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Fish oil rich in omega-3 fatty acids have long been encouraged by doctors as supplements to support heart and joint health, among other benefits. But the supplements have a dark side. Michigan State University Researchers found it induced severe colitis and colon cancer in mice.

Jennifer Fenton, a food science and human nutrition researcher at MSU led the research that supported the need to establish a dose limit for docosahexaenoic acid (DHA), one of the omega-3 fatty acids present in fish oil, particularly for people suffering from chronic conditions such as inflammatory bowel disease (IBD).

“We found that mice developed deadly, late-stage colon cancer when given high doses of fish oil,” Fenton said. “More importantly, with increased inflammation, it only took four weeks for the tumors to develop.”

The study findings, published in the October, 2010 issue of Cancer Research , support a growing body of literature implicating harmful effects of high doses of fish oil consumption in relation to certain diseases.

The MSU researchers found an increase in the severity and an aggressive progression of the cancer not only in the mice receiving the higher doses, but also in those receiving lower doses of DHA. The researchers observed the mice were prone to inflammatory-like bowel disease; inflammation is an important risk factor for many types of cancers, including colon cancer and prostate cancer.

The findings were surprising to the researchers who initially hypothesized DHA-enriched fish oil would decrease the cancer risk in the animal models. DHA has been shown in previous studies to have some anti-inflammatory properties.

“We actually found the opposite. These mice were less equipped to mount a successful immune response to bacteria that increased colon cancer tumors,” Fenton said.

Colon cancer rates in the United States are high compared to other nations. The National Cancer Institute estimates that one in 20 people of either sex will be diagnosed with colon cancer during their lifetime.

Individuals with IBD diseases, including ulcerative colitis and Crohn's disease, have an increased risk of developing cancer of the intestinal tract, and when the cancer metastasizes it can be fatal.

Fenton cautioned that people may not need to avoid fish oil entirely as it does have some known benefits. However, individuals already receiving enough omega-3 fatty acids through a well-balanced diet have no need for adding supplements which could increase their risk. But the research community also believe that a substantial number of people are dangerously omega-3 deficient.

The challenge is “with fish oil, we don’t know yet how much is appropriate,” she said.

As such, academics and the food industry have called for establishing dietary guidelines for omega-3 consumption. To accomplish this, Fenton and colleagues plan future tests of omega-3 fatty acids in people with IBD to determine how these findings correlate to human populations and what, if any, amount is safe.

Lynette Summerill is an award-winning writer who lives in Scottsdale, Arizona. In addition to writing about cancer-related issues for EmpowHER, she pens Nonsmoking Nation, a blog following global tobacco news and events.