A team from the Institute of Food Research in Norwich found the vegetable triggered genes that control inflammation and cancer formation in the prostate.

A number of population studies have previously found anti-cancer benefits from one or more serves a week of cruciferous vegetables, such as broccoli, cabbage and brussels sprouts.

They made people less likely to develop prostate cancer and better able to fight a tumour once it developed, according to the studies. However, no one knew why.

The world-first study asked 22 men to eat 400 grams of broccoli a week for a year, in addition to their normal diet.

Tissue samples were taken from their prostate gland before the start of the trial and after six and 12 months, and analysed.

The study found the broccoli had a big effect on the expression of particular genes in the prostate associated with the formation of cancer and inflammation.

The research's leader, Professor Richard Mithen, said other vegetables and fruits also reduced the risk of developing prostate cancer, though probably in different ways.

"Once we understand these, we can provide much better dietary advice on which specific combinations of fruit and vegetable are likely to be particularly beneficial," he said.

"Until then, eating two or three portions of cruciferous vegetable per week … should be encouraged."

The research was published in the online PLoS One journal yesterday.

The team is now planning a bigger study to examine the effects on existing prostate cancer.

The results were welcomed by Professor Ian Olver, chief executive of the Cancer Council Australia, but he said it was a small study and more work was needed to prove the link.

CSIRO nutrition scientist Dr Michael Fenech said the broccoli serves would be of little assistance to someone susceptible to cancer for genetic or environmental reasons.

He said the study's findings on gene expression should not be seen as proving a direct link between eating broccoli and the prevention of prostate cancer.

Almost 3000 Australians die of prostate cancer each year.