Natural pigments that give certain fruit and vegetables a rich red, purple or blue colour act as powerful anti-cancer agents, according to a study by American scientists.

The compounds, found in foods such as aubergines, red cabbage, elderberries and bilberries, restricted the growth of cancer cells and in some cases killed them off entirely, leaving healthy cells unharmed.

The study combined laboratory tests on human cancer cells with experiments on animals that were designed to see whether a diet rich in the foods made a difference to their risk of developing cancer.

Foods with the highest levels of the compounds were most effective at slowing cancer growth, with exotic purple corn and chokeberries stopping the growth of colon cancer cells and killing 20% in lab tests. Foods less enriched with the pigments, such as radishes and black carrots, slowed the growth of colon cancer cells by 50% to 80%.

The findings bring scientists closer to unravelling the key ingredients responsible for giving fruit and vegetables their cancer-fighting properties.

Because the pigments, which belong to a class of antioxidant compounds known as anthocyanins, are not easily absorbed by the bloodstream, they travel through the stomach to the gastrointestinal tract, where they are taken up by surrounding tissues.

Their survival through to the lower part of the intestine may be the key to their role in preventing cancers in the tract, the scientists believe.

Researchers led by Monica Giusti, an expert in plant nutrients at Ohio State University, extracted anthocyanins from a variety of exotic and more common fruits and vegetables that all had a deep red, blue or purple hue and added them to flasks containing a suspension of human colon cancer cells.

When the team calculated how much of each extract was needed to reduce cancer cell growth by 50%, they found anthocyanin from purple corn to be the most potent. Chokeberries and bilberries were nearly as effective, while radish anthocyanin required nine times as much - or 131 micrograms per millilitre of cancer cell solution to cut cell growth by half.

In a second study, the researchers fed rats with colon cancer a diet of anthocyanin extracts from bilberries and chokeberries, which are most often used as flavourings in jams and fruit drinks. Colon tumours in the rats fell by 60% to 70% compared with a control group that were not given anthocyanin.

"These foods contain many compounds and we're just starting to figure out what they are and which ones provide the best health effects," said Dr Giusti.

"All fruits and vegetables that are rich in anthocyanins have compounds that can slow down the growth of colon cancer cells, whether in experiments in laboratory dishes or inside the body."

The research was presented yesterday at the annual meeting of the American Chemical Society in Boston.

The team are now investigating whether it is possible to modify the structure of the pigment compounds to make them even more potent. Tentative results so far suggest that grafting an extra sugar or acid molecule to the anthocyanins improved their effectiveness.

The work is part of a long-term investigation aimed at a greater understanding of the 600 anthocyanins found in nature. "We're just beginning to scratch the surface of understanding how the body absorbs and uses these different structures," Dr Giusti said.

In June, market researchers reported that sales of anthocyanin-rich blueberries had doubled in the past two years. The berries joined a growing list of what associations marketing the products call "superfoods", alongside oily fish, brazil nuts and tomatoes. Anthocyanins have previously been linked to helping towards a healthy heart and with treating skin conditions.