Dark-coloured fruits and vegetables may help to protect against colon cancer, research has shown.

Scientists found that the chemicals that give foods such as grapes, radishes, purple carrots and bilberries their colour significantly slow the growth of colon cancer cells.

Evidence from experiments on rats and on human colon cancer cells suggests that anthocyanins, the compounds that colour most red, purple and blue fruits and vegetables, slow the growth of the cells by anything from 50 to 80 per cent.

Chokeberries and other dark-coloured fruits are rich in anthocyanin.

The findings take scientists a step closer to figuring out what gives fruits and vegetables their cancer-fighting properties.

"These foods contain many compounds, and we're just starting to figure out what they are and which ones provide the best health benefits," said Monica Giusti, the lead author and assistant professor of food science at Ohio State University, Columbus, who presented her findings yesterday at the national meeting of the American Chemical Society in Boston.

In studies on human colon cancer cells grown in laboratory dishes, the researchers tested the anti-cancer effects of anthocyanin-rich extracts from fruits and vegetables with deep colours.

Anthocyanin pigments from radishes and black carrots slowed the growth of cancer cells from 50 to 80 per cent.

Pigments from purple corn and chokeberries (almost black berries from shurbs native to North America) not only stopped the growth of cancer cells, but also killed roughly 20 per cent of the cancer cells while having little effect on healthy cells.

This was confirmed by measurements which showed the amount of anthocyanin that was needed to reduce cancer cell growth by half.

Extract derived from purple corn was the most potent, in that it took the least amount of this extract (14 micrograms per millilitre of cell growth solution) to cut cell numbers in half. Chokeberry and bilberry extracts were nearly as potent.

Radish extract proved the least potent, as it took nine times as much to cut cell growth by 50 per cent.

"All fruits and vegetables that are rich in anthocyanins have compounds that can slow down the growth of colon cancer cells in experiments in laboratory dishes and possibly inside the body," Giusti told The Daily Telegraph.

In animal studies, rats induced with colon cancer cells were fed a daily diet of anthocyanin extracts either from bilberries and chokeberries, which are used as flavourings or to make jams and juices.

The anthocyanin extracts reduced signs of colon tumours by 70 and 60 per cent, respectively, when compared with control rats.

Giusti says the results suggest that anthocyanins may protect against certain gastrointestinal cancers.

But she stops short of recommending one kind of fruit or vegetable over another.

"There are more than 600 different anthocyanins found in nature," she said.

"While we know that the concentration of anthocyanins in the gastrointestinal tract is ultimately affected by their chemical structures, we're just beginning to scratch the surface of understanding how the body absorbs and uses these different structures."