Fish Oil's Link to Prostate Cancer Unproven

July 26, 2013

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Hello. I'm Dr. Gerald Chodak for Medscape. This week I want to talk about a controversial study that suggested that the intake of omega-3 fatty acids increased a man's risk of developing prostate cancer. Brasky and coworkers[1] published a study in the Journal of the National Cancer Institute in which they measured levels of omega-3 and omega-6 fatty acids in a group of men who had participated in the SELECT[2] trial. SELECT was a study testing whether vitamin E alone or in combination with selenium could prevent prostate cancer. Patients who were diagnosed with prostate cancer were selected from that study, and the control group consisted of men from that study who were not diagnosed with prostate cancer. The authors found that increasing quartiles of omega-3 fatty acids resulted in an increasing risk for diagnosis of prostate cancer, and high-grade disease in particular.

The question is whether the conclusions and the study itself are valid. I think there are a number of concerns about the study design that raise serious questions. First, this was not a prospectively designed trial in which some men were randomized to receive omega-3 fatty acids at different intakes to determine whether the omega-3 fatty acids did result in an increased risk for disease. Making firm conclusions from the study design is impossible. More importantly, there are some questions about the design of the study that raise serious questions about the validity of the data.

The men in the control group were not diagnosed with prostate cancer. However, over 400 sites participated in the SELECT trial, and it wasn't mandated how often patients had a prostate-specific antigen (PSA) test, how that PSA test was managed, whether they had a prostate biopsy, or how the prostate biopsy was done. Furthermore, if a patient had a negative biopsy, there was no standardized procedure for determining whether a second biopsy should be performed. The bottom line is that the control group is not reliable or valid. We could have had a study in which all men in the control group had a biopsy to make sure that they didn't have cancer, and it's likely that the inclusion of this control group would have resulted in a bias in terms of how men were assigned to the different categories or groups that they were in.

Another design flaw is that they assigned Gleason 3+4 tumors into the low-risk category and Gleason 4+3 tumors into the high-risk category, when the standard approach is to have Gleason 7 assigned to an intermediate-risk group of patients. Why they did that is unclear, although the senior author suggested that they may have needed more events or more cancers in order to assess the data. Nevertheless, trying to make that claim with Gleason 4+3 in a high-risk group is not valid. Another problem is that this is a single point in time measurement. Blood samples were obtained at the beginning of the SELECT trial when patients were entered, and that is when the measurements were made. There was no standardized method for how men achieved their level of omega-3 fatty acids. Was it diet or was it supplements? Which supplements were used and what dosages were used? All of these problems contribute to an uncertainty and potential bias in the results.

The bottom line is that we cannot determine from this study design whether the intake of omega-3 fatty acids will cause prostate cancer and raise a man's risk for high-grade disease. The media has taken this and sensationalized the risk associated with omega-3 fatty acid intake, but I believe that the attention is overplayed and the concerns about the study design were not mentioned at all. At the end of the day, this study does not prove that intake of omega-3 fatty acids causes prostate cancer or increases a man's risk for high-grade disease. We would need better-designed trials that are prospective and randomized to be able to make such a claim. Until that is done, we will have to weigh the pros and cons of taking omega-3 fatty acids in terms of its other potential health benefits to decide what to do. Whether it causes prostate cancer is not determined by the results of this trial. I look forward to your comments.


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