A new study suggests the consumption of fish oil can reduce at least half the effectiveness of chemotherapy in cancer patients.

Dr. Emile Voest of Netherlands Cancer Institute in Amsterdam and his team found an alarming connection between fish oil and their effects on drugs used in chemotherapy.

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The team found that herring, mackerel and three other fish oils raised blood levels of a fatty acid called 16:4(n-3) in cancer patients. This, in turn, can lessen the effects of chemotherapy drugs.

For the study, Dr. Voest and his team analyzed fish oil consumption among 118 cancer patients undergoing chemotherapy. The researchers also looked at the levels of fatty acids in 50 healthy participants after they consumed either oily fish or fish oil supplements

 Among those with cancer, 35 said they regularly took fish oil supplements, and 13 said they used supplements with omega-3 fatty acids.

Levels of fatty acid 16:4(n-3) increased in the participants after the recommended daily amount of 10 ml of fish oil was administered.

Blood levels returned to normal eight hours after the dose, but took longer when the participants used 50 ml of fish oil.

In addition, eating 100 grams of herring and mackerel raised blood levels of 16:4(n-3) compared with tuna, which didn't alter the blood levels. Salmon also showed only a short-lived peak.

The findings are in line with a growing awareness of some fatty acids and their receptors. Researchers said they can also raise concern about the simultaneous use of fish oil and chemotherapy drugs.

Based on the results, Dr. Voest and his team suggested that patients undergoing chemo should avoid eating oily fish or taking fish oil supplements before or after chemotherapy until further research has been carried out.

Dr. Voest is planning to further study the biological connection between fatty acids and chemotherapy drugs. He's also launching a clinical trial to find if a compound that has the functions of fatty acids might work to improve the effects of chemotherapy in cancer patients taking fish oil.