A new look at an existing study
The Selenium and Vitamin E Cancer Prevention Trial (known SELECT), was a randomized, placebo-controlled trial that tested whether selenium and vitamin E, alone or combined, reduced prostate cancer risk (they didn’t). The men in SELECT were followed closely for several years. They provided blood samples, and information on their diet, health habits, and other lifestyle factors for the study.
From this group, researchers identified 834 men who had been diagnosed with prostate cancer and compared them with 1,393 similar men who had not been diagnosed with prostate cancer. The researchers adjusted for factors that may have differed between the men, including education, history of diabetes, family history of prostate cancer, and what group they had been assigned to in the original SELECT study.
Compared with men who had the lowest levels of omega-3 fatty acids in their blood at the start of the SELECT trial, those who had the highest omega-3 levels were 44% more likely to have been diagnosed with low-grade (less aggressive) prostate cancer, 71% more likely to have been diagnosed with high-grade (more aggressive) prostate cancer, and 43% more likely to have been diagnosed with any type of prostate cancer.
Fishing for answers
These findings were widely reported as a signal that omega-3 supplements are harmful; however, as Alan Gaby, MD, Chief Medical Editor for Aisle7 points out, “This is a case-cohort study, not a controlled trial, and it cannot prove cause and effect. The findings are intriguing, but do not prove a causative pathway between long-chain omega-3s and prostate cancer risk.”
Before you drop omega-3 supplements from your self-care plan, consider the following caveats about this new study:
Consider study strength: In addition to being based on case-cohort information, which is a type of study that may provide associations, but not prove causality, SELECT was not originally designed to study connections between omega-3 supplements and prostate cancer risk. This increases the likelihood of reaching erroneous conclusions when the data are analyzed for this connection after the fact. In fact, tracking omega-3 supplement use was not part of the study at all. Further, the study looked backward in time to assess future prostate cancer risk. It is possible that something about the disease process itself caused higher omega-3 blood levels rather than the other way around. Only a controlled trial can prove whether omega-3 supplements increase, decrease, or have no effect on prostate cancer risk.
Consider the big picture: The Mediterranean diet pattern, noted for generous omega-3 fats from a variety of sources, is associated with a significantly decreased prostate cancer risk. Many health experts have noted that if omega-3 fats were associated with increased aggressive prostate cancer risk, you would see more cases of the disease in countries with high fish intake. This is not the case, and notably, men living in many countries with the highest omega-3 intakes, such as Japan, have lower risk of prostate cancer. Though the authors state they are confirming results of their earlier work, other studies have shown no increased prostate cancer risk, and in some cases a decreased risk, with higher intakes of omega-3 fats.
Consider your overall health goals: Among its many science-supported benefits, omega-3 supplements are prescribed to reduce cardiovascular disease risk, and it's notable that the top killer of prostate cancer survivors is heart disease, not prostate cancer. If you take omega-3s to manage heart disease risk, it may not be a good idea to stop, based only on the assertions from this study’s results. Talk to your doctor about what is best for you, based on your medical concerns and issues.
(J Natl Cancer Inst
; July 10 2013 [Epub ahead of print; accessed July 11 2013])