The darker the berry, the sweeter the juice, so goes the saying.

But it turns out the darker berry or plum or grape, for that matter, the stronger the cancer-fighting properties.

Researchers conducting a recent study found that the compounds that give some fruits and vegetables their rich colors are powerful cancer deterrants.

Evidence from laboratory experiments on rats and on human colon cancer cells also suggests that anthocyanins, the compounds that give color to most red, purple and blue fruits and vegetables, also slow the growth of colon cancer cells.

“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 of the study and an assistant professor of food science at Ohio State University, in a news release.

The findings, which Giusti presented August 19 at the national meeting of the American Chemical Society in Boston, also bring scientists a step closer to figuring out what gives fruits and vegetables their cancer-fighting properties.

Giusti and her colleagues found that in some cases, slight alterations to the structure of anthocyanin molecules made these compounds more potent anti-cancer agents.

In their studies on human colon cancer cells grown in laboratory dishes, the researchers tested the anti-cancer effects of anthocyanin-rich extracts from a variety of fruits and vegetables. They retrieved these anthocyanins from grapes, radishes, purple corn, chokeberries, bilberries, purple carrots and elderberries.

The plants were chosen due to their extremely deep colors and high anthocyanin content.

The researchers found that the amount of anthocyanin extract needed to reduce cancer cell growth by 50 percent varied among the plants. Extract derived from purple corn was the most potent and used the least amount of extract (14 micrograms per milliliter of cell growth solution) to cut cell numbers in half.

Chokeberry and bilberry extracts were nearly as potent as purple corn. Radish extract was the least potent and it took nine times as much (131 milligrams per milliliter) of this compound to cut cell growth by 50 percent.

“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,” Giusti said.

In additional laboratory studies, researchers found that anthocyanin pigments from radish and black carrots slowed the growth of cancer cells anywhere from 50 to 80 percent.

Pigments from purple corn and chokeberries not only completely stopped the growth of cancer cells, but also killed almost 20 percent of the cancer cells while having little effect on healthy cells.