Flawed Study Links Fish oil to Prostate Cancer

by Mary Perry, RD

For the past week, the media and headlines have been proclaiming:  “Study Links Fish Oil to Prostate Cancer.”   This is based on a new study, Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial, published online in the Journal of the National Cancer Institute.  Not so fast.

The report published July 10 is based on observational studies and not randomized controlled studies.  Observational studies simply look back in time at data.  Researchers used data from the Selenium and Vitamin E Cancer Prevention Trial to make their conclusions.  These assertions were not based on the gold standard of randomized controlled studies in which one group gets the treatment and the other group gets a placebo.  Instead, inferences were based on a study whose purpose was not even to address the question related to the relationship between omega-3 fatty acid levels and prostate cancer.

The researchers compared blood levels of omega-3 fatty acids in more than 800 men later diagnosed with prostate cancer with blood samples from nearly 1,400 men who did not develop the disease.  This study looked at blood levels of long-chain fatty acids such as those found in fish (EPA and DHA).  The association between higher blood levels and prostate cancer was only found for DHA. No association was found between EPA or ALA (the plant-based form of omega-3 found in flaxseeds).

It’s important to keep in mind that blood levels of omega-3 fatty acids are not a reliable indicator of long-term omega-3 consumption since it is subject to significant day-to-day variability.  These levels can be influenced by a single meal or timing of a supplement.  In addition, only one blood sample was taken at the start of the study and not when participants were diagnosed.

These new findings don’t show a cause-and-effect relationship between prostate cancer and omega-3 fatty acids. Instead, these studies point to associations.  It is important to remember that correlation is not causation. 

Researchers did not “control” for such variables as family history, age and race and most importantly diet.  A number of other factors might have come into play:

  • 53 percent of the subjects with prostate cancer were smokers.
  • 64 percent of the cancer subjects regularly consumed alcohol.
    • 30 percent of the cancer subjects had at least one first-degree relative with prostate cancer.
    • 80 percent of the cancer subjects were overweight or obese.

We need to keep in mind that there have been numerous positive studies published on omega-3 fats showing their health benefits.  It’s also important to remember that nutrition is a highly complex and nuanced area. 

As a dietitian I recommend focusing on food first and continuing to eat good food sources of omega-3 fatty acids from fatty, oily fish (such as wild-caught salmon or sardines), walnuts, leafy green vegetables (such as collard greens), grass-fed beef, and ground flaxseed.  As Hippocrates said: “Let food be thy medicine and medicine be thy food.”  Good advice.

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