Pre-menopausal women who eat large amounts of fiber could halve their breast cancer risk, a UK study has suggested.
The University of Leeds researchers, who studied 35,000 women, found those who ate 30g of fiber a day had half the risk of those who ate less than 20g.
They said women should try to increase their fiberintake.
Experts said the International Journal of Epidemiology study was more evidence of the benefits of a healthy diet.
The average person in the UK eats 12g of fibre a day.
To eat 30g, a person would need to eat a high-fiber cereal for breakfast, switch from white or brown bread to wholemeal and ensure they have five portions of fruit and vegetables a day.
A team from the University of Leeds Centre for Epidemiology and Biostatistics have been monitoring the eating habits and health of more than 35,000 women for seven years.
They were aged 35 to 69 at the beginning of the study.
Diet was assessed using a 217-item food questionnaire.
Unlike other studies looking at fiber intake and breast cancer risk, the women studied had a range of diets including groups who were wholly vegetarian or who did not eat red meat.
Just under 16,000 women were pre-menopausal when they entered the study, with 18,000 post-menopausal.
257 pre-menopausal women developed breast cancer during the study, which was initially funded by the World Cancer Research Fund.
However, the effect was not seen in the post-menopausal group, in which 350 developed breast cancer.
The researchers say this may be because fiber affects the way the body processes and regulates the female hormone oestrogen.
Levels of the hormone are higher in pre-menopausal women.
Professor Janet Cade, who led the research, said: "Our study found no protective effect in the older group, but significant evidence of a link in the pre-menopausal women.
"The relevant exposure may be earlier in life, explaining why the protective effect was not shown in the post-menopausal group."
She added: "In addition, post-menopausal women with high body mass indexes [who are overweight or obese] have an increased risk of breast cancer.
"Their weight may over-ride any other effects such as benefits from fiber."
Professor Cade added: "It goes along with the general healthy eating advice to make sure that you are getting plenty of fiber in your diet through breakfast cereals, bread, pasta, fruit and vegetables."
Ed Yong, cancer information officer at Cancer Research UK, said: "We already advise eating a diet rich in fiber to reduce the risk of bowel cancer. "This study suggests that it could help protect against breast cancer in younger women too."
He added: "Until now, the evidence that fiber could reduce the risk of breast cancer has been inconsistent.
"This study suggests that this is because any protective effects are limited to women before their menopause.
"It further highlights the importance of eating a healthy diet for reducing the risk of cancer."
Dr Sarah Cant, of the charity Breakthrough Breast Cancer said separating out the individual effects of different food was difficult.