Fish Oil and Prostate Cancer – The Whole Story

It seems the misinformation about fish oil supplementation has reached a new (low) level. You may have read the recent media headlines purporting that fish oil supplements increase the risk of prostate cancer. Once again, they have it wrong, and this time, terribly so. Let’s take a look at the whole story.

The study in reference1 was a case-controlled study nested within another study—SELECT (the Selenium and Vitamin E Cancer Prevention Trial).2 The SELECT trial tested whether selenium and vitamin E, either alone or combined, reduced prostate cancer risk. You might notice this trial had nothing to do with omega-3 intake. The researchers chose this group of people because they were already being evaluated for prostate cancer risk, but a main flaw of this study is that it was not designed to evaluate the relationship between omega-3 fatty acid levels and prostate cancer. Thus, the results are less reliable.

The researchers evaluated 834 men diagnosed with prostate cancer and 1393 randomly selected matches from the SELECT trial. They analyzed baseline plasma fatty acid levels of the participants and found a 71 percent increased risk of high-grade prostate cancer and a 43 percent increase in risk for all prostate cancers in those participants with the highest concentrations of plasma EPA, DHA, and DPA.

Plasma fatty acid levels, however, only reflect the past 24 to 48 hours of dietary intake of fatty acids. The study did not assess dietary intake, via food or supplements, of fatty acids, however. The difference in omega-3 fatty acid levels between those who developed prostate cancer and those who did not was 0.2%. According to Duffy McKay, vice president of scientific affairs at the Council for Responsible Nutrition, “This change literally could have occurred if somebody ate a fish sandwich on their way to get their blood drawn.”

Without monitoring dietary and supplemental omega-3 intake, we don’t know how much fish or fish oil these patients were really taking—if they were taking any at all. Another possibility, the patients with cancer may have begun taking fish oil after they were diagnosed because they were trying to get healthy. The researchers simply did not control for these possibilities.

Even more importantly, without measuring red blood cell (RBC) fatty acid levels of omega-3s, which most accurately reflect long-term intake of omega-3s as well as incorporation of the omega-3s into tissues, we do not have an accurate picture of what is really happening. This is a common and fundamentally detrimental mistake when studying the health effects of omega-3 fatty acids, as I often mention.

An essential and metabolic fats RBC membrane test is available from several companies including HDL Labs, Genova/Metametrix, and Doctor’s Data. These are very thorough tests which are only obtained through a practitioner’s office and usually are covered by insurance. The simplest, least costly way to know your total body fatty acid balance is to obtain a do-it-yourself, at-home, finger-stick blood test from OmegaQuant.com or LabTestingDirect.com.

These tests give a good approximation of the levels of total omega-3 and omega-6, as well as a comprehensive list of individual fatty acids over about a three-month period. (Red blood cells live for three to four months.) The most significant test may well be the Omega-3 Index which measures the EPA + DHA levels compared to the total fatty acid levels in the red blood cell membrane. An Omega-3 Index under 4 percent indicates the person would need to take 1 to 3 grams of EPA + DHA daily. If the Omega-3 Index is over 8 to 10 percent, the person’s omega-3 intake is probably fine unless a significant inflammatory condition is present, in which case the Omega-3 Index could go higher under the care of a practitioner.

Those who choose to do research in the area of essential omega-3 and omega-6 fatty acids would be wise to measure something more meaningful than a random single omega-3 blood plasma test. It is important to remember that this study found an association—a link. Yet an association does not prove causation. The way to strengthen association studies is to do a good job controlling for confounding factors. Unfortunately, these researchers failed to correctly control for such factors, as noted in a statement by Robert Roundtree, MD:

  • 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.

The researchers even note in their introduction that obesity is associated with increased inflammation and higher risks of high-grade prostate cancer and prostate cancer death. Why did they not control for this factor? It’s a good question.

Further evidence to the weakness of the suggestions in this study lies in the many studies that were actually designed to find a link between omega-3 intake and prostate cancer. In one major review and meta-analysis, although they were unable to find an association between fish consumption and reduced prostate cancer, they did find a 63 percent reduction in prostate cancer-specific death in those with the greatest consumption of fish.3 In one of the best-designed studies of this meta-analysis, as noted by Dr. Michael Murray, ND, high levels of EPA + DHA (as measured in red blood cell membranes) were associated with a 40 percent reduced risk of prostate cancer.4

I spoke with William Harris, PhD, a prominent omega-3 researcher, today and he has kindly offered a link to his blog. He has added several other interesting points on why the connection between fish oil and prostate cancer is inaccurate. He states, “After reading these alarmist headlines without adequate clinical context or medical oversight, patients might take healthcare decisions into their own hands. A patient at increased risk for heart disease may decide to cut fish from his diet, or those with elevated triglycerides may stop taking a prescription omega-3, all in order to ’reduce’ his risk for prostate cancer. But at what cost?”

GOED (the Global Organization for EPA and DHA Omega-3s) sums it up nicely by saying that if the findings of this study were true, “then prostate cancer would be rampant in any country with high seafood consumption (Scandinavia, Japan, etc.) and conversely, low level consumption should be protective. Clearly this is not the case.” The lesson, as always, is that we must be sure to know the whole story before drawing the wrong conclusions.

 

References

  1. Brasky TM, Darke AK, Song X, et al., “Plasma phospholipid fatty acids and prostate cancer risk in the SELECT trial.” J Natl Cancer Inst. 2013 Jul 10. [Epub ahead of print]
  2. Brasky TM, Till C, White E, et al., “Serum phospholipid fatty acids and prostate cancer risk: results from the prostate cancer prevention trial.” Am J Epidemiol. 2011 Jun 15;173(12):1429-39.
  3. Szymanski KM, Wheeler DC, Mucci LA, “Fish consumption and prostate cancer risk: a review and meta-analysis.” Am J Clin Nutr. 2010 Nov;92(5):1223-33.
  4. Norrish AE, Skeaff CM, Arribas GL, et al., “Prostate cancer risk and consumption of fish oils: a dietary biomarker-based case-control study.” Br J Cancer. 1999 Dec;81(7):1238-42.
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