DHA and Prostate Cancer: More Illumination, or More Smoke?
Sometimes nutrition research helps us to make good food choices. Other times, it muddies the waters, creating confusion. Today in the Journal of the National Cancer Institute, Brasky and co-workers have published another nested case-control study within a larger intervention trial that has linked plasma DHA with prostate cancer. This more recent study uses data from the SELECT trial of selenium and vitamin E in men aged over 50 who had no history of prostate cancer and non-elevated prostate-specific antigen (PSA) levels. 1674 cases were identified during the study, and these were matched by age and race to 1364 controls.
In case-control studies, it is important to identify confounders at the start of the study. Cases were more educated, had a higher PSA score, had more relatives with prostate cancer. Diabetes incidence was lower in low-grade cases. This indicates that there are potential confounding factors associated with the case and control groups that should be accounted for in the estimation of the relationship between fatty acids and prostate cancer risk.
Hazard rations were calculated based on an adjusted model that included age, race, education, diabetes, family history, and SELECT trial arm assignment. PSA levels were not included in the model. For this study, like the previous one published in 2011, the authors found that there was an increase in prostate cancer risk with increasing levels of DHA, however the increase was found in the low-grade cancer group for this study rather than the high-grade cancer group in the 2011 publication. Related long chain omega-3 fatty acids EPA and DPA were also associated with increased risk of prostate cancer in the current study. In addition, and different from previous work, trans fatty acids were not protective against prostate cancer. If consumers started buying hydrogenated vegetable oils after the 2011 publication, they will have to throw them out again!
The authors also conducted a small meta-analysis, the methodology of which was not reported, to compare their results with other published research. The meta-analysis showed that the omega-3 fatty acids were also associated with prostate cancer risk, but these results contradict another systematic meta-analysis by Szymanski, Wheeler and Mucci, which included a greater number of studies and found that omega-3 fatty acids were protective, including a 63% reduction in prostate cancer mortality with high intakes of fish.
These results differ from other published research that found omega-3 fatty acids especially DHA to be protective (Mamalakis, Yang) or have no effect on prostate cancer (Christensen). On the other hand, other researchers have found that DHA is linked to greater risk of high-grade prostate cancer (Sorongon-Legaspi). The authors do not suggest a potential mechanism by which omega-3 fatty acids could promote prostate tumors.
Where does this lead? Are these results enlightening, or will they cause the average person to question the value of nutrition advice as it seems to keep changing? Considering the benefits of including sources of DHA in the diet (Lorente-Cebrián), we can hardly suggest a move back to low intakes of omega-3 rich seafood based on limited epidemiological research. It is premature to suggest changing recommendations without better quality evidence.
Theodore M. Brasky, Amy K. Darke, Xiaoling Song, Catherine M. Tangen, Phyllis J. Goodman, Ian M. Thompson, Frank L. Meyskens, Jr, Gary E. Goodman, Lori M. Minasian, Howard L. Parnes, Eric A. Klein, and Alan R. Kristal. Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial. JNCI J Natl Cancer Inst first published online July 10, 2013 http://dx.doi.org/10.1093/jnci/djt174