Two food ingredients already recognised separately as having some cancer prevention properties might be as much as 13 times more powerful when put together, researchers said yesterday.

They found that the impact of the plant-derived chemical sulforaphane and the mineral selenium on genes that influence the growth of cancerous tumours was far stronger when they were used as a combined treatment.

Human trials of the substances might begin next year, according to the team, who published their work in the journal Carcinogenesis.

Evidence of success outside the laboratory, even though it may take years toestablish, could have important conse quences for manufacturers of "health foods" as well as for consumers.

"It opens up new possibilities for functional foods, food supplements or simply new guidelines for healthy eating," said Dr Yongping Bao, senior researcher at the Institute for Food Research in Norwich.

Sulforaphane, seen as a potentially useful curative cancer drug, is found in high concentrations in broccoli, sprouts, cabbage, watercress and rocket salad while foods rich in selenium include nuts, poultry, fish, eggs, sunflower seeds and mushrooms.

Selenium intake by Britons has halved over the last 20 years, despite the fact that a lack of it is associated with cancers such as prostate cancer, whose incidence is rising.