Fish oil supplements containing omega-3 fatty acids may protect against breast cancer, new research suggests.
Scientists found that women who regularly took the supplements reduced their risk of the disease by 32pc.
The effect was restricted to invasive duct breast cancer, the most common form.
Other supplements commonly taken by women for menopausal symptoms did not appear to offer any protection.
It is the first time a link has been found between fish oil supplements and reduced breast cancer risk.
Studies looking at whether dietary consumption of fish or omega-3 fatty acids has any impact on breast cancer have been inconclusive.
"It may be that the amount of omega-3 fatty acids in fish oil supplements are higher than most people would typically get from their diet," said lead researcher Dr Emily White, of Seattle, Washington.
The Vitamins and Lifestyle (Vital) study recruited 35,000 post-menopausal women who had no history of breast cancer.
All were asked to fill out a 24-page questionnaire about their use of dietary supplements and monitored for six years.
Over that period, 880 of the women went on to develop breast cancer.
The new findings were published today in the journal Cancer Epidemiology, Biomarkers & Prevention.
Dr White said women should treat the results with caution. "Without confirming studies specifically addressing this, we should not draw any conclusions about a causal relationship," she said.
Professor Edward Giovannucci, a US nutrition expert at Harvard, said: "It is very rare that a single study should be used to make a broad recommendation. Over a period of time, as the studies confirm each other, we can start to make recommendations."
Fish oil is already said to combat heart disease, reduce the symptoms of arthritis, and improve brain health.
Harvard researchers are now recruiting patients for a new trial which will assess the impact of fish oil supplements and vitamin D on cancer, heart disease and stroke.
Dietary intake of omega-3 fatty acids and fish has been shown to lower blood concentrations of two inflammation-linked proteins that have been associated with breast cancer.