There is much more conclusive evidence that eating fruit and vegetables helps prevent other forms of cancer

Dr Victoria Wilson, Cancer Research Campaign

Some studies have suggested that fruit and vegetables do have a protective effect against breast cancer.

UK and US guidelines encourage people to eat five servings of fruit or vegetables a day to protect against cancer.

Dr Smith-Warner said her study showed that adult women who added another portion of fruit or vegetables to their daily diet reduced their risk by just 1%.

She said: "Other strategies need to be identified to reduce breast cancer risk."

The findings were based on an analysis of eight studies covering 351,825 women tracked for between six and 15 years.

Inconclusive evidence

Dr Victoria Wilson, a science information officer for the Cancer Research Campaign, said the evidence was inconclusive.

She told BBC News Online: "The link between breast cancer and diet is not well understood.

"However, there is much more conclusive evidence that eating fruit and vegetables helps prevent other forms of cancer as well as heart disease and obesity. We also know that people who are obese have much higher rates of breast cancer."

Dr Wilson said it was probably more important to eat a healthy diet during the teenage years when the breast tissues are still developing.

A spokesperson for the British Heart Foundation said: "Fruit and vegetables contain high levels of antioxidant vitamins such as A, C and E and these are beneficial to the heart.

"Eating at least five portions of fruit of vegetables each day is an excellent way to reduce the risk of coronary heart disease - probably by helping to prevent hardening of the arteries.

"Fruit and vegetables also contain many other chemicals or minerals that help to maintain heart health, so overall they form the backbone of a heart healthy diet."

The research is published in the Journal of the American Medical Association.