It's long been said that an apple a day keeps the doctor away, but New Zealand scientists say they have identified exactly what ingredient of fruit and vegetables is so healthy.

Research from the government's Plant & Food Research institute suggested that compounds in fruits and vegetables support the natural beneficial microorganisms in the gut that help us digest food and keep our bodies in balance.

Polyphenols, produced naturally in plants and including the compounds that give fruits and vegetables colour, were one of only a few classes of compounds that reach the colon without being digested, said a statement from Plant & Food Tuesday.

Researchers had demonstrated that the compounds were broken down in the colon to produce molecules that had a positive effect on microorganisms naturally found in the digestive system that were known to support digestion and the immune system.

"Dietary fibre and polyphenols both reach the colon and therefore can have an effect on colonic microbiota," research leader Dr. Shanthi Parkar said in the statement.

"Our research has shown that the polyphenols found in fruits and vegetables are broken down by bacteria in the colon. Our studies in the lab have shown that these simpler products influence the microorganisms in the digestive system to support optimum gut health."

The study showed that it might be possible to extract plant polyphenols and use them as ingredients in functional foods that promote gut health, he said.