A new study from the journal JAMA Oncology finds that cancer patients who take certain fish oils may increase the risk of becoming resistant to chemotherapy.

More an 1.6 million people will be diagnosed with cancer this year, according to the American Cancer Society.

Many cancer patients once diagnosed make lifestyle changes such as eating healthier and taking supplements. For example, a study from the Netherlands Cancer Institute in Amsterdam, identified a threefold increase in food supplement use among patients who had received a cancer diagnosis.

The researchers note that omega-3 fatty acids are commonly used supplements among cancer patients, with around 20% of cancer patients using them in the US - most commonly in the form of fish oil. The amount of fish oil intake may affect treatment outcomes for cancer patients.

The study noted:

"We show that fish oil contains substantial levels of 16:4(n-3), a fatty acid with potent chemotherapy-negating effects in preclinical models, and that intake of low doses of fish oil interferes with chemotherapy activity in mice.

Ingestion of the recommended daily amount of fish oil by healthy volunteers rapidly increased 16:4(n-3) plasma levels. Since low concentrations of 16:4(n-3) were still active in mice, and since 11% of patients undergoing cancer therapy in our center used omega-3 supplements, these findings may have important clinical implications."

These findings further show fish oils may affect cancer patients undergoing chemotherapy. Researchers recommend pateints avoid taking fish oil supplements from the day before their treatment until the day after.