Women who ate soy regularly as children have a lower risk of breast cancer, American researchers report. And men who eat fish several times a week have a lower risk of colon cancer, a second team of researchers have told the American Association for Cancer Research.

The studies add to a growing body of evidence about the role of diet in cancer. Cancer experts now believe that up to two-thirds of all cancers come from lifestyle factors such as smoking, diet and lack of exercise.

The US National Cancer Institute and researchers at the University of Hawaii found that women who ate the most soy-based foods, such as tofu and miso, when aged 5 to 11, reduced their risk of developing breast cancer by 58 per cent. It was not clear how soy might prevent cancer, though compounds in soy called isoflavones have estrogen-like effects.

A second study showed that men who ate fish at least five times a week had a 40 per cent lower risk of developing colorectal cancer compared with men who ate fish less than once a week.

Many kinds of fish are rich in omega-3 fatty acids, which interfere with the cyclooxygenase-2 or COX-2 enzyme. COX-2 affects inflammation, which may play a role in tumour growth.