Fish oil supplements may hinder the effectiveness of chemotherapy treatment in cancer patients, according to a new study from the Netherlands Cancer Institute in Amsterdam.

A pair of particular fatty acids found in the popular supplements are believed to suppress actions of the drugs used in chemotherapy. Researchers examined the supplements in an effort to determine if these organic compounds were present in the pills.

Though one of the fatty acids wasn't found in the fish oil supplements, the other, called 16:4(n-3), was detected in large doses.

Omega-3 fatty acids are consumed by approximately 20 percent of cancer patients, through both food and in the form of pills and liquids.

Researchers examined six major varieties of fish oil supplements, and also conducted surveys as part of their investigation. A total of 118 cancer patients submitted data on their intake of nutritional supplements. About 35 percent reported using some form of supplement, including 11 percent who specifically listed consumption of omega-3 pills.

The study also examined 30 cancer-free subjects as they consumed fish oil supplements. Concentrations of 16:4(n-3) in their blood peaked roughly four hours after consuming 10 mL (0.34 oz) doses, returning to normal four hours later. The same effect was found in mice suffering from cancer.

Consumption of fish was also studied in 20 volunteers. Herring and mackerel were found to raise levels of 16:4(n-3), while tuna and salmon had no discernible effect.

The researchers suggest that cancer patients taking part in chemotherapy treatments should avoid fish oil supplements, as well as the consumption of mackerel and herring on the days just before and after their treatments. This suggestion is supported by the Dutch National Working Group for Oncologic Dieticians and the Dutch Cancer Society.

"It's an intriguing observation, which warrants further investigation," said Powel Brown from the Department of Clinical Cancer Prevention at the University of Texas M.D. Anderson Cancer Center. "In general, when giving chemotherapy, we discourage the use of additional supplements. It's very well known that some supplements affect the metabolism of chemotherapy." 

In addition to fish oils, omega-3 fatty acids are found in walnuts, chia seeds, flax seeds, hemp, eggs, and a range of other foods. The substance is vital to food health, and cannot be produced by the human body.

Further investigations will be required to precisely assess the effect of these fatty acids in patients. One challenge will be a moral dilemma. Experiments are designed to test a given condition against a control group, and it is considered immoral to purposely provide a substance, such as fish oil, to a patient, knowing it could hinder recovery.

Analysis of the role of 16:4(n-3) on chemotherapy treatments was profiled in the journal JAMA Oncology.

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