Scientists have published hundreds of studies on omega-3 fatty acids and cancer.

And the evidence clearly suggests that omega-3 fatty acids from seafood tend to discourage cancer.

Conversely, excess intakes of omega 6 fatty acids from plant foods tend to promote cancer growth.

In a rare exception to the rule, a study published in 2013 struck fear into some men taking omega-3 fish oil supplements.

Before we examine new evidence refuting that study – and three new studies affirming the positive role that omega-3s played against cancer – we will quickly summarize the research on omega-3s and cancer.

Omega-3s and cancer: An overview
The effects of omega-3s on human cancer have been probed in depth.

Scientists have examined these effects in hundreds of laboratory studies, dozens of large epidemiological studies, and a more limited number of clinical studies.

(Epidemiological studies are ones in which researchers compare people's diets, or the levels of nutrients in their blood, to their health status. No single epidemiological study can prove a cause-and-effect relationship between a food or nutrient and a health condition.)

Evidence from epidemiological (population), laboratory, and clinical studies consistently link higher intakes or blood levels of omega-3 and lower rates of breast and colon cancers, among others.

Taken together, the results of all these studies clearly indicate that the omega-3 fatty acids found in seafood and in human cells – EPA and DHA – tend to exert anti-cancer effects.

As Iranian medical researchers wrote earlier this year, "… during recent years, a plethora of studies has demonstrated that omega-3s possess therapeutic role against certain types of cancer. It is also known that omega-3s can improve [the] efficacy and tolerability of chemotherapy.” (Nabavi SF et al. 2015) 

We've reported on many such studies over the years … see the Omega-3s & Immunity section of our News Archive.

Omega-3s appear to discourage cancer by suppressing production of pro-inflammatory molecules, exerting beneficial influences on gene expression, and reducing hormone-related cell growth and the production of free radicals.

In contrast, diets overloaded with omega-6 fatty acids tend to encourage tumor growth by generating pro-inflammatory molecules as well as cancer-promoting genetic signals.

(Most of the influences exerted by omega-3 and omega-6 fatty acids are exerted by the breakdown products that result when they are digested and metabolized … compounds called "metabolites”.)

Until recently, these encouraging indications only began to gain support from epidemiological studies over the past 10 years.

Around 2005, the first epidemiological studies to measure participants' blood levels of omega-3 EPA and DHA began to appear.

The majority and largest of these studies linked higher EPA (and possibly DHA) blood levels with lower cancer risk.

Prominent among these encouraging findings were the results of the European Prospective Investigation into cancer and nutrition (478,040 men and women) and the U.S.-based Physicians Health Study (22,071 men).

Both studies affirmed the hypothesis will that fish intake has a favorable effect on cancer risk.

Prostate-cancer study fanned groundless fears
Two years ago, a team led by Professor Theodore Brasky of Ohio Sate University analyzed the results of a 2013 epidemiological study ... and their analysis linked higher intakes of omega-3s to higher risk for aggressive prostate cancer (Brasky TM et al. 2013).

The fears sparked by that study – and a similar analysis published last year (Brasky TM et al. 2014) – caused many men to stop taking fish oil.

Sadly, the national negative headlines stuck in people's heads, despite immediate criticism from experts in prostate cancer and statistical analysis, who decried the obvious flaws in the authors' analysis.

We covered the flawed 2013 study (and its expert critics) in Fishy Prostate News, conveyed the opposite findings of (far more credible) clinical trials in Omega-3 Diet Reduced Prostate Risk Factors, and Omega-3 Curbed Prostate Cancer Growth.

Then, earlier this year, scientists reported finding a new way in which omega-3s may oppose the growth and spread of prostate cancer ... see Bogus Prostate/Omega-3 Study Takes Another Hit.

Last year, the European Food Safety Authority (EFSA) came to a reassuring conclusion that further undermines the clearly unjustified alarms raised by the flawed 2013 study: "on the basis of available data, there is no evidence for a role of EPA and/or DHA intake in the development of prostate cancer”. (EFSA 2014)

That opinion followed on the heels of another reassuring EFSA scientific opinion issued three years ago: "supplemental intake of EPA and DHA combined at doses up to 5 g/day does not give rise to safety concerns for adults”. (EFSA 2012)

Also in 2014, a large team of U.S. and international researchers – which included Theodore Brasky, the lead author of the flawed 2013 analysis – concluded that omega-3s pose no risk of prostate cancer.

Their meta-analysis, published in the Journal of the National Cancer Institute, examined the findings from seven studies that followed people over a period of years.

While the results showed that men with higher blood levels of omega-3 EPA had a slightly higher risk of prostate cancer, the authors acknowledge that the nature of these epidemiological studies precluded establishing a cause-effect relationship.

Overall, the authors concluded that "there was no strong evidence that circulating fatty acids are important predictors of prostate cancer risk.” (Crowe FL et al. 2014)

We hope that the broad-based critiques and rebuttals of the flawed 2013 study will finally put to rest bogus fears about omega-3s and prostate cancer.

In the meantime, 3 new studies support the many prior indications that dietary omega-3s tend to discourage cancer development and growth. 

Study number 1: Omega-3s aided tamoxifen against breast cancer
It's becoming clear that diets rich in omega-3s tend to discourage breast cancer.

Breast cancer comes in many forms, and omega-3s appear most helpful with regard to estrogen-dependent and hard-to-treat "triple-negative” breast cancers ... see Omega-3s Target Toughest Breast Tumors and the links in it.

As Italian researchers said four years ago, "Population and pre-clinical studies have suggested that omega-3s from seafood inhibit breast cancer growth and improve treatment outcomes (Signori C et al. 2011).

And, three years ago, Chinese researchers reported that omega 3 DHA and EPA from seafood shifted the dangerous effect of estrogen on hormone-dependent breast cancer to a positive (anticancer) effect in certain human breast cancer cells.

As they wrote, their findings show that omega-3s help induce "suicide” in breast cancer cells, and shed new insight into the potential application of omega-3s in breast cancer treatment (Cao W et al. 2012).

Because human breast cancer comes in different forms with different causes and promoters, prevention requires a multi-faceted approach. 

Anti-estrogen drugs such as Tamoxifen and Raloxifene reduce the dangerousness of estrogen-driven tumors.

The complementary effects of Tamoxifen and omega-3s are supported by prior studies (see Fish Fats May Boost Breast Cancer Drug).

Last month, a team of researchers from three major American universities reported that omega-3 fatty acids enhanced the ability of Tamoxifen to reduce the spread of estrogen-dependent breast cancer … as well as its ability to suppress the genetically programmed tendency for cancer cells to commit "suicide” (apoptosis).

As the academic team wrote, "… administration of omega-3 fatty acids allows the use of lower and, hence, likely less toxic doses of Tamoxifen.”
"If these findings are supported in the clinical setting, the combination of omega-3 fatty acids and anti-estrogens may emerge as a promising, effective, and safe chemopreventive strategy …”. (Manni A et al. 2015)

Study number 2: Omega-3s enhanced cancer-drug efficacy
Last July, British researchers found that adding omega-3 fatty acids to an anti-tumor drug improved patients' response and quality of life (Arshad A et al. 2015).

The study, from scientists at the University Hospitals of Leicester, involved 50 patients with advanced pancreatic cancer.

The patients were given 1,000 mg of gemcitabine weekly, followed by intravenous administration of an omega-3 rich emulsion for three weeks, followed by a week without either the drug or the omega-3s.

That regimen was repeated up to six times in each patient.

The results showed that administration of omega threes improved patients response to the drug, helped stabilize the cancer, reduced the size of tumors that had spread to the liver, and improved the patients' self-reported quality of life.

This was the first study to combine omega-3 fatty acids with a chemotherapy drug, and the researchers believe the results are encouraging enough to warrant performance of a randomized clinical trial.

Study number 3: Omega-3 metabolite curbed spread of cancer
Last spring, scientists in Australia discovered that in rodents, a synthetic form of an omega 3 fatty acid completely prevented cancer metastasis.

The synthetic compounds they tested closely resembled the chemical compounds that form when omega-3 fatty acids from seafood are metabolized … substances referred to generically as omega-3 "metabolites”.

Professor Michael Murray from the University of Sydney's School of Medical Sciences had long been intrigued by the fact that omega-3s and their omega-6 cousins – which dominate cheap vegetable oils (soy, corn, safflower, sunflower, and cottonseed) – have distinctly different effects on cancer.

He knew that a metabolite of omega-6 fatty acids from plants helps cancer cells to grow and spread … so he wondered whether omega-3 fatty acids might produce a metabolite that could discourage cancer growth.

One of the synthetic omega-3 metabolites they tested had no effect on the growth of primary tumors in animals … but it completely prevented their spread (metastasis).

Metastasis is the most lethal aspect of most cancers. Existing cancer drugs primarily target cancer growth, and have little effect on metastasis.

But if you can prevent a cancer from spreading, a patient may live a normal life span free of symptoms.

For example, because prostate cancer usually grows very slowly, very few patients die from it or even suffer symptoms before they die from other causes.

The Australian researchers admit that they do not know exactly how their synthetic omega-3 metabolite inhibited metastasis, which is a complex process.

More study needed to detail and confirm the effects of omega-3s  and omega-6s on cancer
Cancer comes in hundreds of different forms, so no one treatment or preventive measure will work for all types.

So it is unwise to rely on any single diet or nutrient to help prevent or treat any single type of tumor. 

But the evidence that diets higher in omega-3s and lower in omega-6s than most Americans eat is persuasive enough for all of us to take heed. 

It's wise to test your blood to learn where you stand on the omega-3/omega-6 balance spectrum, and what dietary changes you may need to make to optimize your omeg-3/6 balance ... and help minimize your risk for major cancers.

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