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.

据美国研究人员报告，儿童时期经常吃豆类食品的女性患乳腺癌的风险较小。另一组研究人员则向美国癌症研究协会提出，一周吃几次鱼的男性患结肠癌的几率较小。

这两项研究进一步证明了饮食对于防癌的作用。癌症防治专家认为，在所有种类的癌症中，多达三分之二都是由吸烟、节食及缺乏锻炼等不健康的生活方式造成的。

美国全国癌症研究所和夏威夷大学的研究人员发现，在5岁至11岁时经常吃豆腐、味噌等豆类食品的女性患乳腺癌的几率要低58%。至于豆类食品为什么可以防癌还不清楚，不过大豆中所含的名叫异黄酮的化合物具有雌性激素的作用。

另一项研究表明，一周吃鱼至少五次的男性患结肠癌的风险要比那些一周都吃不到一次鱼的男性低40%。

很多鱼类中所富含的Omega-3脂肪酸能够抑制环氧化酶2的活性。环氧化酶2会引起发炎，这可能会刺激肿瘤的生长。