According to the Nurses’ Health Study, one of the largest continuing surveys of health and diet, dietary fibre and vegetable fat also reduced the risks, but butter increased them very slightly.

The study has been running since 1976 and draws on details, updated every two years, of diet and other lifestyle factors provided by 120,000 nurses in America. The health of the nurses has been tracked, enabling correlations to be established between particular diseases and diet.

The latest results, published in Breast Cancer Research, an online journal, showed that women who ate, on average, an egg a day were 18 per cent less likely to suffer breast cancer than those who averaged half an egg a day. Fibre may help because it can bind the sex hormone oestrogen within the digestive system.

The authors make no comment on the fact that their results said that hot dogs, not generally considered a health food, are also linked to an 18 per cent reduction in breast-cancer risk.

Less unexpected is the performance of broccoli, every additional daily serving of which reduced breast cancer risk by 26 per cent. Butter increased the risk by 6 per cent.

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