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%. 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.
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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.