Recently, a study was published and taken by the media to show that “fish oil causes a 71% increased risk for aggressive prostate cancer.” This seemed to have created a minor panic (as prostates are useful) and fish oil swung wildly from “definitely take” to “do not touch!” Was this warranted?
As a preface to the study at hand, there was a trial a few years ago called SELECT (Selenium and Vitamin E Cancer Prevention Trial), which was investigating the incidence of prostate cancer and its relation to vitamin E and selenium supplementation. While the initial trial found no protective effect of either supplement, a followup with the participants in that study found an increased risk for prostate cancer associated with Vitamin E supplementation.
The reason the authors investigated the SELECT trial again for a possible association with prostate cancer is because they did something similar with another trial similar to SELECT, and found an increase in rates of high-grade prostate cancer in men already diagnosed (with prostate cancer). Thus the authors decided to replicate the methodology used there with SELECT to give us our current trial.
The current study investigated persons diagnosed with cancer who also participated in SELECT (and then took a group of people also in SELECT who did not have cancer but were matched for other demographics) and measured plasma levels of phospholipids and fatty acids to see if there was an association. There was indeed an association with high-grade prostate cancer, and when comparing the quartile (25% of the participants) with the highest plasma omega-3 levels against the lowest, the highest quartile had a 71% increase in risk of highly aggressive prostate cancers.
A few notables on this study, its methodology, and conclusions are:
This is not an intervention study with a placebo group and drug group. It is a case-cohort study (a one time survey and data crunching of the answers to find correlations). While the method of analysis and only taking participants with baseline values suggests that they did establish a time-dependent relationship, there was still no active fish oil arm of the study and thus this study would be best not used to argue for or against fish oil supplementation; just for plasma fish oil concentrations (something else!)
It’s analyzing omega 3s in bloodstream, not fish oil itself.
- While this study and a past one noted a correlation between fish oil and an increased risk for high-grade prostate cancer in persons already having prostate cancer, studies looking at overall risk (“will I get prostate cancer?”) find no relationships and studies looking at mortality (“will I die from my prostate cancer?”) actually found a decrease in deaths.
Previous studies that explicitly studied fish oil and prostate cancer found that fish oil had no influence in getting prostate cancer, and another found that it decreased the chance of death from prostate cancer.
The 71% increase was the median value; the range of possible values with a 5% margin of error (otherwise known as the 95% confidence interval) was 0-192%. The huge variance makes it hard to take the data at face value, and future studies are needed to see whether the rate is closer to 0% or closer to 192% (completely different issues if either are proven to be more likely)
That 71% number is actually a median. The range of values is huge and better studies need to be performed.
Overall, a few preliminary conclusions can be drawn from this study that should be the take-away messages:
Due to past literature finding no influence of fish oil in causing prostate cancer in people without tumors, and a reduction in mortality associated with fish oil supplementation, the overall clinical relevance of this new study is not clear. There doesn’t appear to be a need for persons without prostate cancer to omit supplementation right now, but in those with prostate cancer supplementation seems to currently be associated with increased disease progression yet less mortality (somewhat contradictory).
Even if we assume that this study is the best ever, it does not directly apply to oral supplementation of fish oil and was not an intervention. It also only investigated those already with prostate cancer, so saying fish oil is a cancer augmenting agent rather than a cancer causing agent may be a bit more valid (although even that claim needs more evidence).
Using this study to state that fish oil causes prostate cancer is unethical. It poses interesting research questions, but it’s mostly relevant to spurring on new research, not as an indictment on fish oil itself.