Q&A with Dr. Alan Kristal about omega-3 fatty acids and prostate cancer risk
Dr. Alan Kristal and colleagues at Fred Hutchinson Cancer Research Center recently published a second large, prospective study in the Journal of the National Cancer Institute that confirmed previous studies linking high blood concentrations of omega-3 fatty acids to an increased risk of prostate cancer.
Their latest findings indicated that high concentrations of EPA, DPA and DHA – the three anti-inflammatory and metabolically related fatty acids derived from fatty fish and fish-oil supplements – are associated with a 71 percent increased risk of high-grade prostate cancer. The increase in risk for high-grade prostate cancer is important because those tumors are more likely to be fatal. Their study also found a 44 percent increase in the risk of low-grade prostate cancer and an overall 43 percent increase in risk for all prostate cancers. Read the news release here.
The study has generated numerous questions, the most common of which Kristal has addressed below.
Should men stop taking fish oil supplements?
Unless your physician has prescribed them for a specific purpose, I would say yes, men should stop using them. The evidence that taking omega-3 supplements prevents cardiovascular disease or diabetes, or is useful in treating diseases such as arthritis, is poor. According to Dr. Mario Kratz, also a nutrition researcher at Fred Hutch, much of this evidence is based on animal models, and these models do not reflect well what occurs in humans. Against such weak evidence for benefit, we now have reasonably consistent evidence that high intakes of omega-3 fatty acids increases prostate cancer risk.
Should men stop eating fatty fish such as salmon?
That is a more difficult question, but I believe the answer is that men can eat fatty fish, in moderation. It is difficult to know from our research how much fish might be “too much.” I think that two servings per week would be reasonable, but more research would be needed to answer this with any certainty.
What about vegetarians who don’t eat fish? If they shouldn’t consume supplements, how can they get essential fatty acids in their diet?
Some of the fatty acids found in flaxseed, walnuts and oils made from soy contain an omega-3 fatty acid called alpha-linolenic acid. It contains 18 carbons, compared to 20 and 22 for the fatty acids found in fish. Humans can convert alpha-linolenic to longer fatty acids, but it is very inefficient and from a practical perspective not important. Vegetarians do tend to have lower levels of long-chain omega-3 fatty acids, but I know of no evidence that this is harmful. The only vegetarian food source of long-chain omega-3 fatty acids I know of is seaweed; supplements derived from algae also provide such fatty acids.
Did researchers account for pollutants, such as mercury, that could have been inside the fish that study participants consumed?
No, and this is always a possibility. But beyond industrial exposure to cadmium, we have no evidence that pollutants that concentrate in fish affect the risk of prostate cancer.
What is a common-sense approach for men who still want to reap the benefits of omega-3’s but avoid increasing their risk of prostate cancer?
As I noted, the benefits are far from certain. The fatty acids found in fish are required for good health, but in small amounts. A serving or two of fish a week meets my definition of common sense.
Are there any studies that show a correlation between omega-3 levels and women’s cancers?
To my knowledge, this research is inconclusive.
What are the mechanisms behind the association between high levels of fatty acids in the blood and increased risk of prostate cancer?
We have no data on this question and can only speculate. Inflammation, which omega-3 fatty acids tend to decrease, is generally considered something that contributes to cancer, because it stimulates cell growth. However, inflammation is a mechanism the body uses to destroy damaged tissues, which would include a cancer cell. There is probably on optimal balance for inflammation, beyond which too little or too much could be harmful.
Are there other key questions about fatty acids and prostate cancer risk that still need to be answered?
Indeed, the question we need to understand is why. If we understood the mechanism behind the association, we might learn some important facts about prostate cancer biology.
Who funded your research study?
The National Institutes of Health funded this research via the National Cancer Institute and the National Center for Complementary and Alternative Medicine.
Dr. Alan Kristal is associate head of the Cancer Prevention Program and a member of the Public Health Sciences Division at Fred Hutchinson Cancer Research Center.