Why You Shouldn't Worry Too Much About Fish Oil

Written by Dr. Mitchell Gaynor

When poorly designed studies with dubious statistical analyses combine with journalists who are often ill-prepared to appropriately interpret complex scientific results, a flurry of sensationalized news headlines ensue, like, “Is fish oil safe during chemotherapy?” or “Can fish oil supplement hinder cancer treatment?” A recent study in JAMA Oncology triggered the latest surge of headlines when the authors reported that fish oil disrupts the effects of chemotherapy. But does this mean fish oil is bad for you, that you should stop taking it in spite of the strong support for its many health benefits?

Let’s take a look at the study in question: In the preclinical portion of the study, researchers administered fish oil supplements to mice with tumors that were being treated with chemotherapy and found that the supplement reduced the efficacy of chemotherapeutic drugs by at least 50%.

During the clinical portion of the study, 400 cancer patients were asked to complete a survey regarding their fish oil supplement use; only 118 participants (or less than 30%) actually completed the questionnaire. Although the study enrolled and evaluated healthy volunteers in the study, the researchers were unable to assess the effects of fish oil in cancer patients.

In both mice and healthy human volunteers, the consumption of fish oil led to elevated blood levels of a fat called 16:10(n-3) several hours later. The study’s findings purported to show that fish oil consumption negates the effect of chemotherapy in mice due to the presence of 16:10(n-3) in some fish oils.

Everything appears pretty straightforward, right? Well, no. Actually, once you dig a little deeper into the study design, you’ll see that it’s flawed in many ways:

1. Animal study findings don’t translate into effects that can be seen in humans.

As the authors point out, they didn’t evaluate the effects of fish oil in people with cancer. The chemoresistant-inducing effect of fish oil was only determined in mice. Animal studies provide preliminary insights that help scientists hypothesize on how disease may develop in humans or how a drug may affect the body, but animals don’t develop diseases in the same way in which people do. To test medical therapies in humans, controlled clinical studies work best.

2. The findings make large leaps.

The researchers found that cancer stem cells in both humans and mice secrete a fatty acid associated with resistance to platinum-based chemotherapeutic drugs. The authors state that since this substance is found in minute amounts in some fish oils, fish oil would also cause resistance in cancer stem cells exposed to platinum-based chemotherapy.

That's quite a leap, considering the stem cells produce intracellular levels of this substance, and there has never been data showing that this substance is able to move from exogenous fish oil into human cancer stem cells. So how can one conclude that fish oil supplements which contain trace amounts of this fatty acid could have induced the chemoresistant effect?

3. Researchers didn't isolate what was causing chemoresistance.

In the study, the authors found that this substance in fish oil triggered the activation of specialized white blood cells in mice to release a chemical that induced chemoresistance. Is that the only chemical in the fish oil that could have caused the problem? No. The researchers didn’t assay the supplements used in the study for contaminants like PCBs or mercury, which are known tumor promoters. Without assaying for these possible contaminants, it’s unclear whether or not another chemical could have contributed to the effects observed in mice.

4. The researchers didn't use the highest quality fish oil possible.

The authors used mackerel, which is considered a fatty fish but is known to have a high mercury content, as well as cured herring, which I also tell my patients to avoid because the pickling process includes chemicals that are carcinogenic. Most health-conscious consumers already know to avoid these two types of fish and fish oil derived from them. Yet the authors of this study apparently did not.

So what does this all mean? For one, the authors did not demonstrate that fish oil impairs chemotherapy in humans. Yet the widespread media attention of this poorly designed study has given credence to these unfounded claims.

Bottom line: continue to eat foods that boost the immune system and detoxification. These help fight disease, curb toxic overload, and help stave off cancer. Fish and fish oil continue to be included in this group of foods.

Ready to learn more about how to unlock the power of food to heal your body, prevent disease & achieve optimal health? Register now for our FREE web class with nutrition expert Kelly LeVeque.

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