Fish oil and chemo together cause more oxidative damage in tumors than either alone, but that’s not the whole reason the tumors shrink — something else (like GPX suppression) matters more.

Feeding mice a diet rich in fish oil changes the fat composition in their breast tumors, and those tumors grow slower when treated with a common cancer drug.

Fish oil makes breast cancer tumors in mice produce less of an enzyme that protects them from oxidative damage, and this makes the cancer cells more vulnerable to chemotherapy.

Even when given an antioxidant (vitamin E), tumors in fish oil-fed mice still get more oxidative damage after chemo, meaning they’re already under more stress and can’t recover.