The Dutch scientists, who were investigating how tumours develop resistance to treatments, found that two fatty acids found in fish oils make the tumours resistant to treatment.

These fatty acids, according to them, are also produced by stem cells in the blood, the BBC reported. Experiments on mice showed that stem cells in the blood responded to the widely-used cancer drug cisplatin. The cells started producing two fatty acids, known as KHT and 16:4(n-3).

These fatty acids begin a series of chemical reactions, which mean cancerous cells become resistant to chemotherapy. Using drugs to block the production of the fatty acids prevented this form of resistance which “significantly enhances the chemotherapy,” the researchers said.

However, they warned that these fatty acids were “abundantly present in commercially available fish oil products”. They showed that off-the-shelf fish oil supplements, given to mice, could stop chemotherapy working against some tumours.

Lead researcher Prof Emile Voest said: “We 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.

“Whilst waiting for the results of further research, we currently recommend that these products should not be used whilst people are undergoing chemotherapy.”

Jessica Harris, of Cancer Research UK, said the study “suggests one possible option for stopping cancers becoming resistant to treatment, but it’s at an early stage and much more research would be needed.”