Eating a portion of tuna, salmon, sardines or other oily fish once or twice a week reduces the risk of breast cancer, according to a review published by the British Medical Journal (BMJ).
Researchers based in China looked at 26 previously published studies covering more than 800,000 volunteers in the United States, Europe and Asia whose health was monitored and who gave details about their eating habits.
Oily fish is rich in so-called n-3 polyunsaturated fatty acids, or n-3 PUFAs, which are involved in the immune system, blood-vessel activity and chemical messaging in the brain.
The group of n-3 PUFAs has four members, known by their initials of EPA, DPA and DHA, which are mainly found in oily fish, and ALA, which is chiefly found in nuts, seeds and leafy vegetables.
The analysis showed that women with a high intake of n-3 PUFAs had a 14-percent reduction in risk of breast cancer compared with those who had a low intake.
But the protective effect came only for fatty acids that come chiefly from fish, and not for ALA acids.
Those who showed the most benefit were women in Asia, whose diet was richer in fish than in Europe and America.
In statistical terms, every 0.1-gramme increase in fish fatty acids per day was linked to a five-percent reduction in breast cancer risk.
As a guide for daily living, this means one or two portions of oily fish per person per week.
Breast cancer has been dubbed a "silent killer" of women because it is often diagnosed too late.
The disease accounted for 23 percent of total cancer cases among women and 14 percent of cancer deaths in 2008, according to figures in the study.
Genetic heritage also plays a role in breast cancer, most notoriously in variants in the BRCA1 and BRCA2 genes, although how this interacts or not with food, lifestyle and environmental factors is unclear.