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Sunday, July 21, 2013

Does Fish Oil Cause Prostate Cancer?


A recent study published in the Journal of the National Cancer Institute reported that high blood levels of omega-3 fats were associated with a higher risk of developing prostate cancer (Brasky 2013).

It was a retrospective observational study, not a randomized controlled trial, so it didn't prove that taking fish oil causes prostate cancer.

There were several problems with this study:

  • The data was taken from a past study designed for a completely different purpose. 
  • The researchers did not take into account the recognized risk factors for prostate cancer like age, race, body mass index, PSA blood level, or rectal exam findings.
  • Researchers did not track how much fish and seafood the study participants ate or whether or not they took fish oil supplements. 
  • The conclusions are based on a single blood test that showed negligible differences in blood levels of omega-3 fatty acids between the men who had prostate cancer and those who didn't, only 0.2%. The actual levels of specific omega-3 fatty acids in the blood (like EPA and DHA found only in fish) were not reported at all.
  • The study participants were not all healthy. Some of the men, approximately 1 in 5, already had advanced prostate cancer. Some men had first degree relatives with prostate cancer. Some smoked cigarettes and/or consumed alcohol regularly. Most of the men (80%) were overweight or obese.
  • Blood levels of omega-3 fats are a good measure of recent intake but a bad measure of long term intake. Blood levels fluctuate from day to day, depending on what we eat, so long term intake should be based on the omega-3 index which measures levels of omega-3 fats inside cells.

This study also comes into conflict with several other studies which have found fish oil to be protective against prostate cancer. For example:

  • An Icelandic study that followed 2,268 men for 9 years concluded that "salted or smoked fish may increase risk of advanced prostate cancer, whereas fish oil consumption may be protective against progression of prostate cancer in elderly men. In a setting with very high fish consumption, no association was found between overall fish consumption in early or midlife and prostate cancer risk" (Torfadottir 2013).
  • A 14-year study of 47,866 cancer-free men in the United States found that "EPA and DHA intakes may reduce the risk of total and advanced prostate cancer" (Leitzmann 2004).
  • A 30-year study of 6,272 Swedish men concluded that "men who ate no fish had a two-fold to three-fold higher frequency of prostate cancer than those who ate moderate or high amounts" (Terry 2001).
  • In a meta-analysis of 24 studies, researchers found that eating fish was associated with a 63 percent lower risk of dying from prostate cancer (Szymanski 2010).

So don't change your consumption of fish or fish oil based on this study. Continue eating fish and seafood and taking fish oil if your naturopathic doctor has recommended it.

When you're selecting fish and seafood, choose smaller species that live low on the food chain. Use the Environmental Defense Fund's Seafood Selector to get the most up-to-date information on toxicity alerts and sustainability ratings.

References:

Brasky TM, Darke AK, Song X, Tangen CM, Goodman PJ, et al. Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial. Journal of the National Cancer Institute. 2013 Jul 10. [Epub ahead of print]

Leitzmann MF, Stampfer MJ, Michaud DS, Augustsson K, Colditz GC, et al. 2004. Dietary intake of n-3 and n-6 fatty acids and the risk of prostate cancer. American Journal of Clinical Nutrition 80:204-216.

Szymanski KM, Wheeler DC, and Mucci LA. 2010. Fish consumption and prostate cancer risk: a review and meta-analysis. American Journal of Clinical Nutrition 92(5):1223–1233.

Terry P, Lichtenstein P, Feychting M, Ahlbom A, and Wolk A. 2001. Fatty fish consumption and risk or prostate cancer. Lancet 357(9270):1764-6.

Torfadottir JE, Valdimarsdottir UA, Mucci LA, Kasperzyk JL, et al. 2013. Consumption of Fish Products across the Lifespan and Prostate Cancer Risk. PLoS One 8(4): e59799.

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