Natural chemicals found in soya beans and vegetables such as broccoli, cabbage and cauliflower boost the body's ability to repair damaged DNA and may prevent cells turning cancerous, scientists said yesterday.

Studies have suggested that eating vegetables appears to provide some protection against certain cancers, but until now the reason why has been a mystery.

Researchers at Georgetown University in Washington DC believe the answer lies with two naturally occurring compounds. The first, indole-3-carbinol or I3C is abundant in vegetables including broccoli and cabbage, while the second, genistein, occurs naturally in soya beans.

The researchers found that when the chemicals were added to cells they boosted the activity of two genes, known as BRCA1 and BRCA2, both of which play a crucial role in detecting damaged DNA and marshalling a cell's response to fix it. Mutations in either of these genes often lead to breast, ovarian and prostate cancer because they are unable to prevent damaged DNA being passed on to the next generation of cells.

Writing in the British Journal of Cancer, the researchers said that since very low levels of BRCA proteins are seen in cancerous cells, higher levels might prevent cancer developing.

"We know that one of the functions of the BRCA1 and BRCA2 genes is to maintain genomic integrity, and to do that, they first have to be able to detect DNA damage and signal to the cell that it is there, and then become involved in repairing it," said Eliot Rosen, the lead author of the study.

He said that the ability of I3C and genistein to boost the number of BRCA proteins could explain their apparent protective effect. "Studies that monitor people's diets and their health have found links between certain types of food and cancer risk," he said. "However, before we can say a food protects against cancer, we have to understand how it does this at a molecular level."

Professor Rosen's team exposed breast and prostate cancer cells to increasing levels of the natural chemicals. Depending on the dose, they boosted the activity of the DNA repairing genes by 10-15 times.

Genistein, which is found in soya beans, improved the activity of the BRCA genes at doses, equivalent to those found in the blood of people who regularly eat soya-based products. Questions were raised over its safety last month when researchers discovered it can disrupt sexual development in mice. "Although we are not entirely certain about how these animal studies on genistein translate to the human population, there is some reason to be cautious," said Dr David Schwartz, director of the National Institute of Environmental Health Sciences.

The second chemical, I3C, is broken down by acid in the stomach to form half a dozen compounds that are believed to offer protection against cancer. In previous studies, the researchers had discovered that other natural chemicals increased the activity of BRCA1, but the latest study reveals that two easily available compounds boost both DNA repair genes.

"It is now clear that the function of crucial cancer genes can be influenced by compounds in the things we eat," Prof Rosen said. The discovery allows researchers to verify whether supplements based on genistein and IC3 work. "There are a lot of supplements out there that claim to help prevent cancer. We can now look at those to see if they upregulate the activity of these genes," he added.