07/26/2013 05:33 pm ET Updated Sep 25, 2013

Can Fish Oil Cause Prostate Cancer?

Whenever a newly-published health study challenges current thinking, you can bet it won't be long before the news media starts ratcheting up the drama and jumping to conclusions. This is true of a recent study called "Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial," published in the July 2013 issue of the Journal of the National Cancer Institute. This study suggests a higher risk of prostate cancer among men who eat omega-3 fatty acids, such as those found in fatty fish like sardines and salmon or in fish oil supplements.

Because I encourage my patients and readers to get plenty of omega-3s, I want to respond to these reports and offer my answer to the question they've raised: Can fish oil cause prostate cancer? But first, let's examine the findings.

What the Study Found

The study, which was conducted at the Fred Hutchinson Cancer Research Center in Seattle, claims a link between increased blood levels of omega-3 fatty acids and increased incidence of prostate cancer. The highest blood plasma levels of these polyunsaturated fatty acids, specifically EPA, DHA and DPA, were associated with the highest risk. The research also showed that higher levels of linoleic acid (or omega-6 fatty acids, which most Americans eat too much of) were actually associated with a lowered risk. This would suggest that the more fish or fish oil a man included in his diet, the greater the chances he would develop prostate cancer. It would also mean that increasing his omega-6 fatty acid intake would be a good idea.

So, have I led you astray by telling you to eat your fatty fish and limit your intake of processed vegetable oils that contain omega-6 fatty acids? Should I warn you against taking fish oil and instead tell you to eat more cottonseed and sunflower seed oils? Let's look at the facts and decide.

A Closer Look at the Study

This study used what is called a retrospective case controlled cohort design. Simply put, to make their conclusions, researchers from the Fred Hutchinson Cancer Research Center used data from a previous study conducted in 2011 called the SELECT (Selenium and Vitamin E Cancer Prevention Trial). It's important to note that the original SELECT study did not have the same objective as this current one. It wasn't designed to determine whether fish oil led to prostate cancer. The fact that both studies didn't have the same goal calls into question whether the old data is even relevant to the new study.

What we can be sure of is that association does not prove cause and effect. If this had been an intervention design study, where half the participants got fish oil and half didn't and they were followed for 20 years to see if they got prostate cancer, then you can say pretty definitively that they are connected. Bottom line, this type of study does not prove cause and effect. If I did a study on sunrise and humans waking up, I would find 100 percent correlation, but that doesn't mean that the sun came up because you woke up. Correlation, yes; causation, no.

Another problem with the study is that the researchers did not address whether the men who were studied got their omega-3 fatty acids from eating fatty fish or from taking supplements. Also, there was no regard for their health status before starting the study. Did they start using fish oil as a therapy once diagnosed with prostate cancer or had they been taking it all along?

And what about the myriad other factors that can lead to the onset and progression of cancer, such as how lifestyle affects genetics? Smoking, nutrition, exercise, environmental toxicity, stress -- none of these things were taken into account. It is too simplistic to reduce a disease as complex as cancer down to one trigger. In fact, perhaps we should be asking if these men were exposed to toxins and heavy metals from eating mercury-containing fish, which can cause cancer. Or did the men smoke or drink to excess? Was there a history of cancer in the family? What was their personal health history prior to diagnosis? Were they overweight or obese, and did they have other symptoms of diabesity?

Another major flaw with this study's design involves the way the researchers got their data. They analyzed blood plasma instead of red blood cells. And they did so with one single blood draw! The conclusions would have been stronger and more reliable had they used red blood cell samples, because those provide a more accurate assessment over the long term (plasma tends to provide only a short-term picture). Because the research was based only on samples of a single blood draw, the red blood cell analysis would have given a better picture of long-term omega-3 intake (a couple months of eating salmon, for example, instead of what happens in the body after a single meal). That's why I suggest people use the omega-3 index test, which measures levels from within the red blood cells.

Consider the Japanese

If it is true that taking fish oil or raising your blood levels of omega-3 phospholipids increases risk for prostate cancer, then why hasn't this been a problem for Japanese men? They certainly eat their fair share of fatty fish and have done so for generations! The Japanese (and other fish-loving cultures) have been studied many times to test this hypothesis, and guess what? Males in Japan, while having some of the highest levels of EPA and DHA, also have some of the lowest rates of prostate cancer. Only in the most recent studies have Japanese men been shown to have an increase in prostate cancer. Could it be that, as the Japanese begin to abandon their traditional diet of fish, seaweed, and other sea vegetables for the typical SAD (standard American diet, high in saturated fat and linoleic fatty acids), their risk of prostate cancer rises?

It seems that for every claim against fish and fish oil, there are several studies that confirm their benefits. One study, "Consumption of Fish Products Across the Lifespan and Prostate Cancer Risk," showed that high blood plasma phospholipids was protective against prostate cancer when fish oil was consumed. Another study showed that omega-3 fatty acids protect against death caused by prostate cancer. And what about the effect of fish oils on the outcome of prostate cancer in men with elevated PSA levels? Again, the literature shows that EPA and DHA have no negative effect.

Personalized Medicine

It's important to stop and remember that each person has a unique inner ecology and external environment. Contributing factors, such as exposure to environmental toxicity, poor nutrition, and other lifestyle variables, as well as genetics, all play a role in the development of cancer. It's a complicated disease, and it would be a good idea to pause and look at the whole picture before drawing any major conclusions.

The simple fact is that countless studies have proven the health benefits of eating a diet rich in antioxidants and fiber from fruits and vegetables. And just as we all know that eating your veggies is good for your health, we are now beginning to prove similar health benefits from including healthy fats in your diet. (For more information on how to increase your intake of healthy fats, please see my discussion here). We also know that limiting omega-6 fatty acids and increasing omega-3 fatty acids has been shown to reduce the risk of diabesity in Western cultures.

So, before we toss good medicine aside, we need to examine carefully the factors that contribute to imbalances in the body. We need to assess what we do know and keep asking questions about what we don't.

We know that a whole foods-based diet, rich in fresh, nutrient-dense fruits and vegetables, healthy fats, and lean protein, does make a positive difference in health outcomes. We know that high-quality, purified fish oils are best. We know that a balanced and varied diet is key for maintaining good health. And we know that moderation is the key to a healthy and sensible relationship to food. Any diet or program promoting an extreme is not realistic, sustainable, or even remotely healthy. Remember, the "dose makes the poison," so just the right amount -- and not too much -- will allow you to reap the intended benefits. In the case of fish oil, 1-2 grams daily is appropriate for most people, though some of you may need more. I strongly suggest you work with a trained functional medicine practitioner to help you determine the appropriate doses you need, not only for fish oil but for all supplements. My nutrition coaches are here to help you transform general guidelines into personalized solutions.

So, where do I stand on whether fish oil causes prostate cancer? I'll be eating sardines in my salad for lunch tomorrow, and I'll be taking my daily fish oil supplement with my dinner tonight. And I hope you will be too!

Now, I'd like to hear from you...

Have you been swayed by recent reports to feel that omega-3s can cause prostate cancer?

Will you limit the amount of omega-6 fatty acids you consume?

What are some of your favorite ways to include fatty fish in your diet?

To your good health!

Mark Hyman, MD

Mark Hyman, MD is a practicing physician, founder of The UltraWellness Center, a six-time New York Times bestselling author, and an international leader in the field of Functional Medicine. You can follow him on Twitter, connect with him on LinkedIn, watch his videos on YouTube, become a fan on Facebook, and subscribe to his newsletter.

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