KEY POINTS:
Muesli at breakfast, 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 during the course of the study 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.
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.
Professor Cade said; "No one is sure of the mechanisms but fibre can influence the production and metabolism of oestrogen, as well as insulin levels in the body which affect cell growth and division. There are reasons why fibre could have an impact [on breast cancer]. There is a rationale to it."
She added: "We don't yet know at which point in life dietary habits impact on a woman's susceptibility to breast cancer. The relevant exposure may be earlier in life explaining why the protective effect was not shown in the post-menopausal group."
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