A new study has been making its rounds online claiming that fish oil causes prostate cancer, and more specifically the claim is that fish oil supplementation causes a 71% increase in high grade prostate cancer.
The study in question is one that appears to be a study based off of the SELECT trials (a large trial initially investigating the link between vitamin E and selenium with prostate cancer) which initially did not find a protective effect of supplementation on prostate cancer, but say an increased risk associated with vitamin E occurred during prolonged follow-up. This led to the current study.
A new study claims that fish oil supplementation causes prostate cancer, and was an observational study that used participants from a previous large scale intervention called SELECT
This will get science-heavy. If it's too heavy for you, just skip to the next section.
In short, this study initially looked at participants of the SELECT trial and got a sample of persons who were diagnosed with prostate cancer (n=834) and made note of how many had advanced cancer (n=156), then 1393 persons from the SELECT study who did not have prostate cancer were selected for comparison. The researchers then measured serum omega-3 fatty acids (EPA, DHA, and their intermediate DPA) and stratified the groups into quartiles to see if there was an association.
The results showed that persons who had prostate cancer were more likely to have higher circulating omega-3 fatty acid levels (excluding ALA, which was not associated) and that omega-6 was unrelated to prostate cancer. Trans fatty acids were mostly unrelated aside from a possible positive relationship with palmitelaidic acid (16:1). When comparing the quartiles against one another (the lowest quartile being set at 1.00 as a reference), the highest levels of fish based omega-3 fatty acids (collectively) was associated with increased risk as assessed by Hazard Ratio for low (1.07-1.40), medium (1.07-1.43), and high (1.00-1.54) grade prostate cancer. While DHA had an HR showing an association with low (1.07-1.37), medium (1.06-1.38), and high (1.03-1.54) grade prostate cancer, DPA was only associated with low (1.03-1.46) and medium (1.08-1.57) while EPA was not significantly associated with an increased risk. Alpha-linolenic acid, omega-6 fatty acids, and trans fatty acids were not related.
Note: The above ranges are known as the 95% confidence interval, and show the range of values that the researchers are 95% confident that the true value lies in. So for a range of 1.03-1.54 this means a possible 3-54% increase (with a 5% margin for error), and if the interval crosses 1.00 (the zero point where lower means reduced risk) then the observation is not considered significant
When adjusting for the omega-3 to omega-6 ratio, it seems that the HRs for low (0.98-1.36) and medium (0.97-1.36) grade prostate cancer became nonsignificant while there was still a significant relation with the high (1.40; 1.03-1.92) grade. The average value here, 1.40, may be where media sources are claiming a 40% increase in prostate cancer risk. The '71%' referenced in most publications was a direct comparison of the risk in the highest quartile against the lowest (with a confidence interval of 0-192%) with a 43% (9-88%) overall increase in risk.
Important Note: The actual association for people with high-grade prostate was in the range of 3% to 92% - this gets averaged to 40%. When comparing the highest quartile (25% of sample) to the lowest, the average risk was increased to 71% but became more variable at 0-192%.
The variables that were made note of in the analysis were education, history of diabetes, family history of prostate cancer, and SELECT intervention assignment (so, placebo or vitamin E groups). So despite the increased risk seen with vitamin E previously in SELECT it likely does not influence the results.
This study found that, when comparing the lowest 25% of subjects (assessed by how much fish oil was in their blood) against the highest 25% that the higher group had a higher frequency of prostate cancer at the time of measurement. They conducted a one-time measurement of blood lipids, and there was no supplemental intervention
Stating "fish oil causes cancer" due to this study would be a mistake, as it is a case-cohort study (conducted at one time point only), and a temporal relationship is not made. While unlikely, with the data available, it could also be possible to state "prostate cancer causes a higher n3 concentration in the blood."
The temporal aspect is important, since fish oil supplementation can drastically change serum levels of omega-3s in the blood. It is quite common for people diagnosed with prostate cancer to supplement with fish oil, as it is commonly touted to be cancer-protective (which would mean that prostate cancer precedes fish oil supplementation). A previous study using persons from SELECT using a design that could assess this temporal relationship found no relation (either protective or harmful) with prostate cancer incidence.
Furthermore, this study did not measure mortality. When looking at mortality, fish oil seems to be associated with reduced mortality. In simpler words, it was found to not help prevent prostate cancer, but reduced your chances of dying from it.
Also of interest is the large ranges observed (as in, the 71% value had an actual range of somewhere between 0% and 192% with a 5% margin of error), which either suggests other factors are at play influencing the results or large differences in how one’s body responds to omega-3 ingestion.
At the most, we can state that prostate cancer is associated with higher omega-3 ratios in your blood. This study poses a chicken-egg problem - which causes which?
This study and no other studies in existence can causatively state that fish oil causes prostate cancer. If anything, this study begets a plethora of questions in regards to the relationship between prostate cancer and omega-3 but proves nothing.