A breakfast bowl of muesli, wholemeal sandwiches at lunch and fruit in the evening could halve a woman's risk of developing breast cancer.
Researchers have found that younger women who eat a high-fibre diet appear to be protected against the disease - at least until the menopause.
A study of 35,000 women over seven years found those with the highest fibre intake of 30 grams a day had a 50 per cent lower incidence of breast cancer than those eating 20 grams a day. But the effect was only seen in pre-menopausal women up to the age of about 50. In post-menopausal women, a high-fibre diet offered no protection.
Professor Janet Cade of the University of Leeds, who led the study, said she had been "surprised" at the strength of the association but was confident it revealed a genuine link.
"Thirty grams of fibre a day is high - about twice the normal level. You would have to eat a fibre-rich breakfast cereal, wholemeal bread instead of white and five portions of fruit and vegetables a day.
"But it's not hamster rations - it's do-able."
The results of the study, published in the International Journal of Epidemiology, showed 257 pre-menopausal women developed breast cancer but the risk decreased with increasing fibre intake.
Professor Cade said: "Previous research has not shown a convincing link between increased dietary fibre and a lower risk of breast cancer. But earlier studies didn't draw any distinction between pre- and post-menopausal women.
"Our study found no protective effect in the older group, but significant evidence of a link in the pre-menopausal women."
The research was funded by the World Cancer Research Fund and Kelloggs, the breakfast cereal manufacturer. Professor Cade said Kelloggs provided about £10,000 and had had no influence on the study design. "It was a tiny proportion of the total costs and the results could have gone either way. I suppose it was nice for them."
The strongest protective link was with cereal fibre - wheat and oats found in bread, pasta and breakfast cereals. There was also a link with fruit fibre.
High-fibre foods are rich in vitamins, zinc and other anti-oxidants which are known to protect against cancer. They also smooth out insulin levels, the hormone that regulates the absorption of sugar, which may play a role in cancer.
Dietary fibre has also been shown to regulate levels of the female hormone oestrogen, which is implicated in breast cancer. Oestrogen levels are higher in pre-menopausal women.
A spokesman for Cancer Research UK, said: "The study highlights the importance of eating a healthy diet for reducing the risk of cancer."
Foods that cut cancer risk
* Eating five portions of fruit and vegetables a day reduces the risk of many cancers, especially of the digestive system.
* Eating fish may reduce the risk of bowel cancer.
* Wholegrain cereals, wholemeal bread and fruit can cutthe risk of bowel and breast cancer.
* Vitamins and minerals such as folate, selenium, calcium and vitamins A, C and E may reduce the risk of many cancers.Reuse content