A diet rich in a fat found in oily fish and some seeds may protect men with prostate cancer from developing a more aggressive form, scientists have found.
Oily fish contains omega-3 fats
Prostate cancer is much more likely to be life-threatening if tumour cells migrate and invade other tissues, such as the bone marrow.
Lab tests found omega-3 oil - present in fish like salmon - prevented this.
The results of the study, based at Manchester's Christie Hospital, are in the British Journal of Cancer.
The Paterson Institute researchers tested the effect of two types of dietary fat on prostate cancer cells in the lab.
Previous research has suggested omega-3 fats, which is also found in some plant sources such as pumpkin seeds, may help cut the risk of cancer - and other conditions, such as heart disease.
Omega-6 fats, found in vegetable oils, nuts and seeds, also play a key role in keeping us healthy.
However, while omega-6 fats increased the spread of prostate cancer cells into bone marrow, omega-3 fats blocked this.
Researcher Dr Mick Brown said: "It is possible to have a healthy balance of these two types of fat - we only need about half as much omega-3 as omega-6 - that will still stop cancer cells from spreading."
The researchers believe tumour cells might use omega-6 fats as a high energy source - giving them the energy they need to maintain a high growth rate and to create molecules that control migration.
Omega-3 fats appear to interfere with this process in some way.
Lead researcher Noel Clarke said: "Some tumours develop slowly in the prostate without producing symptoms and sometimes when symptoms do develop, it is because the cancer has already spread.
"Eating a diet with the right balance of omega-3 and omega-6 fats may well help to keep prostate cancer within the prostate gland where it may be monitored safely or more easily treated with surgery or radiotherapy."
Derek Napier, of the Association for International Cancer Research, which part-funded the study, said the findings might aid the development of new ways to block the spread of many types of cancer.
Professor John Toy, of Cancer Research UK, said the work was still at an early stage.
"We would need large population studies to provide the needed evidence to say a change in diet could reduce prostate cancer cells from spreading," he added.
Prostate cancer is the most common male cancer in the UK.
The research was funded by the Association for International Cancer Research (AICR) and the Medical Research Council (MRC).
The Food Standards Agency recommends men can eat up to four portions of oily fish a week.