the scientists: Is there a link between phospholipids (including omega 3) and prostate cancer?

The subjects studied:  Participants were enrolled in a trial of Vitamin E supplementation verse Placebo.  They were all male, from the US, Canada or Peurto Rico, aged 50+ if black (the medical literature uses this description), or 55+ if not, had no history of prostate cancer and with a PSA (prostate-specific antigen) test of <4ng/ml at the start of the study.  They were enrolled between July 2001 and May 2004.  While 35,533 men were enrolled in the trial, in this study only 2273 were studied.  These consisted of 834 patients who had prostate cancer diagnosed prior to 1 January 2008 and 1364 “matched” subjects who had no prostate cancer diagnosed in that time.  This is called a case-controlled study.  The “matching” is a statistical process whereby they make sure the two groups being compared (those with and without cancer) have certain demographic features in common on average.  In this case the groups had similar age ranges and similar ethnicities.  The cancer group was further divided into those with low and those with high grade cancers.

The methods:  Blood samples taken when patients were recruited and the total fatty acid content along with 4 types of Omega-3 fatty acids, 2 types of Omega-6 fatty acids, and 3 types of Trans-fatty acids were measured. The mean (average) proportions of each of the types of fatty acids (compared with total fatty acid) were compared between the No cancer and the Prostate Cancer groups.

The results:  Those with cancer had on average a greater proportion of each of  three of the kinds of Omega-3 fatty acids than those without cancer.  The p values were 0.03, <0.001, 0.006 (see here for an explanation of p values).  The p values for the two Omega-6 were higher (therefore more likely to be arrived at by chance) at 0.17 each.  The Trans-Fatt p values were 0.048, 0.08, 0.002. At this point it is very important to remember that not all those with cancer had high proportions of Omega-3 – it was the average that was higher.  An analysis comparing the 25% of subjects with the lowest Omega-3 (combination of the three Omega-3s) values with those with the highest 25% showed that the risk of prostate cancer was between 9 and 88% greater (with 95% confidence that this was not just by chance), ie a Hazard Ratio of 1.43 (95%CI 1.01 to 1.88).  Considering only those with the highest grade of cancer the Hazard Ratio was 1.71 (95%CI 1.0 to 2.94).

The authors performed a multivariable analysis.  That is when they check to see if other factors may be influencing the results.  They say that for Omega-3:

The continuous multi-variable-adjusted hazard ratios predicting total, …prostate cancer risk, [was] 1.16 (95% CI = 0.98 to 1.36),

This means that Omega-3 proportions changed the risk of getting prostate cancer by between a 2% decrease (100*(1-0.98)) and 36% increase (100*(1.346-1)) when other factors (not stated what) are accounted for.  This is what the 95% CI (Confidence interval) suggests.  The 1.16 is merely somewhere near the middle of the change in risk (16% higher).  It is the confidence interval that matters.  When it crosses 1, as it does here, it is not normally considered very important (ie not “statistically significant” as is often said).

The authors then conducted a meta-analysis for the Relative Risk of getting prostate cancer for two types of Omega-3 (DHA and EPA) and Omega-3 total fatty acid.  A meta-analysis is where they gather up all the studies and combine the results together.  In this case there were 7 studies (including the present one) which reported DHA and EPA and 4 which reported totals.  The results were

EPA:  RR = 1.07 (95%CI 0.95 to 1.21)
DHA: RR=1.16 (95%CI 1.03 to 1.31)
Total: RR=1.14 (95% CI 0.99 to 1.32)

Remember it is the 95% CI that is most important.  In this case only DHA creeps above 1 for the 95% CI.  Remember also that RR (Relative Risk) is a comparison of the rates of cancer between those with the level of Omega-3 among the lowest 20% and among the highest 20%.

The Conclusions:  The authors conclude

…these findings contradict the expectation that high consumption of long-chain ω-3 fatty acids and low consumption of ω-6 fatty acids would reduce the risk of prostate cancer.

This sounds reasonable under the assumption that consuming omega-3 (eg in supplements) actually increases the proportion of omega-3 in the blood.  They also state

It is unclear why high levels of long-chain ω-3 PUFA would increase prostate cancer risk,

What the media said:  TV3 borrowing from Sky, had a graphic with the word “Supplements” prominent and they talked of a 71% increased risk of high grade prostate cancer and 43% increased risk overall.  As we’ve seen these numbers are not what is relevant, the confidence intervals are – this adds a lot more uncertainty to the results (but not such good TV).  Also, they ignored the meta-analysis entirely (numbers not so big or interesting). They said nothing about the age range etc.  Finally, and most importantly, the study was not a study of supplements!  We have no idea why some participants had higher Omega-3 than others.  Some may have been because of supplements, some because of fish eating, some simply because of their own body composition and own metabolism.

My conclusion:  The study did not show that supplementation of Omega-3 is risky.  Nor did it show that supplementation is beneficial. It simply was not a study of supplementation. It did show that elevated proportions of Omega-3 fatty acids are possibly associated with increased risk of prostate cancer in men 50+ (black) and 55+ (non-black). Remember, too, that this is talking about relative risk.  The overall prostate cancer risk during the study period was just 2.35%.  If I’ve done my math right, then those in the top 25% of Omega-3 have an absolute risk of 2.77% (95%CI 2.12% to 3.65%).


Tagged: cancer, Fatty acids, Omega-3, Phospholipids, prostate cancer, supplements

0 Responses to “Prostate cancer and omega 3”

  • Thankyou, I’ve been looking for the entire study, thinking that, as usual, the media have gone for drama over substance. It’s an interesting area, certainly worthy of more research, but as you say, the absolute risk increase is not that alarming when stated in real world terms. I will link this up on my blog.