Examine.com makes some very good points on this:
The whole page is worth reading, but here is the bottom line:
What Should I Know?
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.
It was good being the party of Robin Hood. Until they morphed into the Sheriff of Nottingham