PHILADELPHIA (CBS) – We all know “an apple a day keeps the doctor away” but for the first time we know why.

The secret, gleaned from a Dutch study of 20,000 people over 10 years, seems to be a substance called a flavonoid, present in white-ish fruit and vegetables like the insides of apples and pears, plus cauliflower and cucumbers.

The study found that, for each 25 grams (that’s two or three apple slices) a participant ate per day, stroke chance went down nine percent.

“We want you to have at least an apple or pear every day. You want to have more than one, that’s fine, but you should also mix it up with other fruits of other colors, because we believe these other pigments and other metabolites also have benefits in reducing certain diseases,” says Dr. Emil Matarese, director of the stroke center at St. Mary Medical Center in Langhorne.

High consumption of apples and pears lowered stroke risk by more than half.

For more information, go to http://www.strokeassociation.org.

Reported By John Ostapkovich, KYW Newsradio