WASHINGTON — Fatty acids found in fish oil supplements may block chemotherapy from attacking tumors and patients should stop taking them, said a study by Dutch researchers on Monday.

Fish oil supplements of omega-3 and omega-6 fatty acids are sold worldwide, and are touted by manufacturers as a way to boost heart and brain health.

However, researchers at The Netherlands' University Medical Center Utrecht report in the journal Cancer Cell that two types of fatty acids, known as platinum-induced fatty acids or PIFAs, were shown to block one type of chemotherapy from working in animal tests.

"Whilst waiting for the results of further research, we currently recommend that these products should not be used whilst people are undergoing chemotherapy," said Professor Emile Voest, a medical oncologist at UMC Utrecht.

Voest supervised the research, which showed that a form of chemo called cisplatin, often used to treat lung, bladder, ovarian and testicular cancer, was rendered impotent by these two PIFAs, which are made by stem cells in the blood and are also present in fish oil supplements.

In tests on mice with tumors under the skin, researchers found that animals injected with the fatty acids, described as "normal amounts of fish oil," became insensitive to chemotherapy.

"Where resistance to chemotherapy is concerned, we usually believe that changes in the cancer cells themselves have occurred. Now we show that the body itself secretes protective substances into the blood that are powerful enough to block the effect of chemotherapy," said Voest.

"These substances can be found in some types of fish oil."