The finding is based on a small study of about 40 patients in the journal Cancer.
Those taking fish oil containing omega-3 fatty acids did not lose weight and they gained muscle mass.
Researchers speculate that fish oil my interfere with some pathways that are altered in advanced cancer.
The finding raises hope of a simple but effective treatment for cancer related malnutrition.
Still, doctors say this is not enough to recommend people start consuming huge amounts of fish oil.