Fish Oil’s Cancer Benefits May Depend on Your Genes

Millions of people take fish oil supplements believing they protect against cancer and heart disease. But new research suggests the benefits may not apply to everyone—and for some, the supplements could actually be harmful.

Scientists at the University of Michigan and the University of Texas MD Anderson Cancer Center found that whether fish oil helps prevent colorectal cancer depends on whether your body carries a gene called ALOX15. The discovery challenges the assumption that omega-3 supplements work the same way for everyone.

Background

About 19 million adults in the United States take fish oil supplements, often hoping to reduce their risk of chronic diseases including cancer. Fish oil contains two omega-3 fatty acids called EPA and DHA that have been studied for their potential health benefits for decades.

When consumed, EPA and DHA are supposed to be converted by the body into compounds called resolvins. These molecules reduce chronic inflammation, which plays a major role in cancer development. The conversion process depends entirely on the ALOX15 enzyme.

However, researchers discovered that ALOX15 is frequently switched off in cancer cells. This raised an important question: if cancer cells lack this enzyme, can fish oil still help prevent tumors from forming?

Key Details

To answer this question, researchers compared mice fed a fish oil-enriched diet with mice given a standard diet. The results were surprising. In mice without the ALOX15 enzyme, fish oil actually increased the number of colon tumors.

"Not all fish oil supplements are the same," said Imad Shureiqi, a professor of internal medicine at the University of Michigan Rogel Cancer Center.

The type of omega-3 mattered. EPA, one of the two main omega-3s in fish oil, performed better than DHA. Mice fed diets rich in EPA developed fewer tumors than those given DHA. The form of the supplement also made a difference. Lovaza, a prescription medication containing ethyl ester forms of EPA and DHA approved by the Food and Drug Administration to treat high triglyceride levels, reduced both the number and size of tumors—but only in mice that had active ALOX15. DHA variants did not prevent tumor growth in mice lacking the enzyme.

How the Process Works

When ALOX15 is present and active, it breaks down EPA and DHA into resolvins. These molecules then reduce inflammation by lowering levels of harmful compounds that promote tumor growth. The process appears to strengthen the immune system's ability to fight cancer cells.

Without ALOX15, this conversion never happens. Instead of being broken down into protective resolvins, the omega-3 fatty acids may be converted into other compounds that could potentially promote inflammation and tumor growth.

What This Means

The findings raise important questions for patients and doctors considering fish oil supplements as a cancer prevention strategy.

People with colon polyps who do not have active ALOX15 may not receive the same protective benefits from EPA and DHA. For them, the supplements could be less effective at slowing tumor growth—or potentially harmful.

The research team is now developing medications designed to boost ALOX15 levels in cancer cells. Their goal is to enhance the body's ability to process EPA and DHA, potentially strengthening efforts to prevent colon cancer in people who currently lack the enzyme.

Experts recommend that patients speak with their doctors before starting fish oil supplements. A doctor can help determine whether supplements are appropriate based on individual health factors and, potentially in the future, genetic testing.

While most of the data comes from animal studies, the findings demonstrate that genetic differences between people can dramatically affect how supplements work. This suggests that the one-size-fits-all approach to supplements may not be accurate. The same pill that protects one person from cancer might do nothing for another—or potentially cause harm.