Talk about confusing. We recently reported on perhaps the best evidence yet for the health benefits of fish and the omega-3 fatty acids they contain. That well-designed study found that people over 65 who had the highest blood levels of omega-3s were 27 percent less likely to die during a 16-year period than those with the lowest blood levels.
Then, a headline-making study in the Journal of the National Cancer Institute (JNCI) linked those same moderately high blood levels of omega-3s to a 44 percent elevated risk of low-grade prostate cancer and 71 percent increased risk of high-grade cancer in men over 50.
How can these findings be reconciled? Here are my thoughts. First off, it’s hard to compare the two studies, since one looked at death rates, the other at prostate cancer incidence. The decreased mortality rate associated with higher levels of omega-3 fats in the first study was almost completely due to fewer deaths from cardiovascular disease, which is consistent with many previous studies. There was no statistically significant impact on cancer mortality, and in any case, prostate cancer was not separated out.
In contrast, the JNCI study did not look at death rates. But it’s fairly safe to assume that more high grade prostate cancers would mean more deaths.
I’ve been reading the studies on the effect of fish or marine omega-3s on prostate cancer (and on cancer in general) and have seldom seen such contradictory findings. It has been hoped that omega-3s would be protective, largely because of their anti-inflammatory potential (inflammation plays a role in many cancers). And, indeed, most observational studies have linked fish intake to reduced risk of prostate cancer and/or improved survival rates.
But some have found no effect. And a few studies have suggested increased risk, including an earlier one by the authors of the new JNCI study, who acknowledged that their initial findings were “counter to expectations.” Even among the studies showing increased cancer risk, there are conflicting data—notably regarding which specific fatty acids were involved and the stage of prostate cancer they were associated with.
I’m not convinced by the new cancer study, though it raises some important concerns that merit further research. For one thing, it used data from an older study not designed to evaluate the effect of omega-3s on prostate cancer, which can complicate the interpretation of the results.
More important, there are too many inconsistencies in the totality of research, and no apparent explanation for an increased risk. Plus, if fish increased the risk of prostate cancer, then why do Japanese men, who eat a lot of fish, have a low rate? Why is it very rare among Inuit in Greenland and Canada, who live largely on fish? More important, even if the cancer risk were confirmed, it’s very likely that for most men the cardiovascular benefits would outweigh this.