Patients undergoing treatment for cancer could be harming their chances of survival by using fish oil supplements, experts warned yesterday.

Dutch researchers say they have discovered that fatty acids in fish oil capsules reduce the effectiveness of nearly all types of chemotherapy, making cancer cells insensitive to the treatment.

The study, by the University Medical Centre (UMC) Utrecht, in the Netherlands, also found that stem cells in patients’ blood produced the same fatty acids that desensitised tumours to chemotherapy.

Scientists are advising cancer patients against using natural remedies that include fish oil during chemotherapy.

Professor Emile Voest, a medical oncologist at UMC Utrecht, said: “Where resistance to chemotherapy is concerned, we usually believe that changes in the cancer cells themselves have occurred.

“Now we can show that the body itself secretes protective substances into the blood that are powerful enough to block the effect of chemotherapy.

“These substances can be found in some types of fish oil. While waiting for the results of further research, we currently recommend that these products should not be used while people are undergoing chemotherapy.”

The study looked at the effect of platinum-induced fatty acids, or PIFAs, in mice and human cells.

The mice had tumours under their skin which, under normal conditions, would decrease in size following chemotherapy.

However, the tumours were found to be insensitive to treatment after the mice were given commercially produced fish oil supplements containing omega-3 and omega-6 fatty acids.

Stem cells in the blood of patients were also found to produce fatty acids during chemotherapy that desensitise tumours to treatment.